Author: Mike Busch (page 1 of 4)

Wonderful news for Continental 520/550 owners!

Mandatory Service Bulletin MSB05-8B (camshaft gear) downgraded to non-mandatory. FAA will not issue AD.

In April, my blog post “Continental’s War on Camshaft Gears” I wrote about Continental Motors’ issuance of Mandatory Service Bulletin MSB05-8B intended to compel owners of Continental 520- and 550-series engines (and a few IO-470s) to preemptively replace the older-style camshaft gears with a newer-style gear that is .060” thicker (about the thickness of a penny).

MSB05-8B would have mandated that engines with the older-style gear would need to be disassembled and the new-style gear installed “within 100-hours of operation, at the next engine overhaul (not to exceed 12 years engine time in service), or whenever camshaft gear is accessible, whichever occurs first.” This would have meant that many thousands of low-time-since-overhaul engines would need to be torn down within 100 hours, and that any engine overhauled more than 12 years ago would need to be torn down before further flight.

Owners push back

When MSB05-8B hit the streets, aviation type club forums were awash with cries of disbelief, expletives, and demands for class-action lawsuits—both against Continental Motors and against the overhaul shops that elected to overhaul engines without installing the new-style camshaft gears. The uncertainty also took its toll on the resale market for Continental-powered aircraft.

My company Savvy Aviation joined a group of stakeholder representatives including AOPA, American Bonanza Society, Cirrus Owners and Pilots Association, and Twin Cessna Flyer. Our group prepared a 10-page response to the FAA on this subject that was submitted to the FAA Engine & Propeller Directorate and to the Atlanta Aircraft Certification Office on May 1. We argued forcefully that the extremely low crankshaft gear failure rate did not rise to the level of “an unsafe condition” required to justify the issuance of an AD.

May and June passed with no word from Continental. Then in early July, AOPA’s Dave Oord emailed the members of our group to say that Continental was about to issue the long-awaited revised MSB and had asked to discuss it with us prior to publication. We were surprised and delighted that Continental was reaching out to us, and agreed to an electronic meeting on July 13th.

Can we talk?

Continental told us that they were planning to release a new MSB05-8C the next day, calling for repetitive visual inspections of older-style camshaft gears at every annual or 100-hour inspection (whichever was applicable to the aircraft), with gear replacement mandated at the next overhaul or case-splitting event. We learned that the FAA and Continental had uncovered only seven documented camshaft gear failures from 1964 to the present, the majority of which were unrelated to any in-flight engine anomalies. We also learned that an estimated 26,000 engines would be affected by the MSB.

Alarmingly, this was issued as a Mandatory Service Bulletin (MSB), which Continental defines as one that “has been incorporated in whole or in part into an Airworthiness Directive (AD) issued by the FAA or have been issued at the direction of the FAA by the manufacturer requiring compliance with an already-issued AD.” It was conspicuously NOT issued as a Critical Service Bulletin (CSB), which Continental defines to be a “candidate for incorporation into an FAA Airworthiness Directive.”

We made a strong appeal for Continental to issue its revision as a CSB rather than an MSB, given that the FAA had not yet decided whether an AD was warranted. We also urged the FAA to think carefully about whether such a tiny number of gear failures over such a long time period (most of which had no safety consequences) really rose to the level of “an unsafe condition” under the FAA’s guidelines for when an AD should be issued. We further argued that the repetitive inspections Continental was proposing would be staggeringly costly to owners and would not prevent a single engine failure.

To put all this in perspective, there has been only ONE in-flight camshaft gear failure in the past 53 years, and that one resulted in an uneventful on-airport forced landing. This makes the camshaft gear arguably the most reliable and least failure-prone component of the engine.

Wonderful news!

The next day, July 14th, each of us individually received a call from Continental Vice President Emmanuel Davidson, who gave us wonderful news: After carefully considering our comments and conducting further discussions with the FAA, Continental had decided to issue its revision as a non-mandatory CSB, and the FAA had decided that no AD was warranted at this time.

This was a marvelous outcome for owners of Continental-powered aircraft, and it was achieved through the most constructive and cooperative interaction I’ve ever seen between owners, a manufacturer, and the FAA (and I’ve been doing this for a long time). I sincerely hope this will become a model for how such situations are dealt with going forward. Kudos to Continental and the FAA for listening with open minds, and ultimately doing the right thing.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.

It’s Different For Cars

The owner of the late-model Cessna T206 Turbo Stationair was livid.

Takata Airbag

Takata Airbag

“Imagine your car is equipped with a Takata airbag system whose faulty inflators often rupture and spray shrapnel into drivers and passengers, resulting in at least 11 deaths in the U.S. and hundreds of injuries,” his email to me began. “But instead of being recalled by the manufacturer, you were instructed by the government that to continue driving your car legally, you were required to take it to a mechanic every 500 miles for a costly inspection at your expense. If your airbag system didn’t pass the inspection, you would be required to pay about $1,200 for a new airbag inflator, again at your expense.”

The Stationair owner was reacting to a just-issued Airworthiness Directive against the exhaust system of his Lycoming TIO-540-AJ1A engine. There are 758 of these engines currently in service. AD 2017-11-10 was apparently prompted by reports of exhaust leaks that could result in excessive carbon monoxide (CO) getting into the cabin. Some of these leaks were caused by cylinder exhaust port studs coming loose, while others were caused by cracked exhaust pipe weld joints.

Lycoming TIO-540-AJ1A

Lycoming TIO-540-AJ1A

The AD requires an initial exhaust system inspection and fastener torque check within 10 hours, and then repetitive exhaust system inspections every 25 hours and torque checks every 100 hours. The FAA estimated the cost of compliance to be $85 (one hour of labor) per required inspection, but that doesn’t take into account the burden on the owner of having to take his airplane to an A&P mechanic every 25 hours (which is half the normal oil-change interval).

Lycoming issued a service bulletin which allowed the repetitive inspection interval to be extended from 25 to 50 hours if the aircraft was equipped with a carbon monoxide detector, but inexplicably the AD does not include this provision. Lycoming presently doesn’t have a fix for this problem (although they claim they’re working on one), so there’s no way of knowing how long the owners of the 758 affected Lycoming TIO-540-AJ1A engines will be required to do these 25-hour exhaust inspections.

“Why do aircraft owners put up with this?” the Stationair owner continued. “For decades, Cessna and Lycoming have been building the same product, and they still can’t get it right. Why? Because they have no incentive to do so. If they make a design or manufacturing mistake, they just pass the costs on to their customers. Nice scam.”

I felt the owner’s pain. I fly a Cessna 310 with a cabin heater made by Stewart-Warner (Southwind), and was just hit with an AD against my heater that will force me to replace it with a new AD-free heater at my upcoming annual in October, at a cost of $6,000 in parts and probably $2,000 in labor. And as I discussed in my last blog post, the FAA is threatening to issue an AD against the camshaft gears in my two Continental TSIO-520-BB engines (and tens of thousands of other engines) that could cost owners like me a bundle.

It’s gotten so bad that when a colleague of mine recently told me he was looking to buy an airplane and was thinking about an older Mooney, I suggested he look into buying an amateur-built experimental airplane instead in order to get out from under the AD burden that has been plaguing us owners of certified airplanes.

It’s different for cars

NHTSAThe Stationair owner was right to point out that the rules are very different for motorists than they are for aircraft owners. In 1966, Congress passed The National Traffic and Motor Vehicle Safety Act (49 USC 301) that gave the Department of Transportation’s National Highway Traffic Safety Administration (NHTSA) the authority to issue vehicle safety standards and to require manufacturers to recall vehicles that have safety-related defects or do not meet Federal safety standards.

Effectively, these NHTSA motor vehicle recalls are the automotive equivalent of Airworthiness Directives. But there’s a big difference: In most cases, the auto manufacturers are required by law to bear the cost of fixing the vehicles. The burden usually doesn’t fall on the vehicle owners.

Why isn’t there a similar law for aircraft? I’m guessing that there are just not enough folks in Congress who care about aircraft owners to support such legislation. By contrast, every member of Congress is a motorist, so the laws are spring-loaded in favor of protecting motorists. But even if the laws protecting motorists were extended to aircraft owners, the lion’s share of the AD burden wouldn’t go away.

Recall NoticeWhy? Well, for one thing, 49 USC 301 requires automobile manufacturers to bear the expense of recalls only for vehicles that are less than 10 years old. While this covers most cars that are recalled, the overwhelming majority of today’s GA fleet consists of airplanes that are more than 10 years old, frequently much more.

The lion’s share of piston GA aircraft were built in the 1960s and 1970s, and then production all but stopped in the 1980s and never came close to recovering to the levels seen in the salad days of piston GA. (My 1979 Cessna 310 is 38 years old, and it’s a “recent model” as Cessna 310s go.) So for most GA airplanes, the manufacturer would be off the hook.

In addition, the law only requires the manufacturer to pay for repair of recalled vehicles if those repairs are performed by an authorized dealer of the manufacturer. Relatively few GA owners have their maintenance performed at authorized dealers, and many makes of GA aircraft no longer have dealers; indeed, many were manufactured by companies that aren’t in business anymore. In general, GA has a far less robust support system compared to automobiles.

Changes I’d really like to see

Bureau of Automotive RepairThe laws I’d most like to see extended to GA are the ones that deal with repair facilities. Automotive repair facilities are typically regulated by the states, not by the feds. Most states require automotive repair facilities to be licensed, and have lots of state laws protecting motorists from unscrupulous repair shops. In most cases, an automotive repair shop cannot work on your car until they’ve given you a written work order itemizing the work they will do and providing a cost estimate (including parts, labor, and outside work), and obtained your signature approving the work order and estimate. Then they are required not to charge you significantly more than the agreed-to estimate.

In the event that the shop runs into something that might result in exceeding the original estimate, they are required to stop work, furnish you with an explanation and a revised estimate, and secure your approval of the new estimate before they may continue. These rules ensure that there will never be any surprises when you receive the final invoice.

To comply with these rules, most auto repair shops use a flat-rate price list for all the most common maintenance tasks they perform. You’ll typically pay the same price for, say, an oil change or a brake job or a tire rotation regardless of how much time the technician spent doing the work.

By contrast, most maintenance work on airplanes is done on a time-and-materials basis, often with no paperwork until the job is done. Sticker shock is rampant as a result, because often the owner doesn’t have a clue what the work will cost until it’s done. This is a bad system, and often results in hard feelings and arguments when owners feel they’ve been charged too much.

I’d love to see the state laws that govern auto repair extended so that they cover aircraft maintenance shops as well, but I’m not holding my breath. Few state legislators give a fig about aircraft owners. In the meantime, it’s up to the owner to demand a written estimate before permitting any shop to work on their aircraft, and to hold the shop to that estimate unless there’s an awfully good reason that it was exceeded.


Not long after this blog post was published by AOPA, I received a phone call from Lycoming and learned that there is some good news and some light at the end of the tunnel for owners of TIO-540-AJ1A-powered Cessna T206s affected by AD 2017-11-10.

First, Lycoming requested and the FAA approved an Alternative Means of Compliance (AMOC) that extends the exhaust inspection interval from 25 hours to 50 hours for aircraft that are equipped with a suitably sensitive carbon monoxide (CO) detector. That will halve the inspection burden and promote CO detector installation, both of which are good things.

I also learned that Lycoming has redesigned the TIO-540-AJ1A exhaust system in order to provide a permanent solution to the problem and a terminating action for the AD. The new exhaust system will be made of 321 stainless steel, in contrast with the Inconel used in the current system. Inconel has outstanding high-temperature characteristics, but it is much more brittle and harder to work with than 321 stainless, and has a much less desirable failure mode. Lycoming concluded that the Inconel system was not sufficiently flexible to deal with the dimensional changes and thermal stresses that occur as the exhaust system heats up and cools down. Most other turbocharged Lycoming engines used a stainless steel exhaust system, and Lycoming believes the new system will prove much more durable and less failure-prone than the current one.

No word yet on when production quantities of the new exhaust parts will be available or what they will cost. But it does look like the repetitive inspections mandated by AD 2017-11-10 won’t have to go on forever. –MDB

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.

Continental’s War on Camshaft Gears

Correction: AOPA has corrected a statement regarding Continental’s mandatory service bulletin and FAA airworthiness directive activity. The FAA has confirmed that it has not received any communications from Continental seeking an airworthiness directive. AOPA regrets the error.

At the end of March 2017, Continental Motors issued Mandatory Service Bulletin MSB05-8B that would require tens of thousands of Continental IO-470/520/550 engines to be torn down prematurely to replace the camshaft gear with slightly thicker gear. It required compliance within the next 100 hours of operation, or at no more than 12 years since overhaul, whichever comes first.

The replacement camshaft gear is 0.06″ thicker, and costs$1,200. Removing and reinstalling the engine is typically a $5,000 job and the engine teardown typically costs around $10,000. So this would be a very big deal for affected owners. The number of affected owners is huge. Almost every Bonanza, Baron, Cessna 200/300/400, Cirrus SR22, and Continental-powered Mooney is targeted by this expensive MSB, plus a bunch of other makes and models.

Compliance with manufacturer’s service bulletins is not generally compulsory for Part 91 operators. The FAA would have to issue an Airworthiness Directive (AD) to compel owners to comply.

Continental first introduced this thicker camshaft gear in August of 2005 and started installing it in its factory new and rebuilt engines. But it wasn’t until November 2009 that it started asking field overhaul shops to install the newer-style gear at engine overhaul. Since it did so in a service bulletin (SB97-6B), almost all overhaul shops considered it to be non-compulsory, and therefore almost all field overhauled engines had their older-style camshaft gears reused.

This means that if the FAA were to issue an AD mandating compliance with Continental’s MSB05-8B, a whole lot of low-time (and even no-time) engines would need to be torn apart. Naturally, Continental would not be picking up any of the cost of doing this (as would be the case if this was an automotive recall rather than an aviation recall). Aircraft owners would be hit with the cost, which would be in the tens of millions of dollars.

Where’s the beef?

We talked with the FAA about this, and were told that Continental had provided the FAA data about only three camshaft gear failures in support of its request for an AD. The FAA also indicated that none of these failures were new ones. A search of the FAA’s Service Difficulty Report (SDR) database uncovered a total of 13 SDRs involving the older-style camshaft gears, most of which did not involve actual failures of the gears. That’s an awfully small number considering that tens of thousands of these gears have been in service for more than 40 years.

Even if all 13 of those SDRs represented gear failures (which they did not), my back-of-the-envelope calculations suggests that this would be one failure every 7,000,000 flight hours, which would make the camshaft gear one of the most reliable components of the engine. I’m almost certain that connecting rods and crankshafts fail more often than camshaft gears do.

If these gears have been in service for more than 40 years, and if there’s no new data indicating that they’re starting to fail at an accelerated rate, why would Continental suddenly conclude that a safety issue exists that warrants asking the FAA for an AD that would cost owners tens of millions and put thousands of airplanes on the ground for weeks or months?

By the way, we checked with three major parts distributors (Aviall, Omaha, A.E.R.O.) and learned that the newer-style camshaft gear is presently backordered for two months. We expect the situation to get worse fast as news of MSB05-8B and a possible AD spreads throughout the owner and mechanic communities.

As owners started learning about MSB05-8B through type clubs, online discussion groups and the electronic aviation press, there was “panic in the streets.” Sales of affected aircraft started falling through as prospective buyers walked away from deals. Owners started talking about class-action lawsuits against overhaul shops who reused older-style camshaft gears instead of installing new ones. Some owners with aircraft undergoing annual inspections started asking whether they should be having their engines overhauled. Other owners started worrying whether it was safe to fly their aircraft.

When we contacted numerous well-known engine overhaul shops, we were surprised to find that all of them told us they had been seeing no problem with the older-style camshaft gears, and all had been reusing the gears at overhaul once they passed inspection (which almost all did). The shops were unanimous that they saw no unsafe condition that would warrant an AD.

So what the heck is Continental’s reason for making a federal case of this now? I honestly don’t know, although as you might imagine there’s an awful lot of speculation and a few conspiracy theories floating around.

Late-breaking developments

On April 20, Continental issued a press release indicating their intention to walk back the Draconian compliance requirements of MSB05-8B. In the press release, Continental promised another revision of the MSB within 15 days, and indicated that the revised document would:

  • Eliminate the preemptive camshaft gear replacement in favor of a mandatory repetitive visual inspection procedure allowing on-condition operation until the engine is overhauled or removed for some other reason.
  • Change the 100-hour or 12-year gear replacement requirement to something that owners can live with more easily.
  • Provide an alternative procedure for replacing the camshaft gear that would not require complete engine disassembly (although it would require removal from the aircraft and partial disassembly).

We sincerely hope that Continental’s revised MSB is something that aircraft owners can live with. If not, there’s going to be a huge battle. After all, we’ve been living with these gears for more than four decades, and the overhaul shops are not seeing a problem with them.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.

It’s Baffling

The email from a Cessna T210 owner read:

Suggested baffle holes

The owner of this T210 suggested making some baffle modifications to improve cooling of cylinders #5 and #6 by “giving them more air.” This would NOT have been a good idea, and would almost certainly have made things worse instead of better.

I recently had my engine rebuilt and had a new baffle kit installed. The CHTs for cylinders #5 and #6 are always 20ºF to 30ºF hotter than the rest. During climb the difference gets even bigger. Cylinder #5 and #6 CHTs are very difficult to keep below 400ºF during a climb, even with the cowl flaps open and rich mixture. Should I consider giving them some air? On cylinder #6, why not cut one or more holes in the white aluminum baffle in front of the cylinder? On cylinder #5, why not drill one or more holes in the horizontal aluminum plate located behind the oil cooler?

I replied that cutting holes in the baffles was definitely NOT a good idea, and that doing so would undoubtedly make the cooling problems worse, not better. It was apparent that the T210 owner didn’t understand how the powerplant cooling system in his aircraft works, or what the function of the baffles is. He’s not alone—some A&P mechanics don’t fully understand it, either!

Cooling: then and now

Spirit of St. Louis

Early aircraft engines were ‘velocity cooled’ by passing the slipstream over the finned cylinders. However, this simple approach to cooling is simply not practical for today’s high-performance engines and low-drag airframes.

In the early days of aviation, aircraft designers took a simple approach to the problem of cooling aircraft engines. The engines were mounted with their finned cylinders out in the slipstream and cooled by the horizontal flow of ram air. This design is known as “velocity cooling” and was adequate for cooling the low-compression single-row radial engines of the time.

As engines grew more powerful and multi-row radials and horizontally opposed engines went into service, it became obvious that simple velocity cooling wasn’t up to the job. For one thing, cooling was uneven—front cylinders got a lot more cooling airflow than rear cylinders. For another, sticking all those cylinders out in the breeze created horrendous cooling drag. A better scheme was obviously needed.

That better system was known as “pressure cooling” and is the method used in all modern piston aircraft. Pressure cooling is accomplished by placing a cowling around the engine and using a system of rigid baffles and flexible baffle seals to produce the volume and pattern of cooling airflow necessary to achieve even cooling with minimum drag.

What do baffles do?

Cooling Airflow

The heart of a modern ‘pressure-cooled’ powerplant installation is a set of rigid sheet-metal baffles and flexible baffle seals that, together with the engine cowling, divide the engine compartment into two chambers: a high-pressure area above the engine and a low-pressure area below and behind the engine. Engine cooling depends upon the vertical airflow from the upper chamber to the lower one. Cowl flaps modulate the cooling by regulating the vacuum in the low-pressure chamber.

Our modern piston aircraft are powered by tightly cowled horizontally opposed engines. Inside the cowling, a system of rigid aluminum baffles and flexible baffle seals divide the engine compartment into two chambers: a high-pressure area above the cylinders, and a low-pressure area below the cylinders and behind the engine. Cylinders are cooled by the vertical flow of air from the high-pressure above the engine to the low-pressure below it. Cooling airflow is top-to-bottom, not front-to-back.

The volume of cooling airflow that passes across the cylinders is a function of the pressure differential between the upper (high-pressure) chamber and the lower (low-pressure) chamber of the engine compartment.  This pressure differential is known as “delta-P.” Cowl flaps are often used to modulate the cooling airflow. Opening the cowl flaps reduces the air pressure in the lower chamber, thereby increasing delta-P and consequently the volume of cooling air that passes vertically across the cylinder fins.

It’s important to understand that the pressure differential between the upper and lower chambers is remarkably small: A typical high-performance piston aircraft generally relies on a delta-P of just 6 or 7 inches of water—about 1/4 PSI! Aircraft designers try to keep this delta-P to an absolute minimum, because higher delta-P means higher cooling drag.

…and so what if they don’t?

Baffle Seals

Flexible seals are used to prevent air from escaping through the gaps between the engine-mounted sheet-metal baffles and the cowling. To do their job, they must be oriented so as to curve toward the high-pressure chamber above the engine, so that air pressure pushes them tightly against the cowling.

Because the pressure differential (delta-P) on which engine cooling depends is so very small, even small leaks in the system of baffles and seals can have a serious adverse impact on engine cooling. Any missing, broken, or improperly positioned baffles or seals will degrade engine cooling by providing an alternative path for air to pass from the upper chamber to the lower chamber without flowing vertically across the cylinder cooling fins.  (This is precisely what the effect would have been had the T210 owner cut holes in his baffles, which is why I strongly discouraged the idea.)

Probably the most trouble-prone part of the cooling system is the system of flexible baffle seals. These flexible strips (usually high-temp silicone rubber) are used to seal up the gaps between the sheet metal baffles and the cowling. These gaps are necessary because the baffles move around inside the cowling as the engine rocks on its shock mounts.

To do their job, the seals must curve up and forward into the high-pressure chamber, so that the air pressure differential (delta-P) presses the seals tightly against the cowling. If the seals are permitted to curve away from the high-pressure area—not hard to do when closing up the cowling if you’re not paying close attention—they can blow away from the cowling in-flight and permit large amounts of air to escape without doing any cooling.

I recall some years ago inspecting a Cessna TR182 whose pilots had complained of high CHTs. Upon removing the top engine cowling, I immediately spotted the problem: One of the ignition leads was misrouted and became trapped between the baffle seal and the cowling, preventing the baffle seal from sealing against the cowling. The ignition lead had become severely chafed where it rubbed against the cowling, and an A&P had wrapped the chafed area with electrical tape, but failed to reroute the tape-wrapped lead to keep it away from the baffle seal. Clearly that A&P didn’t understand the importance of an air-tight seal between the baffle seals and the cowling. Repositioning the ignition lead solved both the cooling problem and the chafing problem.

Another common problem is that seals may develop wrinkles or creases when the cowling is installed, preventing them from sealing airtight against the cowling and allowing air to escape. It’s important to look carefully for such problems each time the cowling is removed and replaced, and especially important when new seals have been installed (as was the case with the T210).

Intercylinder Baffles

Inter-cylinder baffles are oddly-shaped pieces of sheet metal that mount beneath and between the cylinders, and force the down-flowing cooling air to wrap around and cool the bottom of the cylinders. (This photo was taken looking up from the bottom of the engine, with the exhaust and induction systems removed to make the baffle easier to see.)

Yet another trouble-prone part of the cooling system is the inter-cylinder baffles. These are small, oddly-shaped pieces of sheet metal mounted below and between the cylinders. Their purpose is to force the down-flowing cooling air to wrap around and cool the bottom of the cylinders, rather than just cooling the top and sides. These baffles are difficult to see unless you know exactly where to look for them, but they are absolutely critical for proper cooling. It’s not at all uncommon for them either to be left out during engine installation or to fall out during engine operation. Either way, the result is major cooling problems.

Awhile back, I noticed that the #3 cylinder of the right engine on my Cessna 310 was running noticeably hotter than its neighbors. I removed the top cowling from the right engine nacelle and carefully inspected all the aluminum baffles and rubber baffle seals, but couldn’t find anything awry. Frustrated, I removed the lower cowlings so that I could inspect the underside of the engine. Sure enough, I discovered that the intercylinder baffle between cylinders #1 and #3 had vibrated loose and shifted about 1/4 inch out of position, creating a significant air leak near the #3 cylinder. Repositioning the baffle properly and tightening its attach bolt to hold it securely in place against the cylinders and crankcase solved the problem.

Why the T210 engine ran hot


Close-up of a fairly significant cooling air leak due to a wrinkle in a flexible baffle seal. This problem was apparent only with the top cowl installed, and could be seen by inspecting through the front intake openings using a flashlight. It’s an excellent idea to look for such baffle seal problems during preflight inspection.

With this as background, I emailed the T210 owner to discourage him from cutting holes in his baffles, and suggested instead that he examine his baffles and seals for existing holes and gaps that could be plugged up to improve cooling. A couple of days later, the owner emailed me back a series of digital photos showing a half-dozen air leaks that he found in his newly installed baffles.

One of those photos revealed a fairly significant cooling air leak due to a wrinkle in a flexible baffle seal. This problem was apparent only with the top cowl installed, and could be seen by inspecting through the front intake openings using a flashlight. Savvy pilots who understand the importance of baffles and seals look for this sort of thing during pre-flight inspection. (Since mechanics do most of their inspecting with the cowlings removed, problems like this sometimes escape their detection.)

I studied the photos and continued my email dialog with the Cessna owner. Between the two of us, we managed to identify a dozen leaks in the T210’s new baffle system. Some were small, others more serious. Combined, they accounted for a significant loss of cooling efficiency. With a few well-placed dabs of high-temp RTV sealant and a little trimming of the flexible seal strips, the owner plugged the leaks in short order, and his engine began running noticeably cooler.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.

Why change the oil?

Aeroshell W100 PlusContinental and Lycoming tell us that we must change the oil in our engines every 50 hours or 4-6 months, whichever comes first—and that’s if we have a full-flow oil filter installed. If we have only an oil screen, then the oil change interval goes down to 25 hours. Did you ever wonder why we need to change the oil so often?

It’s not because the oil breaks down in service and its lubricating qualities degrade. The fact is that conventional petroleum-based oils retain their lubricating properties for a very long time, and synthetic oils retain them nearly forever.

Consider, for example, that most automobile manufacturers now recommend a 7,500-mile oil-change interval for most cars and light trucks. That’s the equivalent of 150 to 250 hours of engine operation. In fact, oil analysis studies have shown that a synthetic automotive oil like Mobil 1 or Amsoil can go 18,000 miles without appreciable degradation, and that’s the equivalent of 400-600 hours.


No, the reason we change oil in our aircraft engines every 25 to 50 hours is not because it breaks down. It’s because it gets contaminated after 25 to 50 hours in an aircraft engine. In fact, it gets downright filthy and nasty.


Dihydrogen monoxide (DHMO) is a highly corrosive chemical that is produced in copious quantities during combustion, and can cause great harm to costly engine components when it blows by the piston rings and contaminates the engine oil. You may be more familiar with DHMO’s common chemical formula: H2O.

Compared with automotive engines, our piston aircraft engines permit a far greater quantity of combustion byproducts—notably carbon, sulfur, oxides of nitrogen, raw fuel, partially burned fuel, plus massive quantities of the corrosive solvent dihydrogen monoxide or DHMO (see graphic)—to leak past the piston rings and contaminate the crankcase. This yucky stuff is collectively referred to as “blow-by” and it’s quite corrosive and harmful when it builds up in the oil and comes in contact with expensive bottom-end engine parts like crankshafts and camshafts and lifters and gears.

To make matters worse, avgas is heavily laced with the octane improver tetraethyl lead (TEL), which also does nasty things when it blows by the rings and gets into the crankcase. (If you’re as old as I am, you may recall that back before mogas was unleaded, the recommended oil-change interval was 3,000 miles instead of 7,500 miles.)

So one of the most important reasons that we need to change the oil regularly in our Continentals and Lycomings is to get rid of these blow-by contaminants before they build up to levels that are harmful to the engine’s health.


Another reason we need to change the oil regularly—arguably even more important than disposing of contaminants—is to replenish the oil’s additive package, particularly its acid neutralizers. When sulfur and oxides of nitrogen mix with DHMO, they form sulfuric acid and nitric acid. If you remember these dangerous corrosives from your high school chemistry class, then you’ll certainly appreciate why you definitely don’t want them attacking your expensive engine parts.

OIl analysisTo prevent such acid attack, aviation oils are blended with acid neutralizer additives. These are alkaline substances that neutralize these acids, much as we might use baking soda to neutralize battery acid. These acid neutralizers are consumed by the process of neutralizing acids, so it’s imperative that we replenish them before they get used up to an extent that might jeopardize our hardware. Of course, the way we replenish them is to change the oil.

How can we tell when the acid neutralizers in the oil have been used up? It turns out that there’s a laboratory test that measures the level of unneutralized acid remaining in the oil. This is known as the “total acid number” or “TAN” test. Some oil analysis firms can perform this test on your oil samples. However, it’s not routinely done as part of the normal oil analysis report, so you need to specially request a TAN test when you send in your oil sample (and be prepared to pay extra for it).


Tach w/hourmeterMost owners don’t bother with the hassle and expense of TAN testing, and simply change their oil at a conservative interval that’s guaranteed to get the junk out and fresh additives in before anything untoward is likely to occur.

On my own airplane, what I do (and generally recommend to my clients) is to change the oil and filter every 50 hours or 4 calendar months, whichever comes first. This means that operators who fly at least 150 hours a year can go 50 hours between oil changes, but operators who fly less will use a proportionately reduced oil-change interval.

This recommendation assumes that the aircraft has a full-flow (spin-on) oil filter installed, that it operates primarily from paved runways, and that it has decent compressions and relatively low blow-by past the rings. Engines that have only an oil screen (no filter) should have the oil changed every 25 hours. Engines that operate in dirty or dusty conditions and ones that have high oil consumption due to high blow-by should have more frequent oil changes.

My friend Ed Kollin—lubrication engineering wizard who used to head Exxon’s lubrication lab and who developed ASL CamGuard—is even more conservative. He preaches that oil should be changed no less frequently than every 30 hours, and frowns when I suggest that it’s okay to go to 50 if you fly a lot.


InsolublesAnother important indication of oil condition can be found in standard oil analysis report provided by some labs—notably the one I prefer, Blackstone Laboratories in Ft. Wayne, Indiana—is the “insolubles” test. This test is performed by placing the oil sample in a centrifuge to separate out all solids and liquids in the sample that are not oil-soluble.

Virgin oil normally contains no insolubles. The insolubles found in drained engine oil come from three sources: (1) oxidized oil that breaks down due to excessive heat; (2) contaminants from blow-by of combustion byproducts; and (3) particulate contamination caused by poor oil filtration. If your oil analysis report reveals above-normal insolubles, it might be indicative of an engine problem—high oil temperature, excessive blow-by, inadequate filtration—and almost certainly means you should be changing your oil more frequently.

By the way, did I mention that I’m a huge fan of laboratory oil analysis? I use it religiously, recommend it strongly to all piston aircraft owners, and believe that it’s one of the most important tools we have—along with oil filter inspection and borescope inspection—for monitoring the condition of our engines and determining when maintenance is necessary.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.

Temperamental Ignition

A funny thing happened to me on a coast-to-coast trip from California to the East Coast in my Cessna 310. I was on a business trip that would take me from my home base in Santa Maria, California, to Frederick, Maryland, then to Atlanta, Georgia, and then home again.

I first became aware of the problem as I was climbing out of Santa Maria on the very first leg of what I expected to be a 30-hour round-trip. I had reduced to my usual 75% cruise-climb power, was climbing at my usual 130 KIAS cruise-climb airspeed, with the usual 110 pounds/hour fuel flow on each engine. This was all standard routine that I’d performed hundreds of times before.

The engines felt smooth. The airplane was climbing nicely at about 1,000 FPM despite being loaded right at max gross.  The air was smooth. My yoke-mounted SeriusXM satellite weather display indicated no significant weather all the way to Tulsa, Oklahoma, where I planned to make an overnight stop before continuing on to Frederick. The SeriusXM audio was tuned to the classical music channel, piping one of my favorite Bach Brandenburg Concertos into my stereo ANR headset. All seemed right with the world.

My reverie was interrupted by a flashing amber annunciator light that told me my digital engine monitor was trying to get my attention. Sure enough, when I looked over at the instrument on the right side of the panel, its display was flashing a high turbine inlet temperature (TIT) alarm on the left engine, and displaying the TIT as 1620°F. I knew from experience that normal TIT in this configuration is around 1570°F and I’d programmed the engine monitor to alarm any time the TIT exceeded 1600°F.

JPI EDM 760 engine monitor

On climbout, the left engine showed excessive TIT and excessive EGT on the #5 cylinder. However, #5 CHT was normal. This suggested that one spark plug might not be firing in the #5 cylinder. An in-flight mag check confirmed that indeed the bottom plug was not firing.

In-flight troubleshooting

Looking carefully at the engine monitor’s digital readouts, I noticed that the EGT on the left engine’s #5 cylinder was noticeably higher than the other cylinders, and definitely higher than what I was used to seeing. The high #5 EGT suggested to me one of two possible problems: (1) a partially clogged #5 injector; or (2) a #5 spark plug that wasn’t firing.

If a clogged injector was causing cylinder #5 to run too lean, I would have expected that cylinder also to have elevated CHT while operating ROP during climb. However, the engine monitor did not indicate that the #5 CHT was elevated; if anything, it seemed to be a bit lower than usual.

That suggested to me that a non-firing spark plug was the most likely cause of the elevated #5 EGT. To confirm this theory, I performed an in-flight mag check. When I shut off the righthand magneto switch for the left engine, the left engine started running quite rough and the #5 EGT bar on the display dropped out of sight.


My problem was definitely a non-firing spark plug in the #5 cylinder.

Which plug wasn’t firing? Because the cylinder went cold when I shut off the right magneto, the non-firing plug had to be the one connected to the left magneto. On my engines (as with most big-bore TCM engines), each magneto fires the top plugs on its side of the engine and the bottom plugs on the opposite side of the engine. Since cylinder #5 is in the right bank of cylinders, its top plug is fired by the right mag and its bottom plug is fired by the left mag. Therefore, I reasoned, my non-firing spark plug had to be the bottom plug on cylinder #5.

(Bottom plugs tend to misfire much more often than top plugs, because the bottom ones are so vulnerable to oil-fouling and contamination with debris.)

That’s odd, I thought. I had done a thorough runup prior to takeoff, including the usual preflight mag check at 1700 RPM. All 24 spark plugs appeared to be working just fine. Why would one decide to quit working now? Definitely odd.

I leveled off at my cruising altitude of 13,000 feet and did the “big mixture pull” to transition to LOP. The engine monitor continued to show elevated DIFF on the left engine, and elevated #5 EGT. During the next couple of hours, I repeated the in-flight mag check a couple of more times and got exactly the same result: The bottom plug on cylinder #5 was definitely not firing.

Sometimes fouled plugs clear themselves spontaneously. But not this time. Darn!

Not to worry, that’s why I always carry a couple of spare spark plugs in the emergency toolkit I keep in my left wing locker, together with the necessary tools to change out a plug on the ramp if necessary. So I knew what I had to do.

Spark plug transplant

I landed at my first planned refueling stop, Saint Johns, Arizona. KSJN is a frequent fuel stop for me going eastbound because it consistently has among the lowest 100LL prices west of the Mississippi. Also, KSJN has a field elevation of 5,736 feet MSL, which shortens the descent for landing and the subsequent climb back to altitude. All in all, it’s one of my favorite places to refuel.

After topping off the tanks, I retrieved my emergency toolkit and proceeded to remove the bottom spark plug from cylinder #5 of the left engine.

The plug had been in service for about 100 hours, and it looked okay to me. But since it clearly wasn’t firing, I decided to swap it out anyway. I installed a brand new spark plug in the bottom plug hole of left engine cylinder #5, torqued it to 360 in.-lbs. using the torque wrench I carry in my emergency toolkit, and reattached the ignition lead.

After closing up the left engine nacelle and stashing my emergency toolkit back in the wing locker, I fired up and taxied out for departure. At the runup area, I performed an extra diligent runup and mag check to verify that all plugs were firing properly—they were. I then took off and turned eastbound toward Tulsa.

Climbing out of KSJN, I tuned the SeriusXM audio to the ‘60s oldies channel and was just getting into the groove when it happened again: The amber light started flashing and the engine monitor started complaining about high TIT on the left engine. A quick cycle of the instrument and a quick in-flight mag check confirmed that the bottom plug on #5 was once again not firing. Yes, the very same brand new spark plug that I’d just installed!


Plug transplant, part deux

I continued on to Tulsa, taxied to the FBO, and broke out my emergency toolkit once again. This time, I removed the newly-installed plug and installed my one remaining spare. I wasn’t sure it would solve my problem, but figured it was worth a shot.

The next day, climbing eastbound out of Tulsa, I actively monitored the engines looking for signs of trouble. Everything seemed to be working fine. After leveling off in cruise and switching to LOP, I tried another in-flight mag check. The left engine continued to run smoothly on each magneto  individually, and the engine monitor confirmed that everything was operating normally now.


I flew nonstop to Frederick at FL210 (to stay above a bunch of rather nasty frontal weather). High-altitude LOP operation is pretty demanding on the ignition system, but the engines didn’t miss a beat and another in-flight mag check at altitude confirmed that all was well.

After completing my business in Frederick, I flew to Charlotte, North Carolina to spend a few days with my in-laws who live there, then proceeded on to Atlanta for another business meeting. After that, I headed home to the west coast with stops in Memphis and Denver. The engines continued to run perfectly, and I pretty much forgot about the earlier ignition problem.

It’s baaaack!

After returning home to Santa Maria and resting up a bit, I decided it was time to do some preventive maintenance on the airplane. I changed the oil, sent oil samples to the lab for analysis, replaced the oil filters, and cut open the old filters for inspection. (No metal.)

Since the spark plugs had over 100 hours on them, I pulled them and sent them to Aircraft Spark Plug Service in Van Nuys for cleaning, gapping, and bomb testing. All of my cleaned/gapped spark plugs passed the bomb test with flying colors and came back a week later, whereupon I reinstalled them in the engines.

After closing up the engine nacelles, I took the airplane out for a post-maintenance test flight. A thorough pre-flight runup indicated that everything was working fine. But the test flight once again revealed elevated DIFF and elevated #5 EGT on the left engine, and an in-flight mag check showed the bottom #5 spark plug was once again not firing. Arggghhh!!!

It was finally starting to dawn on me that the ignition problem must be something other than a bad spark plug. It had to be either a problem with the magneto itself or a problem with the ignition harness.

I tried replacing the insulator (“cigarette”) and contact spring on the bottom #5 ignition lead, but another test flight showed that this did not solve the problem. I pulled the left mag and opened it up, but couldn’t find anything wrong. The distributor cap was clean inside, the contact springs looked good, the point gap was correct and the internal and external mag timing was spot-on.

Harness transplant

By elimination, that left the ignition harness. I examined the #5 bottom ignition lead and couldn’t spot any visual anomalies. But since I was running out of ideas, and since a brand new full harness (for both mags) cost less than $500.00, I decided to order one and install it. Even though the existing harness looked fine, it did have nearly 2,000 hours on it, so presumably it was fully depreciated.

Ignition Harness

A new full harness (for both magnetos) costs only about $500. (A harness for just one magneto is called a “half harness.”) Figure on four hours of labor to install. I prefer the Slick-brand harnesses (shown above) because of their superior construction and flexibility.

There are a variety of ignition harnesses that are PMA approved for my engines, including Champion, Kelly, Skytronics, Continental, and Slick. I have always preferred the Slick harnesses because of their superior construction and flexibility, so I ordered a new Slick M1740 harness to mate with my Continental/Bendix S-1200 magnetos.

Removing the old harness and installing the new one was more time-consuming than I expected. Doing the job correctly involves considerable Adel clamping, grommeting, and tie-wrapping to ensure that the ignition leads cannot vibrate or chafe on anything and have no tight bends. It took me about six hours to complete the job, including retiming both mags.

I am, of course, the world’s slowest mechanic. I imagine a professional A&P could do it in three or four hours.

Finally, it was time to do yet another post-maintenance test flight. This time, I was overjoyed to find that everything was perfect. The engine monitor readings were just as they should be, and a high-power in-flight mag check showed all systems go. Success at last!

Lessons learned

I learned some important lessons as a result of this experience. One is that the usual pre-flight mag check is a laughably inadequate test of ignition system performance. While trying to track down my problem with a non-firing #5 bottom plug, the ignition system repeatedly showed no problems whatsoever during the pre-flight mag check, only to fail immediately and repeatably as soon as the aircraft was in flight.

Clearly, the pre-flight mag check is not a very demanding test of the ignition system, and won’t detect anything but the grossest ignition anomalies. An in-flight mag check is a far more demanding and revealing test. The most demanding ignition system test is a high-power in-flight mag check with the engine leaned aggressively (preferably LOP).

Many pilots have never done an in-flight mag check, and many are afraid to perform one. I’ve even known some experienced A&P mechanics that discourage pilots from shutting off a magneto in flight. Obviously, I don’t agree with that advice. In fact, in the wake of my experience, I now make a point of performing an in-flight mag check on almost every flight, and I heartily recommend that you consider adopting the same practice.

Another lesson I learned here is the tremendous diagnostic value of a modern digital probe-per-cylinder engine monitor. If it hadn’t been for my JPI EDM 760, I’d never have known that my #5 bottom plug was not firing. It’s quite possible that this situation could have gone on for months and hundreds of hours without being detected. Once again, my engine monitor proved that it is worth its weight in gold.

Finally, I learned that ignition harnesses have a finite useful life. They may look perfect upon visual inspection, yet develop internal electrical leaks that seriously compromise ignition system performance. Since a new harness is relatively inexpensive (at least as aircraft parts go), it probably wouldn’t be a bad idea to replace the ignition harness every 1,000 hours or so just on general principles. In fact, I decided to order another new harness and installed it on my right engine, so now both engines have new harnesses.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.

Scope That Jug!

continental-cylinder-removalIn 2002, I did something unfortunate: pulled a perfectly good cylinder off of one of the engines of my Cessna 310. If I had it to do over again, I wouldn’t have touched the cylinder. But at the time, I thought I was doing the right thing.

It was the usual story. I had just downed the airplane for its annual inspection, and the first items on my checklist were performing a hot compression check, draining the oil, sending oil samples to the lab, and cutting open the oil filters for inspection.

All the cylinders had compressions in the low- to mid-70s. All but one, that is. That one measured about 60/80 with air leaking past the exhaust valve.

At the time, the engine manufacturer’s guidance on compression tests was Continental Motors service bulletin M84-15, which instructed mechanics that a jug could leak considerably past the rings and still be considered perfectly airworthy. However, any leakage at all past the valves was considered unacceptable, according to TCM, and required the cylinder to come off for repair or replacement.

So off it came.

Pulling a cylinder is a real PITA. I spent two hours removing cooling baffles and the exhaust and induction plumbing. It took me another hour to remove the rocker cover, rocker shafts, rocker arms, pushrods and pushrod housings. Finally, I used a cylinder base wrenches and a big breaker bar to coerce the eight cylinder base nuts loose. About four hours into the project, I held the offending jug in my arms and carried it over to my workbench to survey the damage.

I inspected the cylinder closely, with special attention on the exhaust valve. Surprisingly, I couldn’t see anything wrong. The valve looked normal, as did the rest of the cylinder. Yet it must have been bad, I thought, because it had clearly been leaking air past the exhaust valve.

I sent the cylinder out for re-valving and honing, installed new rings on the piston, then spent another four hours reinstalling them on the engine and replacing the exhaust, intake and baffles.

Like I said, it was a PITA. It cost me more than $500 plus a full day of sweat equity. (Had I not been doing the grunt work myself, the tab would have been at least $1,500.)

Continental pulls a switcheroo

That episode turned out to be a classic case of bad timing. Had my annual inspection come a few months later, that cylinder would never have been yanked. That’s because not long after my jug came off, Continental radically changed its guidance to mechanics regarding cylinder inspection.

On March 28, 2003, the wizards in Mobile issued service bulletin SB03-3 titled “Differential Pressure Test and Borescope Inspection Procedures for Cylinders.”  This 14-page document is arguably the best guidance ever provided to mechanics on the subject of when a cylinder should be pulled. (SB03-3 was recently incorporated into Continental Motors Standard Practice Maintenance Manual X-0, and is no longer a service bulletin.)

Continental’s guidance in SB03-3 differed from its predecessor M84-15 in two crucial respects. First, it reverses Continental’s previous position that even small amounts of leakage past the valves during a compression check is unacceptable and grounds for pulling the cylinder. Many experienced A&Ps considered the “zero leakage past the valves” standard as being unrealistic and after 19 years Continental finally agreed with that assessment.

The other difference is arguably even more important: For the first time, Continental directed mechanics to perform a borescope inspection of the cylinders at each annual inspection, 100-hour inspection, and any other time a compression check is done. Continental’s language about this is quite emphatic: “Continental Motors REQUIRES a cylinder borescope inspection be accomplished in conjunction with the differential pressure test.”

This was huge.

Although SB03-3 officially applies only to Continental engines, the guidance it offers makes good sense for Lycomings, too.

Borescope choices

Lennox Autoscope

Lennox Autoscope

At the time in 2003, borescopes were expensive and exotic devices whose use was pretty much limited to turbine engine inspections. Relatively few piston GA maintenance shops and A&P mechanics owned a borescope, and even fewer had a clue how to use one or what to look for. In SB03-3, Continental specifically recommended a particular make and model of borescope: the “Autoscope” from Lenox Instrument Company in Trevose, Pa. This was a simple, low-cost rigid borescope developed in the mid-1980s for use by auto mechanics, and cost about $1,000. The Lennox Autoscope had excellent optics and provides a remarkably clear view of what’s going on inside a cylinder. However, it was purely optical, and offered no way to take photos or capture digital images.

Since then, borescopes have become much less expensive and feature-rich. For years, I recommended the BK8000 digital borescope (also about $1,000) which provides excellent image quality and the ability to view images on a screen or capture them as JPEG files on a computer.

Vividia Ablescope VA-400

Vividia Ablescope VA-400

Last year, I purchased a Vividia Ablescope VA-400 from Amazon for less than $200. This is an amazingly inexpensive rigid digital borescope with the unique ability to adjust its viewing angle to anything between 0 and 180 degrees. The unit doesn’t come with an imaging device, but it has a USB cable that can be connected to any notebook computer or Android tablet or phone with a micro-USB port. It comes with both PC software and an Android app. This thing is so cheap that it’s now practical for every aircraft owner to have one.

What to look for

Normal valves

Normal valves

Also A&Ps tend to have little or no training in how to use a borescope, it’s certainly not rocket science. Here’s a photo what valves normally look like. The smaller valve on the left is the exhaust valve, while the larger one on the right is the intake valve. The reddish deposits on the exhaust valve and the brownish ones on the intake valve are typical. These deposits should appear reasonably symmetrical, indicating that the valves are rotating in service as they should be.

Burned exhaust valve

Burned exhaust valve

By way of contrast, here’s a photo of a burned exhaust valve. Note the asymmetrical appearance, especially the highlighted region (white arrows) where the deposits are minimal or absent (because that portion of the valve is running too hot). This is the classic visual signature of a burned valve. If the cylinder leaks air past the exhaust valve during the compression check and if the borescope shows this kind of asymmetrical deposit pattern, you can be relatively certain that the valve is burned and that the cylinder has to come off. But if the valve looks normal under the borescope, some leakage during the compression check is not grounds for removing the cylinder. (Now they tell me!)

Cylinder barrels

Cylinder barrels

The borescope is also a great way to check the condition of the cylinder barrel. Ths photo shows two borescope views of the upper cylinder bore—the so-called “ring step area.” The left view is normal; the right one has abnormal wear and scoring—possibly due to a broken compression ring—and probably needs to come off.

Next time you put your airplane in the shop, ask your mechanic what kind of borescope he uses. If your A&P doesn’t have a borescope or doesn’t know how to use one, educate him (and let him know that it’s now required equipment for any mechanic that works on Continental engines)…or find another mechanic.

Pulling a cylinder without first borescoping it is a lot like performing major surgery without first getting a CT or MRI. Don’t let any mechanic do that to your engine.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.

Checking the Dipstick

Checking the dipstick

There’s a lot more to checking the dipstick than just noting the oil level. The appearance of the oil is at least as important as its quantity.

We’ve been doing it since our earliest days as student pilots. Now that we’re aircraft owners, we still do it as part of our standard preflight ritual. But are we doing it right?

It turns out that there’s a lot more to checking the engine’s oil dipstick properly than just making sure that the oil level is above the minimum-for-flight level listed in the POH. If we really pay attention, we can learn a lot about the condition of our oil and of our engine.

How much oil is needed?

The engines on my Cessna 310 have 12-quart sumps—13 quarts if you include the quart in the spin-on oil filter. When I first acquired the airplane, my mechanic would fill the sump to its maximum capacity at each oil change. It didn’t take me long to discover that the engines didn’t like that, and promptly tossed several quarts out the engine breathers.

My POH states that the “minimum for flight” oil level is 9 quarts. So I asked my mechanic to service the sump to 10 quarts (instead of 12), and I’d add a quart of make-up oil when the level got down to 9 quarts. That worked better, but I was still seeing a fair amount of oil on the underside of the engine nacelles and the outer gear doors.

After I became a mechanic myself and learned about such things, I checked the Type Certificate Data Sheet (TCDS) for my Continental TSIO-520-BB engines, and found that an oil level of 6 quarts was sufficient to make good oil pressure in all flight attitudes from 23° nose-up to 17° nose-down. Armed with this information, I decided to experiment with lower oil levels.

What I discovered was that oil consumption (and the oily mess on the airframe) was drastically reduced if I maintained the oil level at around 8 quarts on the dipstick. Since then, I’ve avoided filling the sump to more than 9 quarts, and I normally do not add make-up oil until the dipstick reads about 7½ quarts. (This still gives me a 1½-quart “cushion” above what the engine needs to operate reliably in all flight attitudes.)

You might wonder why Continental put a 12-quart sump on an engine that requires only 6 quarts. The answer is that FAA certification requirements demand that the engine be designed to hold twice as much oil as it actually needs:

FAR §33.39 Lubrication system.

(a) The lubrication system of the engine must be designed and constructed so that it will function properly in all flight attitudes and atmospheric conditions in which the airplane is expected to operate. In wet sump engines, this requirement must be met when only one-half of the maximum lubricant supply is in the engine.

The TCDS for my TSIO-520-BB engines states that maximum acceptable oil consumption is about one quart per hour. If my engines actually used that much oil, then I’d need to fill the sumps nearly to their maximum capacity to ensure that I had enough oil to make a 5-hour flight without risking oil starvation. But since I know from long experience that my engines use more like 0.1 quart per hour, there’s no reason for me to carry anywhere near that much oil.

Every aircraft engine installation has an optimum oil level at which oil consumption is minimized and the engine is happiest. I would encourage you to experiment to determine what oil level works best for your airplane. Your engine will operate properly at 50% of its maximum oil capacity—guaranteed. As long as you keep the oil level a quart or two above the 50% point, your engine will be happy.

The best time to get an accurate dipstick reading is just prior to the first flight of the day. If you check the oil level shortly after the engine has been run for awhile, the dipstick reading will be noticeably lower because a significant quantity of oil remains adhered to various engine components. Another reading taken 24 hours later will often show an oil level that is ½ to 1 quart higher.

Oil consumption?

Having assured yourself that there’s enough oil in the engine, your next task is to make note of how much oil your engine is using. Keeping track of oil consumption—particularly any significant increase in oil consumption rate—is an important tool for monitoring engine condition.

The most common method of measuring oil consumption is to record how many quarts of make-up oil are added between oil changes, and to divide the total by the number of hours in the oil-change interval. (For example, if the oil is changed after 50 hours and 6 quarts of make-up oil were added during that time, the average oil consumption rate is 50/6 or 8.3 hours per quart.)

Oil consumption graph

Oil consumption isn’t linear—it accelerates as the oil deteriorates over time. The rate of consumption during the first 10 hours after an oil change is a good indication of engine condition.

However, this approach obscures the fact that oil consumption is not linear over the oil change interval. If you keep track of when you add each quart of make-up oil, you’ll find that less oil is consumed at first, and progressively more oil is consumed as the oil’s time-in-service increases.

The reason for this accelerating oil consumption is that the viscosity of the oil decreases as the oil deteriorates. Mineral oils lose viscosity due to a phenomenon called “polymer shearing” in which the long organic molecules are actually broken apart by mechanical action of the engine’s moving parts. Multigrade oils also lose viscosity because their viscosity-index improvers oxidize when exposed to high temperatures.

The increased rate of oil consumption provides tangible evidence that your engine oil is getting “long in the tooth” and ought to be changed soon.

What does your oil look like?

Whenever you check the dipstick, it’s also important to make note of the oil’s appearance—particularly its color and clarity. The oil’s appearance offers valuable clues to its condition and that of your engine.

Oil color

Color and transparency are important indicators of engine condition. When oil becomes dark and opaque, it should be changed. If this happens rapidly, it suggests that the engine has too much blow-by past the rings, or that oil temperature is too hot.

Fresh engine oil has a light amber color and is so transparent that it’s sometimes hard to read the dipstick level. As the oil remains in service, it gradually darkens in color and becomes progressively more opaque.

The darkening of engine oil is caused by contamination and oxidation. Contaminants include carbon (soot), lead salts and sulfur from combustion byproducts that get past the compression rings and into the crankcase (“blow-by”), as well as any dust or dirt that gets past the induction air filter. Oxidation of the oil occurs when it is exposed to high localized temperatures at it circulates through the engine, and results in the formation of coke. Various oil additives are also vulnerable to oxidation, particularly the viscosity-index improvers used in multiweight oils.

Dispersant additives are blended in the oil to help keep these so-called “insolubles” in suspension in order to keep the engine clean and minimize sludge deposits. As the quantity of insolubles in suspension increases, the oil darkens and becomes opaque.

It is important to note how quickly this darkening occurs. If your oil remains relatively light-colored and translucent after 25 hours in service, you can be reasonably confident that your cylinders and rings are in fine condition and that your oil can prudently remain in service for 40 or 50 hours. On the other hand, if your oil gets dark and opaque after 10 or 15 hours, you’d be wise to change your oil more often—perhaps at 25 hours—and you may want to investigate the possibility that one or more cylinders are excessively worn.

Such rapid discoloration is often a good indicator that the oil is distressed. In one study, 90% of oil that appeared abnormally dark on the dipstick was subsequently found by laboratory analysis to be non-compliant with required specifications. Oil that is dark and opaque from blow-by past the rings is very likely to be rich in acids and other corrosive compounds that can attack your cam and lifters, and that’s probably the #1 cause of engines failing to make TBO. Any time your oil appears dark or opaque, you would be wise to drain it and replace it with fresh oil and a new oil filter, regardless of the oil’s time-in-service.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.

Temperature, Temperature, Temperature

I’ve had wonderful luck with piston aircraft engines throughout my nearly 50 years as an aircraft owner. All the engines on my airplanes have made TBO with minimal maintenance along the way, and in recent years they’ve gone far, far beyond TBO.

For decades, I was convinced that the secret of my success was the fact that I “babied” my engines, typically limiting my cruise power settings to no more than 60 or 65 percent power. I felt that sacrificing a little airspeed in exchange for long engine life and reduced maintenance cost was a good tradeoff.

I’ve come to learn, however, that such “babying” is one way to achieve long engine life, but it’s not the only way. That’s because it’s not POWER that damages out engines—it’s TEMPERATURE. It turns out you can run these engines as hard as you like so long as you are obsessive about keeping temperatures under control.

Or as my late friend, powerplant guru and former Continental Motors tech rep Bob “Mose” Moseley used to say, “There are three things that affect how long your engine will last: (1) temperature, (2) temperature, and (3) temperature!”

It’s all about the heat

CHTOur piston aircraft engines are heat engines. They have moving parts—notably exhaust valves and valve guides—that are continually exposed to extremely high temperatures in the vicinity of 1,500°F and sometimes hotter. Since engine oil cannot survive temperatures above about 400°F, these moving parts must function with no lubrication. They depend on extremely hard metals operating at extremely close tolerances at extremely high temperatures with no lubrication. Aluminum pistons and cylinder heads are also exposed to these very hot temperatures, despite the fact that aluminum melts at about 1,200°F. It’s nothing short of miraculous, and a testament to outstanding engineering, that these “hot section” components last as long as they do.

The key to making these critical parts last is temperature control, and the most important temperature is cylinder head temperature (CHT). Mose monitored and overhauled these engines for nearly four decades, and he swore to me that an engine that is operated at CHTs above 400°F on a regular basis will show up to five times as much wear metal in oil analysis as an identical engine that is consistently limited to CHTs of 350°F or less. “It’s amazing how much a small increase in CHT can accelerate engine wear,” Mose said.

As critical as CHT is, many owners don’t have a clue whether their CHTs are 400°F+ or 350°F-. That’s because the engine instrumentation provided by most aircraft manufacturers is pathetically inadequate. The typical factory CHT gauge looks at only one cylinder, and it’s not necessarily the hottest one. Further, the typical factory CHT gauge often isn’t even calibrated, and its green arc extends up to a ridiculously hot 460°F (for Continentals) or 500°F (for Lycomings). Those numbers may be okay as emergency red lines, but they’re horribly abusive for continuous operation. If all you have is factory gauges, you could easily be cooking your cylinders to death while blissfully thinking that all is okay because the CHT gauge is well within the green arc.

To know what’s really going on in front of the firewall, you have to have a modern multiprobe engine analyzer with a digital readout. Such instrumentation isn’t cheap—figure $2,500 for a single or $5,000 for a twin, installed—but if it saves you from having to replace a couple of jugs en route to TBO, it has more than paid for itself. Installing a digital engine analyzer is probably the best money you can spend on your airplane.

Fuel system setup

MaintenanceFor takeoff and initial climb, we normally are at wide-open throttle, full-rich mixture, maximum RPM (if we have a constant-speed prop), and wide-open cowl flaps (if we have those). So there’s not much we can do from the cockpit to affect CHT during these phases of flight.

What does affect CHT is how our full-power fuel flows are adjusted. Unfortunately, it is shockingly common to see damagingly high CHTs due to improperly adjusted fuel flows, particularly in fuel-injected engines. It is not unusual for the fuel flows to be set wrong from the day an engine is installed, and never to be checked or adjusted all the way to TBO. The owner winds up going through cylinders every 500 hours and never knowing why (or blaming the manufacturer).

In part, the problem lies with mechanics who don’t fully understand how critical it is to test and adjust the fuel system setup on a regular basis. Continental recommends that the fuel system setup on its fuel-injected engines be checked and adjusted several times a year to account for seasonal changes. I’ll grant that’s a bit anal, but most Continental-powered airplanes go year after year without this ever being done, and many shops that maintain these airplanes don’t even have the necessary test equipment to do it.

Even when mechanics do test and adjust the fuel system, they often adjust it wrong. For example, Continental Manual M-0 (formerly SID97-3G) contains a lengthy table that specifies full-power fuel flow as a range (minimum and maximum). The “fine print” instructs mechanics to adjust the full-power fuel flow to the high end of the specified range, but many mechanics miss this subtlety and adjust it to the middle of the range. Experience shows that this is simply not enough fuel flow to keep CHTs cool during hot-weather takeoffs.

Lycoming engines with RSA fuel injection have no field adjustments for takeoff fuel flow. If it’s inadequate, the fuel servo has to be sent in to a specialty shop for bench checking and adjustment.

Then there’s the problem of aftermarket engine modifications. For example, engines that have been retrofitted with Superior’s Millennium cylinders often run higher CHTs than they did with their original factory cylinders. That’s because Millennium cylinders have substantially better “volumetric efficiency” than factory cylinders—in other words, they breathe better. Since they breathe more air during every combustion cycle, they need more fuel to maintain the same fuel/air mixture. The full-power fuel flow marked on your fuel-flow gauge may simply not be high enough if you have Millennium cylinders installed.

Even worse are turbocharged engines with aftermarket intercoolers installed. The intercooler reduces the temperature of the air that the cylinder breathes, making it denser. Denser air demands more fuel to maintain the desired fuel/air mixture, so full-power fuel flow must be increased significantly above original factory specifications. Too often this is not done, and the result is fried cylinders.

Many A&Ps are reluctant to adjust takeoff fuel flow above red-line. However, if you have Millennium cylinders, an aftermarket intercooler, or some other “mod” that allows your engine to produce more power than it did when it left the factory, that’s exactly what must be done to keep your CHTs cool and avoid premature cylinder failure.

Enough fuel flow?

MP and FF guage comboHow can you tell if your full-power fuel flow is adequate? If you’re limited to factory gauges, you probably can’t, at least with any precision. About the best you can to is to watch your fuel flow gauge (if you have one).

A good rule of thumb is to multiply your engine’s maximum rated horsepower by 0.1 to obtain the minimum required fuel flow in gallons-per-hour, or by 0.6 for pounds-per-hour. For example, if your engine is rated at 285 horsepower, your takeoff fuel flow should be at least 28.5 GPH; if it’s rated 310 horsepower, the minimum should be 31.0 GPH. If your takeoff fuel flow is significantly less than this, have your mechanic crank it up. And don’t forget that if you have Millennium cylinders or an aftermarket intercooler, your engine might be producing a few percent more horsepower than what the book says, so it might need a few percent more fuel flow.

Now if you have a digital multiprobe engine analyzer, it’s easy to tell if your fuel flow is adjusted high enough. Just make sure none of your CHTs exceed 380°F during takeoff and climb for Continentals or 400°F for Lycomings. Lower is even better.

What about cruise?

Cruise flight represents the lion’s share of our flying time. Just as in takeoff and climb, it’s essential to keep all our CHTs at or below 380°F (for Continentals) or 400°F for Lycomings during cruise to achieve good cylinder longevity, and lower is even better. There are basically three different strategies for keeping CHTs low during cruise:

  • Baby the engine
  • Operate very rich
  • Operate lean-of-peak

All three strategies work, and conscientious use of any of them will give you a good shot at making TBO with minimum cylinder problems. But each has its pros and cons. Let’s take a closer look.

Engine graph

The mixture that many POHs refer to as “recommended lean mixture” is 50°F rich of peak EGT. As this graph shows, using that mixture results in very nearly the highest possible CHT. To reduce CHTs to the level required for good cylinder longevity, you need to do one of three things: (1) reduce power, (2) operate very rich, or (3) operate lean-of-peak.

Baby the engine

Many POHs talk about operating at three alternative mixture settings: “best power mixture” (~125°F rich-of-peak), “recommended lean mixture” (~50°F rich of peak, and “best economy mixture” (~peak EGT). It turns out that “recommended lean mixture” (~50°F ROP) is just about the worst possible mixture setting for keeping CHT low.  If you look at Figure 1, you’ll see that CHT reaches a maximum very close to 50°F ROP. So if you want to operate at “recommended lean mixture” and simultaneously keep CHT low, there’s only one way to get there: reduce power dramatically (generally 65% power or less). In other words, baby the engine.

Both “best power mixture” (~125°F ROP) and “best economy mixture” (~peak EGT) result in somewhat lower CHTs than does “recommended lean mixture.” At either of these mixture settings, you can usually operate at 70% power or so and still keep CHTs in the acceptable range.

In any of these cases, you’re trading power and airspeed for reduced temperatures and increased longevity. For most of us, that’s a reasonable tradeoff to make.

Operate very rich

But what if you are unwilling to sacrifice power and airspeed? Is it possible to go fast and still keep CHTs low?

Sure it is. We already talked about one way to do this in our discussion of takeoff and initial climb: pour lots of 100LL on the problem. In other words, operate very rich.

How rich? Figure 1 suggests that to reduce CHTs by 25°F, you need to enrich the mixture to about 160°F ROP. For each additional 10°F of CHT reduction, you need to enrich an additional 50°F ROP. Using such very rich mixtures, you can go fast and still stay cool. (This is how Reno racers usually operate.) But before you decide to go this route, consider the downsides.

The most obvious downside is that this strategy is very fuel-inefficient. Compared to “best economy mixture,” the very-rich strategy consumes about 25% more fuel, and reduces range by a similar amount. Advocates of very rich mixtures will tell you that “fuel is cheaper than engines,” but don’t be so sure. At today’s avgas prices, using 25% more fuel in a 300 horsepower engine can cost more than $40,000 over the engine’s TBO, and that’s enough to change out quite a few cylinders.

A second and less obvious downside is that very rich mixtures result in “dirty” combustion with lots of unburned byproducts in the exhaust gas. Operating this way for long periods of time tends to cause deposit buildup on piston crowns, ring grooves, spark plugs and exhaust valve stems. Do it long enough and you could wind up with stuck rings, stuck valves, worn valve guides, and fouled plugs.

Operate lean-of-peak

The third way to reduce CHTs is to lean even more aggressively than the POH recommends and operate on the lean side of peak EGT. Figure 1 shows that you can reduce CHTs by 25°F by leaning to about 10°F LOP. For each additional 10°F of CHT reduction, you need to lean an additional 15°F LOP. Using these very lean mixtures, you can go fast, stay cool, and obtain outstanding fuel economy, all at the same time.

What’s the downside of the LOP approach? The only major downside is that if your engine has uneven mixture distribution among its cylinders, it will usually run unacceptably rough at LOP mixture settings.

Uneven mixture distribution can usually be corrected in fuel-injected engines by “tuning” the fuel injector nozzles to eliminate the mixture imbalances. GAMIjectors are tuned nozzles that are STC’d for the majority of fuel-injected Continentals and Lycomings. Continental now offers its own version of tuned injectors on some of its premium engines.

If your engine is carbureted, you have no injector nozzles to tweak. Most carbureted Lycomings have pretty decent mixture distribution and can be run at least mildly LOP without running rough. Some carbureted Continentals (notably the O-470 used in the Cessna 182) have miserable mixture distribution, making it difficult to run those engines LOP without uncomfortable roughness.

Stay cool!

Whatever strategy you prefer, the important thing is to keep a close watch on your CHTs and ensure that they remain cool. The best way to do this is to install a multiprobe digital engine monitor and program its CHT alarm to go off at 390°F (Continentals) or 410°F (Lycomings). If the alarm goes off during takeoff or initial climb, you’re going to have to get your mechanic to turn up the full-power fuel flow.  If it goes off during cruise, either enrich (if ROP) or lean (if LOP) to bring the CHT down to acceptable levels.

If you don’t have a multiprobe digital engine monitor, install one. The cost of such instrumentation (including installation) is usually less than the cost of replacing one cylinder. Failure to install such instrumentation is a classic case of “penny wise, pound foolish.”

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.

Fix It Now!

Sometimes I simply cannot fathom what makes aircraft owners do some of the things they do. Particularly amazing to me are some of the mechanical problems that aircraft owners elect to live with rather than fix.

Now I’m just as averse to spending money as the next guy (and probably more than most). In fact I’ve made something of a crusade out of saving money on aircraft maintenance, and built a company dedicated to helping my fellow aircraft owners how to do the same.

On the other hand, I have always had something close to a zero-tolerance policy about mechanical problems. When something isn’t right on my airplane, it drives me nuts until I fix it. Invariably I fix such problems right away, rather than putting them off.

Nearly five decades as an aircraft owner has taught me that it’s usually cheaper to fix a problem sooner rather than later…sometimes a great deal cheaper. Not to mention that continuing to fly with a known mechanical deficiency can sometimes be hazardous to your health.

Fuel leak

Apparently some aircraft owners don’t share my fix-it-now philosophy. Check out this email I received from an aircraft owner:

Shortly after I bought my airplane last year, I noticed a drip coming from under the aircraft which pooled just to the left of the nosewheel. The drip occurred with the frequency one drip probably every five seconds while the aircraft sat static with the fuel selector on either the left or right tank. Obviously one of the very important shutdown tasks for me was to turn the fuel selector off in order to stop the leak. I never established whether the fuel leaked while the engine is running.

After not flying for the past month, I went out to my airplane last week. The aircraft was leaking fuel despite the selector being in the off position. There was a big pool of avgas beneath the airplane, and the fuel gauges indicated that I had lost almost all the fuel in my tanks…at $4.75 a gallon!

Not understanding why the fuel now leaked regardless of fuel selector setting, I started the aircraft, taxied it around to warm-up the engine and then left it at the maintenance hanger.

I am being told by the very competent maintenance supervisor that originally it was simply a fuel selector gone bad. However, they are now telling me that given that the aircraft now leaks in any position, it’s also a bad engine driven fuel pump. Usually I’d say let’s fix the selector and see if that resolves the problem altogether but I am concerned about the fuel pump going out at some critical time. Please advise.

Here we have an owner who knowingly flew his airplane for a year with a known significant fuel leak in the engine compartment. He only brought it to the attention of his mechanic when he could no longer stop the leak when the aircraft was parked by turning off the fuel selector. Now he’s asking whether it would be okay to fix the fuel selector and continue flying with the fuel leak in the engine compartment unaddressed.

Fuel Leak

A running fuel leak is NOT something that can prudently be deferred. Fix it now!

Good grief! I cannot imagine operating my LAWNMOWER with a known fuel leak, much less my airplane. What is this owner thinking?

Exhaust leak?

While still scratching my head over that one, I heard from the owner of a Cessna 340 that made me start scratching my head again:

I don’t push the engines hard, running at 65% power or lower most of the time. However, despite a published service ceiling of 27,000 feet, the engines really don’t perform well over 15,000 feet. I routinely fly over that altitude, but the cylinder head temperatures get a little high, and the engines burn more oil.

Sometimes I have trouble with the wastegates functioning properly at altitude, too, and I get some bootstrapping of manifold pressures (needle separation), which is unpleasant at best (because the engines get out of sync), and is dangerous at worst (because the bootstrapping could be due to an exhaust manifold leak). So as a practical matter, I only climb over 21,000 if it is absolutely necessary.

It baffles me how this owner can be sufficiently knowledgeable to recognize that his aircraft has a turbocharging problem that prevents it from operating properly at altitude, and even understands that the problem could well be due to an exhaust leak, yet continues to fly the aircraft with that known deficiency.

Exhaust Leak

An exhaust leak at the cylinder exhaust port if caught early can often be fixed with a cheap gasket. If you let it go, you’re probably looking at a costly cylinder rework job, or in extreme cases (as shown here) a whole new cylinder.

Doesn’t he understand that turbocharged twin Cessnas have a ghastly history of exhaust-related accidents, many of them fatal? Doesn’t he know about AD 2000-01-16 that requires repetitive inspection of his exhaust system every 50 hours, and pressure testing at every annual inspection? What is this owner thinking? (For that matter, what is his A&P thinking?)

Starter adapter slipping

The beat goes on. Here’s a post I saw recently on a popular Internet aviation forum:

On my departure from Pensacola on Sunday afternoon, I turned the key to start the engine (a Continental IO-520) and I could hear the starter motor, but the prop wouldn’t turn. It did actually turn slightly, but then just sat there.

I have noticed frequently in the past that the prop turns a little and then stops and then a second or two later it continues. Once the prop starts turning, the engine usually fires on the first turn and starts right up.

On my previous airplane, my A&P told me to turn the prop until I hear the click and it would help to start. So I turned everything off, got out of the plane and turned turn the prop by hand until I heard it click. I turned it again until I heard it click a second time just for good measure. I then got back in the plane and it fired right up like normal.

When I stopped for fuel at Zephyrhills on the way home, the engine started right up with out having to do the prop trick. I figured I would monitor it and if it acted up again to call in my A&P for a surgical procedure, but after thinking about it this morning I thought I would come to the forum here and see what others have to say.

Replies to this owner’s post explain that he was suffering from the classic symptoms of a TCM starter adapter that is severely worn and slipping. What bothers me is that the owner’s description makes it obvious that he’s been aware of this slippage problem for a long time, yet did nothing about it. Even after the slippage got so severe that he nearly found himself stranded in Pensacola, his first thought was to “monitor it” and only bring it to the attention of his A&P “if it acted up again.”

Continental Starter Adapter

If you try to start a Continental engine and the starter motor turns but the prop doesn’t, you can bet that the starter adapter is slipping (top). This indicates that the spring and shaftgear (lower left and right) are worn. If you catch the problem early, it can be repaired for a few hundred dollars by installing an undersize spring. If you let it go, you may wind up buying a new shaftgear for thousands of dollars, or perhaps even a new engine for tens of thousands.

This owner’s approach was clearly to do nothing about the starter adapter slippage until it becomes so bad that he simply cannot tolerate it any more. This is truly a “penny wise, pound foolish” attitude because every time a Continental starter adapter slips, it “makes metal” inside the engine. If the owner is really lucky, most of that metal will be caught by the oil filter and won’t circulate through the engine and contaminate the bearings and plug up the small passages in the hydraulic valve lifters. If he’s not so lucky, he could easily find himself buying a $30,000 engine overhaul.

Yet this owner is hardly alone. Countless owners of Continental-powered aircraft have slipping starter adapters, but elect to live with the problem rather than fix it. Not smart.

Fix it now!

I could go on and on with similar examples, but by now I’m sure you’ve got the idea. Any time you become aware of something on your aircraft that isn’t quite right, the smart thing to do is to bring it to the attention of your mechanic pronto. If the mechanic agrees that the problem is one you can prudently defer fixing until the next scheduled maintenance cycle, fine. But it’s often the case that the fix-or-defer decision is a “pay me a little now or pay me a lot later” proposition.

An exhaust leak at an exhaust riser flange might be solved with a simple gasket if addressed early. If left unaddressed until the cylinder exhaust flange has been severely eroded, the jug will probably have to come off for expensive rework or replacement.

A slipping Continental starter adapter if caught early can usually be fixed for less than $1,000 by installing an undersize spring. If allowed to continue slipping until the shaftgear is worn beyond limits, you’re looking at many thousands of dollars to repair—or if you get unlucky, a new engine.

A fuel leak caught early can often be fixed by tightening a B-nut or replacing a chafed line. If ignored, it can cause a fire, loss of the aircraft, and perhaps even loss of life.

So don’t just scribble the discrepancy on a post-it note so you can squawk it at the next annual inspection. Fix it now—or at least discuss it with your mechanic before making a fix-or-defer decision. It’s the smart and prudent thing to do, and it might just wind up saving you big bucks.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike’s book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions.
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