Posts Tagged ‘time between overhauls’

Hangnails and Hand Transplants

Tuesday, April 12th, 2016
Engine teardown

Here’s what happens to your engine when you send it in for major overhaul. Do you really want to do this?

You know me. I believe in running engines as long as they’re demonstrably healthy, even if that means going beyond the manufacturer’s recommended TBO. Nothing disturbs me more than when I hear about owners who get talked into (or talk themselves into) euthanizing engines that are running just fine.

Case in point: Here’s an email I received from a Bonanza owner seeking a second opinion on what to do about his Continental IO-520 engine:

“The engine is now at 1500 hours (TBO is 1700) and it seems to be running very well. But here’s the bad part: it’s using a quart of oil every 4 hours, and putting a LOT of oil on the belly of the aircraft, even with an air/oil separator installed.

“So what should I do? Should I get a field overhaul, or opt for a factory rebuilt engine? (The engine does NOT have a VAR crank.) Should I consider an STC upgrade to an IO-550? I’m leaning toward using Superior Millennium cylinders, do you agree?”

I took a deep breath and counted to ten. This owner just told me that he as a fine-running engine, yet he’s already concluded it needs to be overhauled or replaced. What was he thinking? It sure wasn’t clear to me that this engine had any major issues, much less anything requiring immediate euthanasia.

Where’s the beef?

So what if it’s using a quart in 4 hours? Is that so terrible?

No, it isn’t. Continental SID97-2B is the bible when it comes to determining the airworthinss of Continental cylinders, and here what it has to say about oil consumption:

Oil consumption can be expected to vary with each engine depending on the load, operating temperature, type of oil used and condition of the engine. A differential compression check and borescope inspection should be conducted if oil consumption exceeds one quart every three hours or if any sudden change in oil consumption is experienced and appropriate action taken.

This guidance indicates that the Bonanza’s oil consumption of a quart in four hours is perfectly acceptable. Even when Continental’s oil consumption threshold of a quart in three hours is exceeded, Continental simply calls for a borescope inspection to determine if there’s really a problem. If the cylinders look okay under the borescope, the engine can remain in service despite the high oil consumption.

SID97-2B also indicates that in February 1997, Continental actually reduced the tension on the oil control rings in its cylinder assemblies to increase oil consumption to achieve improved lubrication of the cylinder bore. A certain amount of oil consumption is essential for maximum cylinder life. When it comes to oil consumption, less is not necessarily always a good thing.

Bottom line is that it’s quite likely that there’s nothing at all wrong with the engine in this owner’s Bonanza. At worst, perhaps it has a couple of worn cylinders that might need to be replaced eventually. Even that’s not clear, since the owner didn’t mention low compression readings. Maybe all he needs is some new piston rings.

A worn jug is like a hangnail

Cracked cylinder head

Cylinder problems (like this head crack) call for cylinder work, not euthaizing the whole engine.

Even if a borescope inspection reveals that the engine has a worn-out jug or two, so what? Both Continental and Lycoming cleverly designed their engines so that the cylinders were bolt-on accessories that can be repaired or replaced without removing the engine from the airframe or splitting the case. If the engine actually does have badly worn cylinders, that’s a reason to repair or replace the jugs, not to tear down the whole engine.

Think about this for a moment. If some other bolt-on engine accessory went bad—say an alternator or vacuum pump or magneto or prop governor—would you let your mechanic remove the engine and have it major overhauled? Of course not.

If you had a hangnail, would you go to a surgeon for an amputation and hand transplant? No, I didn’t think so!

Why would an aircraft owner even consider major overhaul or engine replacement just because one or two cylinders might be worn out? To my way of thinking, it doesn’t matter whether an engine is at 100 hours since new or 100 hours past TBO—a sick cylinder calls for cylinder replacement, not engine replacement.

Euthanasia is a bit much

Here’s what I emailed back to the owner:

“I would NEVER consider overhauling an otherwise good-running engine just because it has high oil consumption. There’s nothing wrong with burning a quart in 4 hours, so long as your sparkplugs aren’t oil-fouled and your compressions are within acceptable limits. If things get bad enough and you find one or more cylinders with unacceptably low compression, you may want to consider replacing them. That’s why Continental makes its engines with bolt-on cylinders: so you can change them without having to overhaul the engine. The ONLY valid reason for overhauling an engine is a problem with the “bottom end” (crankcase, crankshaft, camshaft, gears, main bearings, etc.) that cannot be cured without splitting the case.

“Have you simply tried running the engine at a lower oil level on the dipstick? Big-bore Continental engines are famous for throwing out excess oil if the crankcase is overfilled. The TSIO-520s on my T310R have a 12-quart sump, but I typically run them at 8 quarts on the dipstick.

“Excessive oil on the belly is usually caused by excessive crankcase pressure. Sometimes this is due to worn cylinders that permit excessive blow-by past the rings (in which case your cylinders will show low compression readings and your oil will get dirty very quickly after each oil change). But it can also be due so something as simple as an oil filler cap that isn’t sealing properly (when did you last check the oil cap gasket?) or a leaky front crankcase seal (which is not difficult to change).

“It sounds to me as if you may be a long way from needing to major-overhaul this engine. If you do decide to overhaul it anyway, drop me another email and I’ll offer some suggestions. But I really think that any consideration of rebuild/overhaul at this point is way premature.”

Don’t obsess about the manufacturer’s published TBO. It’s just a suggestion, not a requirement or a life limit. (The engines on my Cessna T310R are made it well past 200% of TBO and were still running magnificently.) When your engine is ready for overhaul, it’ll let you know by starting to make metal or to leak oil or to crack their crankcases or spall their cam lobes or something else obvious to let you know that “it’s time.” That’s the time to overhaul them. Doing it earlier always strikes me as being a capital crime.

What Makes an Engine Airworthy?

Wednesday, July 2nd, 2014

If we’re going to disregard manufacturer’s TBO (as I have advocated in earlier blog posts), how do we assess whether a piston aircraft engine continues to be airworthy and when it’s time to do an on-condition top or major overhaul? Compression tests and oil consumption are part of the story, but a much smaller part than most owners and mechanics think.

Bob Moseley

James Robert “Bob” Moseley (1948-2011)

My late friend Bob Moseley was far too humble to call himself a guru, but he knew as much about piston aircraft engines as anyone I’ve ever met. That’s not surprising because he overhauled Continental and Lycoming engines for four decades; there’s not much about these engines that he hadn’t seen, done, and learned.

From 1993 and 1998, “Mose” (as his friends called him) worked for Continental Motors as a field technical representative. He was an airframe and powerplant mechanic (A&P) with inspection authorization (IA) and a FAA-designated airworthiness representative (DAR). He was generous to a fault when it came to sharing his expertise. In that vein, he was a frequent presenter at annual IA renewal seminars.

Which Engine Is Airworthy?

During these seminars, Mose would often challenge a roomful of hundreds of A&P/IA mechanics with a hypothetical scenario that went something like this:

Four good-looking fellows, coincidentally all named Bob, are hanging out at the local Starbucks near the airport one morning, enjoying their usual cappuccinos and biscotti. Remarkably enough, all four Bobs own identical Bonanzas, all with Continental IO-550 engines. Even more remarkable, all four engines have identical calendar times and operating hours.

While sipping their overpriced coffees, the four Bobs start comparing notes. Bob One brags that his engine only uses one quart of oil between 50-hour oil changes, and his compressions are all 75/80 or better. Bob Two says his engine uses a quart every 18 hours, and his compressions are in the low 60s. Bob Three says his engine uses a quart every 8 hours and his compressions are in the high 50s. Bob Four says his compressions are in the low 50s and he adds a quart every 4 hours.

Who has the most airworthy engine? And why?

Compression/Oil Level

Don’t place too much emphasis on compression test readings as a measure of engine airworthiness. An engine can have low compression readings while continuing to run smoothly and reliably and make full power to TBO and beyond. Oil consumption is an even less important factor. As long as you don’t run out of oil before you run out of fuel, you’re fine.

This invariably provoked a vigorous discussion among the IAs. One faction typically thought that Bob One’s engine was best. Another usually opined that Bobs Two and Three had the best engines, and that the ultra-low oil consumption of Bob One’s engine was indicative of insufficient upper cylinder lubrication and a likely precursor to premature cylinder wear. All the IAs agreed Bob Four’s was worst.

Mose took the position that with nothing more than the given information about compression readings and oil consumption, he considered all four engines equally airworthy. While many people think that ultra-low oil consumption may correlate with accelerated cylinder wear, Continental’s research doesn’t bear this out, and Mose knew of some engines that went to TBO with very low oil consumption all the way to the end.

While the low compressions and high oil consumption of Bob Four’s engine might suggest impending cylinder problems, Mose said that in his experience engines that exhibit a drop in compression and increase in oil consumption after several hundred hours may still make TBO without cylinder replacement. “There’s a Twin Bonanza that I take care of, one of whose engines lost compression within the first 300 hours after overhaul,” Mose once told me. “The engine is now at 900 hours and the best cylinder measures around 48/80. But the powerplant is running smooth, making full rated power, no leaks, and showing all indications of being a happy engine. It has never had a cylinder off, and I see no reason it shouldn’t make TBO.”

Lesson of a Lawn Mower

To put these issues of compression and oil consumption in perspective, Mose liked to tell the story of an engine that was not from Continental or Lycoming but from Briggs & Stratton:

Snapper Lawnmower

If this one-cylinder engine can perform well while using a quart of oil an hour, surely an aircraft engine with 50 times the displacement can, too.

Years ago, I had a Snapper lawn mower with an 8 horsepower Briggs on it. I purchased it used, so I don’t know anything about its prior history. But it ran good, and I used and abused it for about four years, mowing three acres of very hilly, rough ground every summer.

The fifth year I owned this mower, the engine started using oil. By the end of the summer, it was using about 1/2 quart in two hours of mowing. If I wasn’t careful, I could run out of oil before I ran out of gas, because the sump only held about a quart when full. The engine still ran great, mowed like new, although it did smoke a little each time I started it.

The sixth year, things got progressively worse, just as you might expect. By the end of the summer, it was obvious that this engine was getting really tired. It still ran okay, would pull the hills, and would mow at the same speed if the grass wasn’t too tall. But it got to the point that it was using a quart of oil every hour, and was becoming quite difficult to start. The compression during start was so low (essentially nil) that sometimes I had to spray ether into the carb to get the engine to start. It also started leaking combustion gases around the head bolts, and would blow bubbles if I sprayed soapy water on the head while it was running. In fact, the mower became somewhat useful as a fogger for controlling mosquitoes. But it still made power and would only foul its spark plug a couple of times during the season when things got really bad.

Now keep in mind that this engine was rated at just 8 horsepower and had just one cylinder with displacement roughly the size of a coffee cup, was using one quart of oil per hour, and had zilch compression. Compare that to an IO-550 with six cylinders, each with a 5.25-inch bore. Do you suppose that oil consumption of one quart per hour or compression of 40/80 would have any measurable effect on an IO-550’s power output or reliability—in other words, its airworthiness? Not likely.

In fact, Continental Motors actually ran a dynamometer test on an IO-550 whose compression ring gaps had been filed oversize to intentionally reduce compression on all cylinders to 40/80, and it made full rated power.

Common Sense 101Let’s Use Common Sense

I really like Mose’s commonsense approach to aircraft engines. Whether we’re owners or mechanics (or both), we would do well to avoid getting preoccupied with arbitrary measurements like compression readings and oil consumption that have relatively little correlation with true airworthiness.

Instead, we should focus on the stuff that’s really important: Is the engine “making metal”? Are there any cracks in the cylinder heads or crankcase? Any exhaust leaks, fuel leaks, or serious oil leaks? Most importantly, does the engine seem to be running rough or falling short of making full rated power?

If the answer to all of those questions is no, then we can be reasonably sure that our engine is airworthy and we can fly behind it with well-deserved confidence.

On-Condition Maintenance

The smart way to deal with engine maintenance—including deciding when to overhaul—is to do it “on-condition” rather than on a fixed timetable. This means that we use all available condition-monitoring tools to monitor the engine’s health, and let the engine itself tell us when maintenance is required. This is how the airlines and military have been doing it for decades.

Digital borescope (Adrian Eichhorn)

Digital borescopes and digital engine monitors have revolutionized piston aircraft engine condition monitoring.

For our piston aircraft engines, we have a marvelous multiplicity of condition-monitoring tools at our disposal. They include:

  • Oil filter visual inspection
  • Oil filter scanning electron microscopy (SEM)
  • Spectrographic oil analysis programs (SOAP)
  • Digital engine monitor data analysis
  • Borescope inspection
  • Differential compression test
  • Visual crankcase inspection
  • Visual cylinder head inspection
  • Oil consumption trend analysis
  • Oil pressure trend analysis

If we use all these tools on an appropriately frequent basis and understand how to interpret the results, we can be confident that we know whether the engine is healthy or not—and if not, what kind of maintenance action is necessary to restore it to health.

The moment you abandon the TBO concept and decide to make your maintenance decisions on-condition, you take on an obligation to use these tools—all of them—and pay close attention to what they’re telling you. Unfortunately, many owners and mechanics don’t understand how to use these tools appropriately or to interpret the results properly.

When Is It Time to Overhaul?

It takes something pretty serious before it’s time to send the engine off to an engine shop for teardown—or to replace it with an exchange engine. Here’s a list of the sort of findings that would prompt me to recommend that “the time has come”:

Lycoming cam and lifter

Badly damaged cam lobe found during cylinder removal. “It’s time!”

  • An unacceptably large quantity of visible metal in the oil filter; unless the quantity is very large, we’ll often wait until we’ve seen metal in the filter for several shortened oil-change intervals.
  • A crankcase crack that exceeds acceptable limits, particularly if it’s leaking oil.
  • A serious oil leak (e.g., at the crankcase parting seam) that cannot be corrected without splitting the case.
  • An obviously unairworthy condition observed via direct visual inspection (e.g., a bad cam lobe observed during cylinder or lifter removal).
  • A prop strike, serious overspeed, or other similar event that clearly requires a teardown inspection in accordance with engine manufacturer’s guidance.

Avoid getting preoccupied with compression readings and oil consumption that have relatively little correlation with true airworthiness. Ignore published TBO (a thoroughly discredited concept), maintain your engine on-condition, make sure you use all the available condition-monitoring tools, make sure you know how to interpret the results (or consult with someone who does), and don’t overreact to a single bad oil report or a little metal in the filter.

Using this reliability-centered approach to engine maintenance, my Savvy team and I have helped hundreds of  aircraft owners obtain the maximum useful life from their engines, saving them a great deal of money, downtime and hassle. And we haven’t had one fall out of the sky yet.

The Dark Side of Maintenance

Tuesday, June 10th, 2014

The Dark SideHave you ever put your airplane in the shop—perhaps for an annual inspection, a squawk, or a routine oil change—only to find when you fly it for the first time after maintenance that something that was working fine no longer does?  Every aircraft owner has had this happen. I sure have.

Maintenance has a dark side that isn’t usually discussed in polite company: It sometimes breaks aircraft instead of fixing them.

When something in an aircraft fails because of something a mechanic did—or failed to do—we refer to it as a “maintenance-induced failure”…or “MIF” for short. Such MIFs occur a lot more often than anyone cares to admit.

Why do high-time engines fail?

I started thinking seriously about MIFs in 2007 while corresponding with Nathan Ulrich Ph.D. about his ground-breaking research into the causes of catastrophic piston aircraft engine failures (based on five years’ worth of NTSB accident data) that I discussed in an earlier post. Dr. Ulrich’s analysis showed conclusively that by far the highest risk of catastrophic engine failure occurs when the engine is young—during the first two years and 200 hours after it is built, rebuilt or overhauled—due to “infant-mortality failures.”

But the NTSB data was of little statistical value in analyzing the failure risk of high-time engines beyond TBO, simply because so few engines are operated past TBO; most are arbitrarily euthanized at TBO. We don’t have good data on how many engines are flying past TBO, but it’s a relatively small number. So it’s s no surprise that the NTSB database contains very few accidents attributed to failures of over-TBO engines. Because there are so few, Ulrich and I decided to study all such NTSB reports for 2001 through 2005 to see if we could detect some pattern of what made these high-time engines fail. Sure enough, we did detect a pattern.

About half the reported failures of past-TBO engines stated that the reason for the engine failure could not be determined by investigators. Of the half where the cause could be determined, we found that about 80% were MIFs. In other words, those engines failed not because they were past TBO, but because mechanics worked on the engines and screwed something up!

Sheared Camshaft Bevel GearCase in point: I received a call from an aircraft owner whose Bonanza was undergoing annual inspection. The shop convinced the owner to have his propeller and prop governor sent out for 6-year overhauls. (Had the owner asked my advice, I’d have urged him not to do this, but that’s another story for another blog post.)

The overhauled prop and governor came back from the prop shop and were reinstalled. The mechanic had trouble getting the prop to cycle properly, and he wound up removing and reinstalling the governor three times. During the third engine runup, the the prop still wouldn’t cycle properly. The mechanic decided to take the airplane up on a test flight anyway (!) which resulted in an engine overspeed. The mechanic then removed the prop governor yet again and discovered that the governor drive wasn’t turning when the crankshaft was rotated.

I told the owner that I’d seen this before, and the cause was always the same: improper installation of the prop governor. If the splined drive and gears aren’t meshed properly before the governor is torqued, the camshaft gear is damaged, and the only fix is a teardown. (A couple of engine shops and a Continental tech rep all told the owner the same thing.)

This could turn out to be a $20,000 MIF. Ouch!

How often do MIFs happen?

They happen a lot. Hardly a day goes by that I don’t receive an email or a phone call from an exasperated owner complaining about some aircraft problem that is obviously a MIF.

A Cessna 182 owner emailed me that several months earlier, he’d put the plane in the shop for an oil change and installation of an STC’d exhaust fairing. A couple of months later, he decided to have a digital engine monitor installed. The new engine monitor revealed that the right bank of cylinders (#1, #3 and #5) all had very high CHTs well above 400°F. This had not shown up on the factory CHT gauge because its probe was installed on cylinder #2. (Every piston aircraft should have an engine monitor IMHO.) At the next annual inspection at a different shop, the IA discovered found some induction airbox seals missing, apparently left off when the exhaust fairing was installed. The seals were installed and CHTs returned to normal.

Sadly, the problem wasn’t caught early enough to prevent serious heat-related damage to the right-bank cylinders. All three jugs had compressions down in the 30s with leakage past the rings, and visible damage to the cylinder bores was visible under the borescope. The owner was faced with replacing three cylinders, around $6,000.

Sandel SN3308The next day, I heard from the owner of an older Cirrus SR22 complaining about intermittent heading errors on his Sandel SN3308 electronic HSI. These problems started occurring intermittently about three years earlier when the shop pull the instrument for a scheduled 200-hour lamp replacement.


I’ve seen this in my own Sandel-equipped Cessna 310, and it’s invariably due to inadequate engagement between the connectors on the back of the instrument and the mating connectors in the mounting tray. You must slide the instrument into the tray just as far as possible before tightening the clamp; otherwise, you’ve set the stage for flaky electrical problems. This poor Cirrus owner had been suffering the consequences for three years. It took five minutes to re-rack the instrument and cure the problem.

Pitot-Static PlumbingNot long after that, I got a panicked phone call from one of my managed-maintenance clients who’d departed into actual IMC in his Cessna 340 with his family on board on the first flight after some minor avionics work. (Not smart IMHO.) As he entered the clag and climbed through 3,000 feet, all three of his static instruments—airspeed, altimeter, VSI—quit cold. Switching to alternate static didn’t cure the problem. The pilot kept his cool, confessed his predicament to ATC, successfully shot an ILS back to his home airport, then called me.

The moment I heard the symptoms, I knew exactly what happened because I’d seen it before. “Take the airplane back to the avionics shop,” I told the owner,  “and ask the tech to reconnect the static line that he disconnected.” A disconnected static line in a pressurized aircraft causes the static instruments to be referenced to cabin pressure. The moment the cabin pressurizes, those instruments stop working. MIF!

I know of at least three other similar incidents in pressurized singles and twins, all caused by failure of a mechanic to reconnect a disconnected static line. One resulted in a fatal accident, the others in underwear changes. The FARs require a static system leak test any time the static system is opened up, but clearly some technicians are not taking this seriously.

Causes of Accidents

Why do MIFs happen?

Numerous studies indicate that three-quarters of accidents are the fault of the pilot. The remaining one-quarter are machine-caused, and those are just about evenly divided between ones caused by aircraft design flaws  and ones caused by MIFs. That suggests one-eighth of accidents are maintenance-induced, a significant number.

The lion’s share of MIFs are errors of omission. These include fasteners left uninstalled or untightened, inspection panels left loose, fuel and oil caps left off, things left disconnected (e.g., static lines), and other reassembly tasks left undone.

Distractions play a big part in many of these omissions. A mechanic installs some fasteners finger-tight, then gets a phone call or goes on lunch break and forgets to finish the job by torqueing the fasteners. I have seen some of the best, most experienced mechanics I know fall victim to such seemingly rookie mistakes, and I know of several fatal accidents caused by such omissions.

Maintenance is invasive!

Whenever a mechanic takes something apart and puts it back together, there’s a risk that something won’t go back together quite right. Some procedures are more invasive than others, and invasive maintenance is especially risky.

Invasiveness is something we think about a lot in medicine. The standard treatment for gallstones used to be cholecystectomy (gall bladder removal), major abdominal surgery requiring a 5- to 8-inch incision. Recovery involved a week of hospitalization and several weeks of recovery at home. The risks were significant: My dad very nearly died as the result of complications following this procedure.

Nowadays there’s a far less invasive procedure—laproscopic cholecystectomy—that involves three tiny incisions and performed using a videoscope inserted through one incision and various microsurgery instruments inserted through the others. It is far less invasive than the open procedure. Recovery usually involves only one night in the hospital and a few days at home. The risk of complications is greatly reduced.

Similarly, some aircraft maintenance procedures are far more invasive than others. The more invasive the maintenance, the greater the risk of a MIF. When considering any maintenance task, we should always think carefully about how invasive it is, whether the benefit of performing the procedure is really worth the risk, and whether less invasive alternatives are available.

Ryan Stark of Blackstone LabsFor example, I was contacted by an aircraft owner who said that he’d recently received an oil analysis report showing an alarming increase in iron. The oil filter on his Continental IO-520 showed no visible metal. The lab report suggested flying another 25 hours and then submitting another oil sample for analysis.

The owner showed the oil analysis report to his A&P, who expressed grave concern that the elevated iron might indicate that one or more cam lobes were coming apart. The mechanic suggested pulling one or two cylinders and inspecting the camshaft.

Yikes! What was this mechanic thinking? No airplane has ever fallen out of the sky because of a cam or lifter problem. Many have done so following cylinder removal, the second most invasive thing you can do to an engine. (Only teardown is more invasive.)

The owner wisely decided to seek a second opinion before authorizing this exploratory surgery. I told him the elevated iron was almost certainly NOT due to cam lobe spalling. A disintegrating cam lobe throws off fairly large steel particles or whiskers that are usually visible during oil filter inspection. The fact that the oil filter was clean suggested that the elevated iron was coming from microscopic metal particles less than 25 microns in diameter, too small to be detectable in a filter inspection, but easily detectable via oil analysis. Such tiny particles were probably coming either from light rust on the cylinder walls or from some very slow wear process.

I suggested the owner have a borescope inspection of his cylinders to see whether the bores showed evidence of rust. I also advised that no invasive procedure (like cylinder removal) should ever be undertaken solely on the basis of a single oil analysis report. The oil lab was spot-on in recommending that the aircraft be flown another 25 hours. The A&P wasn’t thinking clearly.

Even if a cam inspection was warranted, there’s a far less invasive method. Instead of a 10-hour cylinder removal, the mechanic could pull the intake and exhaust lifters, and then determine the condition of the cam by inspecting it with a borescope through the lifter boss and, if warranted, probing the cam lobe with a sharp pick. Not only would this procedure require just 15% as much labor, but the risk of a MIF would be nil.

Sometimes, less is more

Many owners believe—and many mechanics preach—that preventive maintenance is inherently a good thing, and the more of it you do the better. I consider this wrongheaded. Mechanics often do far more preventive maintenance than necessary and often do it using unnecessarily invasive procedures, thereby increasing the likelihood that their efforts will actually cause failures rather than preventing them.
Mac Smith RCM Seminar DVDAnother of my earlier posts discussed Reliability-Centered Maintenance (RCM) developed at United Airlines in the late 1960s, and universally adopted by the airlines and the military during the 1970s. One of the major findings of RCM researchers was that preventive maintenance often does more harm than good, and that safety and reliability can often be improved dramatically by reducing the amount of PM and using minimally invasive techniques.

Unfortunately, this thinking doesn’t seem to have trickled down to piston GA, and is considered heresy by many GA mechanics because it contradicts everything they were taught in A&P school. The long-term solution is for GA mechanics to be trained in RCM principles, but that’s not likely to happen any time soon. In the short term, aircraft owners must think carefully before authorizing an A&P to perform invasive maintenance on their aircraft. When in doubt, get a second opinion.

The last line of defense

The most likely time for a mechanical failure to occur is the first flight after maintenance. Since the risk of such MIFs is substantial, it’s imperative that owners conduct a post-maintenance test flight—in VMC , without passengers, preferably close to the airport—before launching into the clag or putting passengers at risk. I think even the most innocuous maintenance task—even a routine oil change—deserves such a post-maintenance test flight. I do this any time I swing a wrench on my airplane.

You should, too.

Quest for a TBO-Free Engine

Tuesday, May 13th, 2014

“It just makes no sense,” Jimmy told me, the frustration evident in his voice. “It’s unfair. How can they do this?”

Jimmy Tubbs, ECi’s legendary VP of Engineering

Jimmy Tubbs, ECi’s legendary VP of Engineering

I was on the phone with my friend Jimmy Tubbs, the legendary Vice President of Engineering for Engine Components Inc. (ECi) in San Antonio, Texas. ECi began its life in the 1940s as a cylinder electroplating firm and grew to dominate that business. Starting in the mid-1970s and accelerating in the late 1990s—largely under Jimmy’s technical stewardship—the company transformed itself into one of the two major manufacturers of new FAA/PMA engine parts for Continental, Lycoming and Pratt & Whitney engines (along with its rival Superior Air Parts).

By the mid-2000s, ECi had FAA approval to manufacture thousands of different PMA-approved engine parts, including virtually every component of four-cylinder Lycoming 320- and 360-series engines (other than the Lycoming data plate). So the company decided to take the next logical step: building complete engines. ECi’s engine program began modestly with the company offering engines in kit form for the Experimental/Amateur-Built (E-AB) market. They opened an engine-build facility where homebuilders could assemble their own ECi “Lycoming-style” engines under expert guidance and supervision. Then in 2013, with more than 1,600 kit-built engines flying, ECi began delivering fully-built engines to the E-AB market under the “Titan Engines” brand name.

Catch 22, FAA-style

ECi’s Titan Exp experimental engine

A Titan engine for experimental airplanes.
What will it take to get the FAA to certify it?

Jimmy is now working on taking ECi’s Titan engine program to the next level by seeking FAA approval for these engines to be used in certificated aircraft. In theory, this ought to be relatively easy (as FAA certification efforts go) because the Titan engines are nearly identical in design to Lycoming 320 and 360 engines, and almost all the ECi-built parts are already PMA approved for use in Lycoming engines. In practice, nothing involving the FAA is as easy as it looks.

“They told me the FAA couldn’t approve an initial TBO for these engines longer than 1,000 hours,” Jimmy said to me with a sigh. He had just returned from a meeting with representatives from the FAA Aircraft Certification Office and the Engine & Propeller Directorate. “I explained that our engines are virtually identical in all critical design respects to Lycoming engines that have a 2,000-hour TBO, and that every critical part in our engines is PMA approved for use in those 2,000-hour engines.”

“But they said they could only approve a 1,000-hour TBO to begin with,” Jimmy continued, “and would consider incrementally increasing the TBO after the engines had proven themselves in the field. Problem is that nobody is going to buy one of our certified engines if it has only a 1,000-hour TBO, so the engines will never get to prove themselves. It makes no sense, Mike. It’s not reasonable. Not logical. Doesn’t seem fair.”

I certainly understood where Jimmy was coming from. But I also understood where the FAA was coming from.

A brief history of TBO

To quote a 1999 memorandum from the FAA Engine & Propeller Directorate:

The initial models of today’s horizontally opposed piston engines were certified in the late 1940s and 1950s. These engines initially entered service with recommended TBOs of 500 to 750 hours. Over the next 50 years, the designs of these engines have remained largely unchanged but the manufacturers have gradually increased their recommended TBOs for existing engine designs to intervals as long as 2,000 hours. FAA acceptance of these TBO increases was based on successful service, engineering design, and test experience. New engine designs, however, are still introduced with relatively short TBOs, in the range of 600 hours to 1,000 hours.

From the FAA’s perspective, ECi’s Titan engines are new engines, despite the fact that they are virtually clones of engines that have been flying for six decades, have a Lycoming-recommended TBO of 2,000 hours, and routinely make it to 4,000 or 5,000 hours between overhauls.

Is it any wonder we’re still flying behind engine technology designed in the ‘40s and ‘50s? If the FAA won’t grant a competitive TBO to a Lycoming clone, imagine the difficulties that would be faced by a company endeavoring to certify a new-technology engine. Catch 22.

Preparing for an engine test cell endurance run.

Incidentally, there’s a common misconception that engine TBOs are based on the results of endurance testing by the manufacturer. They aren’t. The regulations that govern certification of engines (FAR Part 33) require only that a new engine design be endurance tested for 150 hours in order to earn certification. Granted, the 150-hour endurance test is fairly brutal: About two-thirds of the 150 hours involves operating the engine at full takeoff power with CHT and oil temperature at red-line. (See FAR 33.49 for the gory details.) But once the engine survives its 150-hour endurance test, the FAA considers it good to go.

In essence, the only endurance testing for engine TBO occurs in the field. Whether we realize it or not, those of us who fly behind piston aircraft engines have been pressed into service as involuntary beta testers.

What about a TBO-free engine?

“Jimmy, this might be a bit radical” I said, “but where exactly in FAR Part 33 does it state that a certificated engine has to have a recommended TBO?” (I didn’t know the answer, but I was sure Jimmy had Part 33 committed to memory.)

“Actually, it doesn’t,” Jimmy answered. “The only place TBO is addressed at all is in FAR 33.19, where it says that ‘engine design and construction must minimize the development of an unsafe condition of the engine between overhaul periods.’ But nowhere in Part 33 does it say that any specific overhaul interval must be prescribed.”

“So you’re saying that engine TBO is a matter of tradition rather than a requirement of regulation?”

“I suppose so,” Jimmy admitted.

“Well then how about trying to certify your Titan engines without any TBO?” I suggested. “If you could pull that off, you’d change our world, and help drag piston aircraft engine maintenance kicking and screaming into the 21st century.”

An FAA-inspired roadmap

I pointed out to Jimmy that there was already a precedent for this in FAR Part 23, the portion of the FARs that governs the certification of normal, utility, aerobatic and commuter category airplanes. In essence, Part 23 is to non-transport airplanes what Part 33 is to engines. On the subject of airframe longevity, Part 23 prescribes an approach that struck me as being also appropriate for dealing with engine longevity.

Since 1993, Part 23 has required that an applicant for an airplane Type Certificate must provide the FAA with a longevity evaluation of metallic  wing, empennage and pressurized cabin structures. The applicant has the choice of three alternative methods for performing this evaluation. It’s up to the applicant to choose which of these methods to use:

  • “Safe-Life” —The applicant must define a “safe-life” (usually measured in either hours or cycles) after which the structure must be taken out of service. The safe-life is normally established by torture-testing the structure until it starts to fail, then dividing the time-to-failure by a safety factor (“scatter factor”) that is typically in the range of 3 to 5 to calculate the approved safe-life of the structure. For example, the Beech Baron 58TC wing structure has a life limit (safe-life) of 10,000 hours, after which the aircraft is grounded. This means that Beech probably had to torture-test the wing spar for at least 30,000 hours and demonstrate that it didn’t develop cracks.
  • “Fail-Safe” —The applicant must demonstrate that the structure has sufficient redundancy that it can still meet its ultimate strength requirements even after the complete failure of any one principal structural element. For example, a three-spar wing that can meet all certification requirements with any one of the three spars hacksawed in half would be considered fail-safe and would require no life limitation.
  • “Damage Tolerance” —The applicant must define a repetitive inspection program that can be shown with very high confidence to detect structural damage before catastrophic failure can occur. This inspection program must be incorporated into the Airworthiness Limitations section of the airplane’s Maintenance Manual or Instructions for Continued Airworthiness, and thereby becomes part of the aircraft’s certification basis.

If we were to translate these Part 23 (airplane) concepts to the universe of FAR Part 33 (engines):

  • Safe-life would be the direct analog of TBO; i.e., prescribing a fixed interval between overhauls.
  • Fail-safe would probably be impractical, because an engine that included enough redundancy to meet all certification requirements despite the failure of any principal structural element (e.g., a crankcase half, cylinder head or piston) would almost surely be too heavy.
  • Damage tolerance would be the direct analog of overhauling the engine strictly on-condition (based on a prescribed inspection program) with no fixed life limit. (This is precisely what I have been practicing and preaching for decades.)

How would it work?

SavvyAnalysis chart

Engine monitor data would be uploaded regularly to a central repository for analysis.

Jimmy and I have had several follow-on conversations about this, and he’s starting to draft a detailed proposal for an inspection protocol that we hope might be acceptable to the FAA as a basis of certifying the Titan engines on the basis of damage tolerance and eliminate the need for any recommended TBO. This is still very much a work-in-progress, but here are some of the thoughts we have so far:

  • The engine installation would be required to include a digital engine monitor that records EGTs and CHTs for each cylinder plus various other critical engine parameters (e.g., oil pressure and temperature, fuel flow, RPM). The engine monitor data memory would be required to be dumped on a regular basis and uploaded via the Internet to a central repository prescribed by ECi for analysis. The uploaded data would be scanned automatically by software for evidence of abnormalities like high CHTs, low fuel flow, failing exhaust valves, non-firing spark plugs, improper ignition timing, clogged fuel nozzles, detonation and pre-ignition. The data would also be available online for analysis by mechanics and ECi technical specialists.
  • At each oil-change interval, the following would be required: (1) An oil sample would be taken for spectrographic analysis (SOAP) by a designated laboratory, and a copy of the SOAP reports would be transmitted electronically to ECi; and (2) The oil filter would be cut open for inspection, digital photos of the filter media would be taken, when appropriate the filter media would be sent for scanning electron microscope (SEM) evaluation by a designated laboratory, and the media photos and SEM reports would be transmitted electronically to ECi.
  • At each annual or 100-hour inspection, the following would be required: (1) Each cylinder would undergo a borescope inspection of the valves, cylinder bores and piston crowns using a borescope capable of capturing digital images, and the borescope images would be transmitted electronically to ECi; (2) Each cylinder rocker cover would be removed and digital photographs of the visible valve train components would be transmitted electronically to ECi; (3) The spark plugs would be removed for cleaning/gapping/rotation, and digital photographs of the electrode ends of the spark plugs would be taken and transmitted electronically to ECi; and (4) Each cylinder would undergo a hot compression test and the test results be transmitted electronically to ECi.

The details still need to be ironed out, but you get the drift. If such a protocol were implemented for these engines (and blessed by the FAA), ECi would have the ability to keep very close tabs on the mechanical condition and operating parameters of each its engines—something that no piston aircraft engine manufacturer has ever been able to do before—and provide advice to each individual Titan engine owner about when each individual engine is in need of an overhaul, teardown inspection, cylinder replacement, etc.

Jimmy even thinks that if such a protocol could be implemented and approved, ECi might even be in a position to offer a warranty for these engines far beyond what engine manufacturers and overhaul shops have been able to offer in the past. That would be frosting on the cake.

I’ve got my fingers, toes and eyes crossed that the FAA will go along with this idea of an engine certified on the basis of damage tolerance rather than safe-life. It would be a total game-changer, a long overdue nail in the coffin of the whole misguided notion that fixed-interval TBOs for aircraft engines make sense. And if ECi succeeds in getting its Titan engine certified on the basis of condition monitoring rather than fixed TBO, maybe Continental and Lycoming might jump on the overhaul-on-condition bandwagon. Wouldn’t that be something?

How Do Piston Aircraft Engines Fail?

Wednesday, April 9th, 2014

Last month, I tried to make the case that piston aircraft engines should be overhauled strictly on-condition, not at some fixed TBO. If we’re going to do that, we need to understand how these engines fail and how we can protect ourselves against such failures. The RCM way of doing that is called Failure Modes and Effects Analysis (FMEA), and involves examining each critical component of these engines and looking at how they fail, what consequences those failures have, and what practical and cost-efficient maintenance actions we can take to prevent or mitigate those failures. Here’s my quick back-of-the-envelope attempt at doing that…


CrankshaftsThere’s no more serious failure mode than crankshaft failure. If it fails, the engine quits.

Yet crankshafts are rarely replaced at overhaul. Lycoming did a study that showed their crankshafts often remain in service for more than 14,000 hours (that’s 7+ TBOs) and 50 years. Continental hasn’t published any data on this, but their crankshafts probably have similar longevity.

Crankshafts fail in three ways: (1) infant-mortality failures due to improper materials or manufacture; (2) failures following unreported prop strikes; and (3) failures secondary to oil starvation and/or bearing failure.

Over the past 15 years, we’ve seen a rash of infant-mortality failures of crankshafts. Both Cnntinental and Lycoming have had major recalls of crankshafts that were either forged from bad steel or were damaged during manufacture. These failures invariably occurred within the first 200 hours after the new crankshaft entered service. If the crankshaft survived its first 200 hours, we can be confident that it was manufactured correctly and should perform reliably for numerous TBOs.

Unreported prop strikes seem to be getting rare because owners and mechanics are becoming smarter about the high risk of operating an engine after a prop strike. There’s now an AD mandating a post-prop-strike engine teardown for Lycoming engines, and a strongly worded service bulletin for Continental engines. Insurance will always pay for the teardown and any necessary repairs, so it’s a no-brainer.

That leaves failures due to oil starvation and/or bearing failure. I’ll address that shortly.

Crankcase halvesCrankcase

Crankcases are also rarely replaced at major overhaul. They are typically repaired as necessary, align-bored to restore critical fits and limits, and often provide reliable service for many TBOs. If the case remains in service long enough, it will eventually crack. The good news is that case cracks propagate slowly enough that a detailed visual inspection once a year is sufficient to detect such cracks before they pose a threat to safety. Engine failures caused by case cracks are extremely rare—so rare that I don’t think I ever remember hearing or reading about one.

Lycoming cam and lifterCamshaft and Lifters

The cam/lifter interface endures more pressure and friction than any other moving parts n the engine. The cam lobes and lifter faces must be hard and smooth in order to function and survive. Even tiny corrosion pits (caused by disuse or acid buildup in the oil) can lead to rapid destruction (spalling) of the surfaces and dictate the need for a premature engine teardown. Cam and lifter spalling is the number one reason that engines fail to make TBO, and it’s becoming an epidemic in the owner-flown fleet where aircraft tend to fly irregularly and sit unflown for weeks at a time.

The good news is that cam and lifter problems almost never cause catastrophic engine failures. Even with a badly spalled cam lobe (like the one pictured at right), the engine continues to run and make good power. Typically, a problem like this is discovered at a routine oil change when the oil filter is cut open and found to contain a substantial quantity of ferrous metal, or else a cylinder is removed for some reason and the worn cam lobe can be inspected visually.

If the engine is flown regularly, the cam and lifters can remain in pristine condition for thousands of hours. At overhaul, the cam and lifters are often replaced with new ones, although a reground cam and reground lifters are sometimes used and can be just as reliable.


The engine has lots of gears: crankshaft and camshaft gears, oil pump gears, accessory drive gears for fuel pump, magnetos, prop governor, and sometimes alternator. These gears are made of case-hardened steel and typically have a very long useful life. They are not usually replaced at overhaul unless obvious damage is found. Engine gears rarely cause catastrophic engine failures.

Oil Pump

Failure of the oil pump is rarely responsible for catastrophic engine failures. If oil pressure is lost, the engine will seize quickly. But the oil pump is dead-simple, consisting of two steel gears inside a close-tolerance aluminum housing, and usually operates trouble free. The pump housing can get scored if a chunk of metal passes through the oil pump—although the oil pickup tube has a suction screen to make sure that doesn’t happen—but even if the pump housing is damaged, the pump normally has ample output to maintain adequate oil pressure in flight, and the problem is mainly noticeable during idle and taxi. If the pump output seems deficient at idle, the oil pump housing can be removed and replaced without tearing down the engine.

spun main bearingBearings

Bearing failure is responsible for a significant number of catastrophic engine failures. Under normal circumstances, bearings have a long useful life. They are always replaced at major overhaul, but it’s not unusual for bearings removed at overhaul to be in pristine condition with little detectable wear.

Bearings fail prematurely for three reasons: (1) they become contaminated with metal from some other failure; (2) they become oil-starved when oil pressure is lost; or (3) main bearings become oil-starved because they shift in their crankcase supports to the point where their oil supply holes become misaligned (as with the “spun bearing” pictured at right).

Contamination failures can generally be prevented by using a full-flow oil filter and inspecting the filter for metal at every oil change. So long as the filter is changed before its filtering capacity is exceeded, metal particles will be caught by the filter and won’t get into the engine’s oil galleries and contaminate the bearings. If a significant quantity of metal is found in the filter, the aircraft should be grounded until the source of the metal is found and corrected.

Oil-starvation failures are fairly rare. Pilots tend to be well-trained to respond to decreasing oil pressure by reducing power and landing at the first opportunity. Bearings will continue to function properly at partial power even with fairly low oil pressure.

Spun bearings are usually infant-mortality failures that occur either shortly after an engine is overhauled (due to an assembly error) or shortly after cylinder replacement (due to lack of preload on the through bolts). Failures occasionally occur after a long period of crankcase fretting, but such fretting is usually detectable through oil filter inspection and oil analysis).They can also occur after extreme unpreheated cold starts, but that is quite rare.

Thrown Connecting RodConnecting Rods

Connecting rod failure is responsible for a significant number of catastrophic engine failures. When a rod fails in flight, it often punches a hole in the crankcase (“thrown rod”) and causes loss of engine oil and subsequent oil starvation. Rod failure have also been known to cause camshaft breakage. The result is invariably a rapid and often total loss of engine power.

Connecting rods usually have a long useful life and are not normally replaced at overhaul. (Rod bearings, like all bearings, are always replaced at overhaul.) Many rod failures are infant-mortality failures caused by improper tightening of the rod cap bolts during engine assembly. Rod failures can also be caused by the failure of the rod bearings, often due to oil starvation. Such failures are usually random failures unrelated to time since overhaul.

Pistons and Rings

Piston and ring failures usually cause only partial power loss, but in rare cases can cause complete power loss. Piston and ring failures are of two types: (1) infant-mortality failures due to improper manufacturer or assembly; and (2) heat-distress failures caused by pre-ignition or destructive detonation events. Heat-distress failures can be caused by contaminated fuel (e.g., 100LL laced with Jet A), or by improper engine operation. They are generally unrelated to hours or years since overhaul. A digital engine monitor can alert the pilot to pre-ignition or destructive detonation events in time for the pilot to take corrective action before heat-distress damage is done.

Head SeparationCylinders

Cylinder failures usually cause only partial power loss, but occasionaly can cause complete power loss. A cylinder consists of a forged steel barrel mated to an aluminum alloy head casting. Cylinder barrels typically wear slowly, and excessive wear is detected at annual inspection by means of compression tests and borescope inspections. Cylinder heads can suffer fatigue failures, and occasionally the head can separate from the barrel. As dramatic as it sounds, a head separation causes only a partial loss of power; a six-cylinder engine with a head-to-barrel separation can still make better than 80% power. Cylinder failures can be infant-mortality failures (due to improper manufacture) or age-related failures (especially if the cylinder head remains in service for more than two or three TBOs). Nowadays, most major overhauls include new cylinders, so age-related cylinder failures have become quite rare.

Broken Exhaust ValveValves and Valve Guides

It is quite common for exhaust valves and valve guides to develop problems well short of TBO. Actual valve failures are becoming much less common nowadays because incipient problems can usually be detected by means of borescope inspections and digital engine monitor surveillance. Even if a valve fails completely, the result is usually only partial power loss and an on-airport emergency landing.

Rocker Arms and Pushrods

Rocker arms and pushrods (which operate the valves) typically have a long useful life and are not normally replaced at overhaul. (Rocker bushings, like all bearings, are always replaced at overhaul.) Rocker arm failure is quite rare. Pushrod failures are caused by stuck valves, and can almost always be avoided through regular borescope inspections. Even when they happen, such failures usually result in only partial power loss.

Failed Mag Distributor GearsMagnetos and Other Ignition Components

Magneto failure is uncomfortably commonplace. Mags are full of plastic components that are less than robust; plastic is used because it’s non-conductive. Fortunately, our aircraft engines are equipped with dual magnetos for redundancy, and the probability of both magnetos failing simultaneously is extremely remote. Mag checks during preflight runup can detect gross ignition system failures, but in-flight mag checks are far better at detecting subtle or incipient failures. Digital engine monitors can reliably detect ignition system malfunctions in real time if the pilot is trained to interpret the data. Magnetos should religiously be disassembled, inspected and serviced every 500 hours; doing so drastically reduces the likelihood of an in-flight magneto failure.

The Bottom Line

The bottom-end components of our piston aircraft engines—crankcase, crankshaft, camshaft, bearings, gears, oil pump, etc.—are very robust. They normally exhibit long useful life that are many multiples of published TBOs. Most of these bottom-end components (with the notable exception of bearings) are routinely reused at major overhaul and not replaced on a routine basis. When these items do fail prematurely, the failures are mostly infant-mortality failures that occur shortly after the engine is built, rebuilt or overhauled, or they are random failures unrelated to hours or years in service. The vast majority of random failures can be detected long before they get bad enough to cause an in-flight engine failure simply by means of routine oil-filter inspection and laboratory oil analysis.

The top-end components—pistons, cylinders, valves, etc.—are considerably less robust. It is not at all unusual for top-end components to fail prior to TBO. However, most of these failures can be prevented by regular borescope inspections and by use of modern digital engine monitors. Even whey they happen, top-end failures usually result in only partial power loss and a successful on-airport landing, and they usually can be resolved without having to remove the engine from the aircraft and sending it to an engine shop. Most top-end failures are infant-mortality or random failures that do not correlate with time since overhaul.

The bottom line is that a detailed FMEA of piston aircraft engines strongly suggests that the traditional practice of fixed-interval engine overhaul or replacement is unwarranted and counterproductive. A conscientiously applied program of condition monitoring that includes regular oil filter inspection, oil analysis, borescope inspections and digital engine monitor data analysis can yield improved reliability and much reduced expense and downtime.

Do Piston Engine TBOs Make Sense?

Thursday, March 13th, 2014

Last month, I discussed the pioneering work on Reliability-Centered Maintenance (RCM) done by United Airlines scientists Stan Nowlan and Howard Heap in the 1960s, and I bemoaned the fact that RCM has not trickled down the aviation food chain to piston GA. Even in the 21st century, maintenance of piston aircraft remains largely time-based rather than condition-based.

mfr_logo_montageMost owners of piston GA aircraft dutifully overhaul their engines at TBO, overhaul their propellers every 5 to 7 years, and replace their alternators and vacuum pumps every 500 hours just as Continental, Lycoming, Hartzell, McCauley, HET and Parker Aerospace call for. Many Bonanza and Baron owners have their wing bolts pulled every five years, and most Cirrus owners have their batteries replaced every two years for no good reason (other than that it’s in the manufacturer’s maintenance manual).

Despite an overwhelming body of scientific research demonstrating that this sort of 1950s-vintage time-based preventive maintenance is counterproductive, worthless, unnecessary, wasteful and incredibly costly, we’re still doing it. Why?

Mostly, I think, because of fear of litigation. The manufacturers are afraid to change anything for fear of being sued (because if they change anything, that could be construed to mean that what they were doing before was wrong). Our shops and mechanics are afraid to deviate from what the manufacturers recommend for fear of being sued (because they deviated from manufacturers’ guidance).

Let’s face it: Neither the manufacturers nor the maintainers have any real incentive to change. The cost of doing all this counterproductive, worthless, unnecessary and wasteful preventive maintenance (that actually doesn’t prevent anything) is not coming out of their pockets. Actually, it’s going into their pockets.

If we’re going to drag piston GA maintenance kicking and screaming into the 21st century (or at least out of the 1950s and into the 1960s), it’s going to have to be aircraft owners who force the change. Owners are the ones with the incentive to change the way things are being done. Owners are the ones who can exert power over the manufacturers and maintainers by voting with their feet and their credit cards.

For this to happen, owners of piston GA aircraft need to understand the right way to do maintenance—the RCM way. Then they need to direct their shops and mechanics to maintain their aircraft that way, or take their maintenance business to someone who will. This means that owners need both knowledge and courage. Providing aircraft owners both of these things is precisely why I’m contributing to this AOPA Opinion Leaders Blog.

When are piston aircraft engines most likely to hurt you?

Fifty years ago, RCM researches proved conclusively that overhauling turbine engines at a fixed TBO is counterproductive, and that engine overhauls should be done strictly on-condition. But how can we be sure that his also applies to piston aircraft engines?

In a perfect world, Continental and Lycoming would study this issue and publish their findings. But for reasons mentioned earlier, this ain’t gonna happen. Continental and Lycoming have consistently refused to release any data on engine failure history of their engines, and likewise have consistently refused to explain how they arrive at the TBOs that they publish. For years, one aggressive plaintiff lawyer after another have tried to compel Continental and Lycoming to answer these questions in court. All have failed miserably.

So if we’re going to get answers to these critical questions, we’re going to have to rely on engine failure data that we can get our hands on. The most obvious source of such data is the NTSB accident database. That’s precisely what brilliant mechanical engineer Nathan T. Ulrich Ph.D. of Lee NH did in 2007. (Dr. Ulrich also was a US Coast Guard Auxiliary pilot who was unhappy that USCGA policy forbade him from flying volunteer search-and-rescue missions if his Bonanza’s engine was past TBO.)

Dr. Ulrich analyzed five years’ worth of NTSB accident data for the period 2001-2005 inclusive, examining all accidents involving small piston-powered airplanes (under 12,500 lbs. gross weight) for which the NTSB identified “engine failure” as either the probable cause or a contributing factor. From this population of accidents, Dr. Ulrich eliminated those involving air-race and agricultural-application aircraft. Then he analyzed the relationship between the frequency of engine-failure accidents and the number of hours on the engine since it was last built, rebuilt or overhauled. He did a similar analysis based on the calendar age of the engine since it  was last built, rebuilt or overhauled. The following histograms show the results of his study:

Ulrich study (hours)

Ulrich study (years)

If these histograms have a vaguely familiar look, it might be because they look an awful lot like the histograms generated by British scientist C.H. Waddington in 1943.

Now,  we have to be careful about how we interpret Dr. Ulrich’s findings. Ulrich would be the first to agree that NTSB accident data can’t tell us much about the risk of engine failures beyond TBO, simply because most piston aircraft engines are voluntarily euthanized at or near TBO. So it shouldn’t be surprising that we don’t see very many engine failure accidents involving engines significantly past TBO, since there are so few of them flying. (The engines on my Cessna 310 are at more than 205% of TBO, but there just aren’t a lot of RCM true believers like me in the piston GA community…yet.)

What Dr. Ulrich’s research demonstrates unequivocally is striking and disturbing frequency of “infant-mortality” engine-failure accidents during the first few years and first few hundred hours after an engine is built, rebuilt or overhauled. Ulrich’s findings makes it indisputably clear that by far the most likely time for you to fall out of the sky due to a catastrophic engine failure is when the engine is young, not when it’s old.

(The next most likely time for you to fall out of the sky is shortly after invasive engine maintenance in the field, particularly cylinder replacement, but that’s a subject for a future blog post…stay tuned!)

 So…Is there a good reason to overhaul your engine at TBO?

Engine overhaulIt doesn’t take a rocket scientist (or a Ph.D. in mechanical engineering) to figure out what all this means. If your engine reaches TBO and still gives every indication of being healthy (good performance, not making metal, healthy-looking oil analysis and borescope results, etc.), overhauling it will clearly degrade safety, not improve it. That’s simply because it will convert your low-risk old engine into a high-risk young engine. I don’t know about you, but that certainly strikes me as a remarkably dumb thing to do.

So why is overhauling on-condition such a tough sell to our mechanics and the engine manufacturers? The counter-argument goes something like this: “Since we have so little data about the reliability of past-TBO engines (because most engines are arbitrarily euthanized at TBO), how can we be sure that it’s safe to operate them beyond TBO?” RCM researchers refer to this as “the Resnikoff Conundrum” (after mathematician H.L. Resnikoff).

To me, it looks an awful lot like the same circular argument that was used for decades to justify arbitrarily euthanizing airline pilots at age 60, despite the fact that aeromedical experts were unanimous that this policy made no sense whatsoever. Think about it…