Author: Mike Busch (page 2 of 5)


Something unusual happened while I was at the AOPA Regional Fly-In in Groton, Connecticut: An air crash lawsuit at which I was scheduled to testify as an expert witness had settled on the eve of trial, leaving me unexpectedly with two unencumbered weeks on my hands. I was on the East Coast with my airplane and now could spend those two weeks however I pleased. For someone who hadn’t taken a vacation in years, this was cool!

Wright Brothers National Memorial

Wright Brothers National Memorial — Kill Devil Hills, NC

I decided to spend the first week exploring the Outer Banks of North Carolina, and the second week visiting friends in Raleigh and family in Charlotte. I also made arrangements with a flight school in Raleigh to get a much-needed Flight Review and Instrument Proficiency Check.

The two-hour flight from Groton to the Outer Banks was uneventful, and the last part of it was beautifully scenic. I spent the week in a small waterfront Airbnb with a balcony overlooking the Albermarle Sound, a few miles south of Kill Devil Hills where Orville and Wilbur first flew in 1903. It was a marvelously enjoyable, productive, restorative week.

Early Sunday morning, I checked out of my Airbnb and drove my rental car back to the airport to fly to Raleigh. I turned in my rental car, taxied my plane to the departure end of the runway of the small untowered airport, picked up my IFR clearance from Cherry Point Approach Control, and performed the usual preflight runup.

Like most piston twins, my Cessna 310 has four magnetos—two for the left engine and two for the right—controlled by four toggle switches. The preflight runup involves turning the mag switches off one at a time and checking for excessive RPM drop, unacceptable roughness, or abnormal EGT indications. My routine is to sequence through the switches from left to right, shutting off the left engine’s left mag first and the right engine’s right mag last. I’ve done this thousands of times in the 30 years I’ve owned this airplane, and I must confess I perform it somewhat robotically. This time, things were different.

Uh oh!

S-1200 magneto

Bendix S-1200 magneto

As I cycle the leftmost mag switch, the left engine quits cold. Yikes! I hastily flip the mag switch back on just in the nick of time to get it running again. I cycle through the remaining three mag switches and everything appears normal. I try the leftmost switch again. The left engine quit again.

Hmmm… Turning off the left mag kills the left engine. That means the right mag must not be producing any spark. Not good.

I briefly consider departing anyway—that’s why this airplane has two mags and two engines, right?—and instantly reject that idea. A wise aviation mentor once taught me that when making aeronautical decisions, I should always think about what the NTSB probable cause report would say. “PIC departed into instrument meteorological conditions with a known mechanical deficiency.” No way.

While taxiing back to the airport ramp, I think about the consequences of scrubbing the mission. It’s Sunday. I could order a replacement magneto first thing Monday morning. If I pay for overnight shipping, the mag might arrive by mid-day Tuesday, and the airplane might be back in the air by late Tuesday afternoon. I’ll have to cancel my Tuesday training appointment in Raleigh. I’ll need to find lodging and ground transportation for two more days on the Outer Banks. There’s a rental car waiting for me in Raleigh that’s probably too late to cancel…

Wait…I’m an A&P mechanic and my emergency toolkit is in the airplane’s wing locker. Maybe I can troubleshoot this mag problem and figure out a way to fix it. Maybe it’s something simple that doesn’t require ordering a replacement mag. Maybe I can improvise some battlefield repair…

I’m grasping at straws now, and realize the chances are somewhere between slim and none. But I’ve got to give it a shot, otherwise my plans for the coming week will fall like a row of dominoes.

An Open Door…

Open hangar door

Open hangar door, toolbox inside

Approaching the transient tiedown ramp, I notice a large hangar off to my right with the door wide open. I can’t believe my luck: Someone’s open on Sunday! Maybe I can get some help? I taxi toward the open hangar and shut down on the ramp in front of it. The huge hangar appears largely empty. I don’t see any people or airplanes inside, just a big red roll-around toolbox and some miscellaneous ground support equipment. A beautiful Waco open-cockpit biplane is parked on the ramp nearby.

I uncowl the left engine nacelle to inspect the right magneto and its associated wiring, but find no obvious defects. I disconnect the P-lead from the right mag, but that doesn’t fix the problem, so the problem must be inside the magneto itself. Ugh!

I walk towards the open hangar door. As I get closer, I spot a fellow puttering around deep in the bowels of the hangar. I walk over to him and muster up my most friendly smile.

“Good morning! I’m Mike, and that’s my Cessna 310,” I say, pointing at my airplane on the ramp.

“Good morning,” replied the fellow with a smile, “I’m Sam.”

“Nice to meet you, Sam,” I said. “I’ve got a problem and I’m hoping maybe you can help me.”

I proceeded to describe my plans to fly to Raleigh and my decision to scrub the takeoff because the right mag on the left engine was inoperative during my preflight runup.

“The mag completely dead?” Sam asked. “Not just fouled plugs?”

“Dead as a doornail,” I said.

“That doesn’t sound good,” Sam said as I nodded in agreement.

“Would it be possible for me to pull the plane into your hangar, so I can work on the problem?” I inquired, gesturing at the huge, vacant structure.

“Nope,” Sam replied curtly. That wasn’t the answer I was expecting, but Sam was still smiling, so I persisted.

“Any chance I could use this toolbox,” I pointed at the big red roll-around, “while I’m working on my airplane on the ramp?”

“Nope, I can’t let you do that,” Sam said, still smiling. My puzzlement continued to grow at Sam’s unexpected non-cooperation. Then I had a thought.

“Sam, are you an A&P?” He nodded in the affirmative. “Would YOU like to try troubleshooting my magneto problem?” It occurred to me perhaps he was viewing me as competition.

“Nope, I don’t have time for that. Gotta take some tourists up for a biplane ride,” Sam said. “Besides, I don’t work on magnetos; I always send them out.”  Sigh.

Ultimately, I managed to persuade Sam to lend me a ½-inch offset wrench and a small stepladder. With those and my emergency toolkit, I was able to remove the ailing magneto from the engine, disassemble it, resolve the problem, and put everything back together. Ultimately, I departed on my flight to Raleigh a few hours late, but my plans for the week remained unscathed.


Over the next few days, my mind kept returning to interaction with Sam. He seemed like such a nice fellow. Why did he act toward me in such an uncooperative fashion? What would it have cost him to let me use his empty hangar and his unused toolbox while he was up flying the Waco?

Liticaphobia means fear of being sued

The only answer I could come up with was liticaphobia: the fear of being sued. Sam undoubtedly saw me as a lawsuit waiting to happen. There’s an old joke among aircraft mechanics that the most dangerous thing in aviation is an aircraft owner with a toolbox. I’m sure that in Sam’s mind, if he facilitated my hairbrained scheme of taking a magneto apart (something he stated he’d never do himself) and then anything bad happened, he would be contributorily negligent and vulnerable to civil litigation.

Sam is not alone. In my experience, most aircraft mechanics who work on GA aircraft have a siege mentality about the possibility of being sued. This fear casts a shadow over every decision they make. It causes them to practice “defensive maintenance”—performing more maintenance than justified on the grounds of safety-of-flight—and to be secretive about errors they make for fear that disclosure might lead to litigation.

Twenty-five years ago, before I became an A&P myself, I had an eerily similar experience at an airport in Northern California. I’d flown there for a business meeting, and when I returned to the airport dressed in coat and tie, I discovered to my horror that my right main tank had been misfueled with Jet A instead of 100LL. The fueling company had no A&Ps on staff, so I started contacting the various maintenance shops on the field looking for someone who would help me get my fuel system purged. Not one was willing to touch my airplane for fear of liability. Finally, I succeeded in persuading one A&P to agree to help me if I signed a blanket waiver agreeing to hold him harmless for anything that might go wrong. This mechanic then wanted to disassemble all sorts of stuff on my airplane that didn’t need to be disassembled in order to purge the system. Ultimately, I was successful in getting my airplane flyable again, but not without a terrible struggle.


These days, I do a good deal of expert witness work in air crash lawsuits, generally on the defense side defending mechanics, shops and aviation manufacturers against claims by air crash victims. I can testify firsthand that aviation is a horribly litigious field, with way too many lawsuits for my taste.


Mechanics’ fear of being sued tends to be greatly overblown.

At the same time, I can also tell you that mechanics’ fear of being sued tends to be greatly overblown. Mechanics are rarely the target of air crash lawsuits, simply because few of them are high-net-worth individuals with enough assets to be worth suing. In the relatively few cases where mechanics and shops do get sued, these suits virtually always settle quickly within the limits of their liability insurance (typically $1 million), simply because the plaintiff lawyers understand that there’s no more money to be had. That’s why these lawsuits almost always target aircraft, engine, and component manufacturers who tend to have deeper pockets.

This paranoia about being sued is not limited to aviation. Doctors have been practicing “defensive medicine” for decades, especially those in high-risk specialties like ob/gyn and anesthesiology. Teachers have become frightened to discipline unruly kids or even give them hugs, while seesaws are disappearing from schoolyards for fear a kid might get injured. Have you purchased a ladder or bicycle or baby carriage lately and seen how many warning placards they now have? The fear level is getting ridiculous.

The incidence of US civil (tort) litigation has remained essentially flat per capita since 1975, but media coverage of litigation has skyrocketed, and that coverage is overwhelmingly skewed toward reporting cases involving huge damage awards. This has created the perception that the risk of being sued is much greater than it used to be, and that the consequences are frequently ruinous for the defendant. That’s seldom the case.

Look at the facts: According to a Harvard University study, for every 100 people hurt in an accident, 10 file a liability claim, 8 are settled within insurance limits, and only 2 actually get to court. Of those that make it to court, the plaintiff wins only 30% of the time, and in those cases the median damage award is $30,000, almost always covered by insurance.

So my appeal to shops and A&P mechanics is to maintain a reasonable amount of liability insurance ($1 million is generally adequate) and then do the right thing without paranoia about being sued if something goes wrong. Enough with the CYA already!

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

Unobtanium and Unaffordium

un·ob·tain·i·um (ˌənəbˈtānēəm)—noun informal—a highly desirable material that is hypothetical, scientifically impossible, extremely rare, costly, or fictional, or has some of these properties in combination.

My 1979 Cessna T310R

My 1979 Cessna T310R

As I write this, I’m just in the final throes of completing the annual inspection on my Cessna Turbo 310. Usually I do this myself, and it takes me three or four weeks; that’s because my turbocharged piston twin has way too many moving parts, and because I’m the world’s slowest aircraft mechanic. This year, my travel schedule didn’t permit that much downtime, so I decided to do the annual at a maintenance shop owned by a colleague, and to enlist the help of a couple of his staff A&Ps to help with the opening, closing, repairs and preventive maintenance in order get everything done in just over one week. I performed and signed off the actual “inspection part of the inspection” myself.

Hartzell/C&D cabin heater

Hartzell/C&D cabin heater

I knew going in that this was going to be a costly ordeal. I got off easy during my 2015 and 2016 annuals, but the law of large numbers caught up with me this year. To begin with, the old Southwind (Stewart Warner) combustion cabin heater got hit with a costly AD that would have required me to spend more than $3,000 overhauling the heater, and thereafter to perform frequent pressure-decay tests. I’ve hated this heater for the 30 years I’ve owned this plane. It has always been a problem child, caused me lots of grief, and I just couldn’t see shoveling more money into it. So I ordered a brand new, far superior heater designed by C&D Associates (and now manufactured by Hartzell Engine Technologies). The new heater cost $6,000 (ouch!) but will hopefully be trouble-free and AD-free going forward.

Installation of the new heater turned out to be a lot quicker and easier than I expected. Other things didn’t.

Prop Sync

One item on my squawk sheet was that the propeller synchrophaser was inoperative. I’d done sufficient troubleshooting to determine that the sensors and servo were okay, and that the problem was in the ARC SP105-B control box. I called Cessna, and was told that all rights to ARC equipment (including my control box) had been acquired by Sigma-Tek. I contacted Sigma-Tek and learned that they had no replacement boxes and no bench repair capabilities, but would sell me a maintenance manual for the SP105-B for $400.

ARC SP105-B Prop Synchrophaser

ARC SP105-B Prop Synchrophaser

I contacted every major avionics shop I could think of with bench repair capabilities, and all responded with some variant of “we used to work on those back in the day, but we no longer have that capability.” Finally, I did an exhaustive search of salvage yards to see if I could find a serviceable SP105-B control box. My search led me to Preferred Airparts in Kidron, Ohio (the largest dismantler of twin Cessnas), who said they had 40 inoperative SP105-Bs in inventory, plus one that was in “as removed” condition and “probably” was in working condition.

I purchased that one for $500 and installed it in my airplane. When I powered it up, the prop sync annunciator illuminated and the slave governor servo centered, which was encouraging. But when I test flew the airplane, I found that turning on the prop sync switch yielded a rapidly flashing annunciator (meaning the control box was not happy about something) and no prop synchronization function. Drat! My next step is to create a test harness so I can measure the sensor pulses going into the prop sync box and its output voltages to the servo motor while airborne and try to figure out what’s going on.

Positronic GM-Series Connector

Positronic GM-Series Connector

To create that test harness, I’ll need a pair of connectors (one male and one female). After an hour of Googling, I determined that the connectors in question are Positronic GM26s. I couldn’t find these connectors listed in the catalogues of any of the electronics supply houses I usually rely upon (Newark, Mouser, Digikey), so I contacted Positronic and learned that the only place that has these connectors in stock is in Puerto Rico and was rendered incommunicado by Hurricane Maria. It may be weeks before I know whether the connectors I need are obtainable. Sadly, I placarded my prop sync “inoperative” and pulled the circuit breaker per FAR 91.213(d). The prop sync saga will continue.

Flap Preselect Cable

One of the items on my inspection checklist for this year was to check all the flight control cable tensions. I was particularly interested in checking the tension on the flap retract cables, because the maintenance manual calls for those to be tensioned to 280±20 pounds. Normal cable tensiometers used in piston GA maintenance only measure up to a maximum of 100 pounds, so my flap retract cable tension hadn’t been checked for years. My colleague said he had a high-range tensiometer that measured up to 300 pounds, so we pressed that into service. As I suspected, the flap retract cable tensions were low (about 225 pounds). That meant that during a high-speed descent, the flaps might not stay fully retracted.

Broken Flap Preselect Cable

Broken Flap Preselect Cable

One of the A&Ps who was assisting me volunteered to adjust the flap retract cables, and I gratefully accepted his offer. That turned out to be a mistake. After the A&P adjusted the turnbuckles to the specified tension, he performed a functional check and found, to his horror, that the flaps would not retract. Further investigation revealed that the flap preselect cable (which runs from the flap actuator under the floorboards to the flap preselect control on the instrument panel) was now severely kinked to the point of being unrepairable. This was clearly a MIF—a “maintenance-induced foul-up”—that had turned a minor issue (low cable tension) into a major one (inoperative flaps).

We checked Cessna’s parts inventory for a replacement flap preselect cable. I expected it to be “unaffordium” but found instead that it was “unobtanium”—Textron Aviation no longer had any, and didn’t plan to make any more, ever. A Google search of the part number uncovered a used-but-serviceable one at B.A.S. Parts & Sales LLC in Greely, Colorado. We ordered it and asked for expedited shipping. Installing the new cable and rigging the flap preselect control wound up being a full-day affair.

More Ouch

Tire and Tube

Tire and Tube

My inspection revealed more costly stuff that needed to be done. Two new main landing gear tires and tubes ($700), two new brake discs ($350), 12 new brake linings ($150), and a bunch of miscellany brought the tab for this year’s “annual ordeal” to nearly $15,000—and that doesn’t include my sweat equity. Aircraft ownership isn’t for sissies…and that goes double for twins. But I do have a feeling of accomplishment that a lot of important stuff got done this time around. With luck, the next year or two will be smooth sailing…

…except for the $6,000 I will soon need to spend installing ADS-B-out…sigh.

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

The Shameful Lycoming Rod Bushing AD Affair

In late July, as tens of thousands of GA aircraft owners were converging on Oshkosh for AirVenture 2017, Lycoming published Mandatory Service Bulletin 632 titled “Identification of Connecting Rods with Non-Conforming Small End Bushings.” This was a very nasty service bulletin affecting Lycoming engines of all models that were built, rebuilt, overhauled or repaired during the past two years. Lycoming quickly published two revisions (632A and 632B) in rapid succession.

SB 632B addressed a problem with small-end connecting rod bushings (part number LW-13923) that were used in Lycoming factory new and rebuilt engines and shipped by Lycoming to overhaul shops and mechanics between November 2015 and November 2016. It turns out that there was a quality assurance problem with these bushings, and many of them had an outside diameter that did not conform with specifications. These bushings are pressed into the small end of Lycoming connecting rod assemblies using a hydraulic press. If the bushings are too small in diameter, the press-fit isn’t secure and the bushings can migrate out of the connecting rod when in service. That’s exactly what seems to have happened to a relatively small percentage of these non-conforming bushings, hence the mandatory service bulletin.

Lycoming Rod Bushings: Good vs Bad

Lycoming rod bushings.

Lycoming used these bushings in-house to build connecting rod assemblies, some of which were sold to overhaul shops and mechanics between November 2015 and February 2017, and most of which went into Lycoming factory new and rebuilt engines. Any engine that has these non-conforming bushings, whether built by the factory or overhauled or repaired in the field, are affected by SB 632B.

Why is SB 632B so nasty?

SB 632B requires that all engines that might possibly contain these non-conforming bushings have all their cylinders removed within the next 10 hours. With the cylinders removed, the securing of all small-end connecting rod bushings then must be tested using a special tool (“ST-531 Connecting Rod Bushing Press-Out Verification Tool”) to apply a calibrated force to each bushing to see if it can be displaced. If the bushing moves during this press-out test, then the connecting rod assembly must be removed from the engine and sent to Lycoming, and a new connecting rod assembly with a known-good bushing must be installed. Lycoming initially estimated that the press-out test will have approximately a 20% flunk rate, but from what we’ve been hearing that estimate may turn out to be way too optimistic.

Lycoming ST-531 Connecting Rod Bushing Press-Out Verification Tool

Lycoming ST-531 Connecting Rod Bushing Press-Out Verification Tool

As someone deeply involved in piston GA maintenance, I find what SB 632B requires to be a horrifying prospect. The requirement to remove all cylinders within 10 hours is bad enough; there is a long history of catastrophic engine failure after removal and replacement of all cylinders in the field that I’ve written about extensively. But the prospect of having 20% or 30% or 40% of the connecting rods removed and replaced in the field represents a far greater risk, because the majority of mechanics have never before performed this operation (notably tightening rod bolts to a specified stretch using a special micrometer). The rod bolts are the most highly-stressed component in the entire engine, and tightening them properly is ultra-critical. In my opinion and the opinion of every highly experienced A&P/IA I’ve spoken with, this is NOT work that should be attempted by line mechanics in the field working on engines mounted in airplanes. It really should be done only by an experienced technician in an engine shop with the engine mounted on a stand with unencumbered access.

In short, I quickly concluded that that the cure called for by Lycoming would very likely be worse than the disease, and that it’s likely that there may be more catastrophic engine failures caused by maintenance errors in performing SB 632B than would be caused by the migrating bushing problem that SB 632B addresses. After conferring with a few very experienced A&P/IAs who have much more experience maintaining Lycoming engines than I do, I also concluded that there is a far less invasive and risky and expensive method that would effectively detect bushing migration and mitigate the safety risk without creating a bigger one in the process.

Owner organizations respond

While in Oshkosh, I spoke to several Lycoming executives who indicated that they expected the FAA’s New York Aircraft Certification Office (ACO) to start the wheels in motion to issue an emergency Airworthiness Directive the following week that would mandate compliance with SB 632B. I then sought out AOPA’s David Oord, with whom I’d recently worked so successfully on dealing with the Continental camshaft gear issue. Dave and I discussed that the FAA had not issued an Airworthiness Concern Sheet (ACS) about this Lycoming bushing issue in order to solicit input from the affected aircraft owner associations, something they had promised to do when we met with the FAA Engine and Propeller Directorate at a GA engine summit meeting in 2015. Dave and I agreed that it would be appropriate for AOPA to ask the FAA to do that so we would have a reasonable time to research this issue and provide the FAA with a thoughtful response before their AD process began, and Dave promised he’d make some calls as soon as he returned to his office.

On Tuesday, Dave phoned to tell me that that he’d heard back from the FAA, and that they said they would not be able to issue an ACS because they considered the issue too time-critical. Dave pressed for input to the process from the aircraft owner community, and the FAA agreed to try to set up a conference call between aircraft owner representatives, key FAA personnel, and representatives of Lycoming. That sounded better than nothing.

The next morning, I was awakened by another phone call from Dave, who told me that the FAA was willing to do a conference call, but it had to be TODAY. Yikes! We identified several other qualified aircraft owner representatives to be on the call to represent Cessna and Piper owners, and all agreed to participate.

The call was scheduled for 3 pm Eastern. I spent two hours drafting a bullet-point document containing our questions, concerns, and proposed alternative solution to SB 632B, and emailed it to Dave, who sent it to all the participants expected to be on the call from the FAA, Lycoming, and the owner associations.


The conference call took place as scheduled and lasted for an hour. However, Lycoming declined to answer ANY of the questions we posed to them, telling us that the information was proprietary and Lycoming was sharing it solely with the FAA and no one else.

Lycoming would not tell us how many displaced bushings have been found, how many connecting rod failures had occurred due to bushing displacement, what the distribution of engine times was when bushing displacement was detected or connecting rod failure occurred. They would not tell us how many engines they expected to be affected. They would not even tell us how much the special ST-531 press-out tool would cost, or how soon they could get enough of these tools out in the field to perform the required test.

The FAA would not tell us, either, saying that they were not permitted to release any of this information without Lycoming’s permission (which clearly was not forthcoming). We spent the better part of an hour asking questions but got no answers. It was absolutely exasperating.

We spent the rest of the time on the call trying to convince Lycoming and the FAA that there was a far less invasive and risky and costly way to deal with the displaced bushing problem, and we described it to them in detail. But it became clear that Lycoming and the FAA had already decided that SB 632B was necessary, despite the maintenance-induced failure risk, and that they were not interested in considering any alternatives.

As the call concluded, I felt totally disgusted with the total lack of cooperation exhibited by Lycoming and the FAA. I have been involved in working with the FAA on numerous Airworthiness Directives during the past two decades, and this was unquestionably the most unreasonable performance I’ve seen.

Shameful and disturbing

On Thursday, we worked with Dave to create a formal joint letter to the head of the FAA’s New York ACO. In it, we expressed our disappointment in how the FAA seemed to be dealing with this issue, and included the bullet-point document I’d created outlining our questions, concerns, and recommended alternative to SB 632B. In the letter, we specifically asked the FAA to approve our proposed minimally-invasive alternative as an Alternate Means of Compliance (AMOC) as outlined at the end of the document.

Needless to say, the New York ACO convened a Corrective Action Review Board (CARB) that rubber-stamped Lycoming’s requested corrective action in record-breaking time. On August 9, the FAA issued AD 2017-16-11 mandating compliance with SB 632B and putting numerous Lycoming-powered GA aircraft on the ground and their owners in jeopardy both moneywise and (IMHO) safetywise.

I find this whole sorry episode very disturbing for several reasons.

In discussing this situation with companies like Rick Romans and Aircraft Specialties Services and Zephyr Aircraft Engines who are in the business of re-bushing Lycoming connecting rods in the field, it appears that the problem with these loose Lycoming connecting rod bushings has been well-known by industry insiders for more than a year. Several of those firms told me that they stopped installing Lycoming-supplied rod bushings many months ago in favor of PMA-equivalent bushings from Superior Air Parts that fit properly. Given that this problem has been known for quite some time, it seems to me that the FAA could and should have taken a bit more time to solicit and consider input from folks who would be most affected, especially alternative methods of addressing the problem in a less risky fashion, before publishing an emergency AD.

Furthermore, I find it unconscionable for the FAA to justify such a draconian rulemaking action on data that it refuses to disclose to the very people who will bear the burden of that rulemaking. I understand that if a manufacturer provides information to the FAA that the manufacturer identifies as proprietary, the FAA is not permitted to disclose it. But it seems to me that the FAA should be forbidden from using such proprietary data to justify issuing an AD. The federal Administrative Procedure Act (APA) guarantees that members of the public who will be affected by federal rulemaking shall have a say in the rulemaking process. I’m not a lawyer, but clearly rulemaking made in the kind of secret “star chamber” fashion that characterized AD 2017-16-11 makes a mockery of the spirit (and perhaps the letter) of the APA.

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).

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 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).
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