Do Piston Engine TBOs Make Sense?

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…

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).


  1. Amen, on condition overhaul makes way more sense. In the airline world we generally see more issues with airplanes and engines when they are new or have just undergone extensive maintenance work. We tend to have everything on condition unless there is a restriction on a part like one of the turbine disk that makes you crack open the core of the engine. A flight school I use to work at had a C-150 with an engine at 6000+ hours on it and it out performed the C-152’s that all had low TTSO. It had awesome compression and no oil consumption. The chief mechanic took great care and pride in the motor and I wouldn’t be surprised to hear the same motor is still in it and still without an overhaul.

    • Indeed, the secret to aircraft engine longevity is regular use, lots of operational and maintenance TLC, but avoiding invasive maintenance (like cylinder removal) as much as possible. If we use them a lot, treat the well, and don’t take them apart, they’ll live long and prosper.

  2. In my careers as mechanical engineer and software architect, nothing has been more true than the “infant mortality” problem — and it’s corollary, “if i ain’t broke, don’t fix it”. This has also been true for me flying when getting things “fixed” away from home (loose screws, etc). The government and industry in general have spend billions studying probabilities of failure based on DATA. Our industry needs to publish it’s data and let’s get on with doing things based on facts, and not fear.

    • Indeed, it’s truly tragic that fear of litigation prevents our aircraft and engine manufacturers (particularly the latter) fro being forthcoming with the extensive data they possess about engine failure history. Perhaps we need Congress to provide them immunity or something.

  3. Mike – another interesting post on RCM!

    I own a Piper Cherokee 180 with a Lyc O-360. It’s a little past TBO, and I was talking with a local AI on the subject of “how long can I go before I overhaul the engine, barring any obvious warning signs of impending failure?” His response was that “Continental engines tend to fail on the top end because of various design flaws related to the oil passages, while Lycomings tend to fail on the bottom end due to an undersized component (the key) on the crankshaft design which is the weak point. Don’t fly your engine beyond 110% of TBO or you risk a crank failure.”

    That response with respect to Lycoming engines didn’t make much sense to me. I am a civil engineer (not a ME) so I understand the principles of stress, including failures due to cyclic stress. It seems to me that a crankshaft in a 4-cylinder piston engine, while experiencing variable stress during the four cycles of internal combustion, still “averages” the internal stress from four cylinders on a more or less continuous basis at mostly constant RPM over most of the engine life, against the fluid resistance of a propeller spinning in air. This stress can be contrasted with the rather different stress experienced by the crank in an automobile engine, which frequently experiences large and sudden changes in throttle while operating against the “hard” resistance from the drive train.

    Therefore, the crankshaft seems rather unlikely to be the weak point in an aircraft engine, as compared to the cylinder, piston, or valve train components. Or as compared to airframe components such as wing spars, which accumulate cyclic stress-related weakening over many thousands of cycles of use. And if it’s going to fail, it seems to me that a crankshaft is far more likely to fail early after an overhaul, due to a material flaw in the steel, rather than after thousands of hours of successful operation later.

    What do you think about this AI’s theory of failure for Lycomings?

    • Pure unmitigated poppycock. We routinely see 4-cyllinder Lycoming 320s and 360s remain healthy for 4,000+ hours so long as they’re flown regularly enough that their cams and lifters don’t suffer corrosion pitting that leads to spalling. What your mechanic suggesting doesn’t pass the laugh test, since a crankshaft failure invariably results in a lawsuit against Lycoming. Lycoming crankshafts routinely last for 14,000 hours (according to a Lycoming study).

      • Mike,
        This brings up similar response to drivers license medical to stop the decrease flyers of GA as well as to cut costs, which is killing GA.
        Why not initiative to make the 100 hr/annual at least a 100 hr or 18 month whichever comes 1st. For us flying 60-75 hrs a year it would save a high expense item over several cycles and we all know our risks are also higher after extensive maintenance or annual.
        Maybe too far out for a 100hr/biennial but who knows til we see if planes start falling from the sky.

        Someone needs to champion this for us little guys


        • Your thoughts mike?

          • Whichever approach could be performed at a savings to ga owner pilots and not compromise safety is a winner for all who’s livelihood depends on this segment of economy.
            To not infringe on the high hour flyers could it not be annual if over 100 hours or upon reaching 100 hr thereafter to a max calendar of 18 or 24 month?

            Just looking for a simpler to institute method, although I would definitely sign up for the phased inspection if option to current system.

            Your thoughts mike?


          • That would be okay, although I’d prefer a phased inspection approach.

        • I would NOT endorse changing the annual inspection to being required at 100 hours or 18 months, whichever comes first. That would penalize folks who fly 200 or 300 hours a year, and I would not support doing that.

          What I would strongly endorse is a phased inspection approach (like what bizjets and airlines do) where various components have differing inspection intervals. For example, I think certain components (i.e., everything firewall-forward plus wheels, brakes, and landing gear retraction systems) need fairly frequent inspection, while many other items could safely go 2 or 3 years between inspections, particularly if the aircraft is hangared or subject to low corrosion risk.

          Any such change that I would endorse would be an ALTERNATIVE to the current annual inspection protocol, not a REPLACEMENT. Owners would get to choose whether they wanted to use the new phased inspection program or whether they wanted to stick with conventional annual inspections. This would be much like the current rules for progressive inspections, which are available as an option to almost all operators but must be elected by the operator in lieu of annual inspections.

  4. Mike, maybe you could publish some stats and indications for moving Props to RCM for a future topic?
    (ref: constant spd, vs fixed or adjustable)

    • Absolutely. For our managed maintenance fleet, we typically overhaul constand-speed propellers and prop governors only when the engine is overhauled, rather than every 5 to 7 calendar years as recommended by Hartzell and McCauley. For propellers exposed to high corrosion risk, we way do a “reseal” every 5-7 years, but for ones whose corrosion-risk exposure is low, we’ll do the reseal only every 10-15 years. Of course all bets are off if the prop suffers a prop strike.

      • Thanks

      • Charles W Newman

        March 15, 2014 at 7:33 am

        Is the reseal done at this stage to keep moisture out or grease in?

        • Neither. It’s done to inspect the parts inside the propeller hub for corrosion damage. In essence, a “reseal” is the same as a full overhaul except that the bades are just visually inspected and then left alone if they look okay. The hub and its interior components typically undergo the same process they would during a full overhaul. A reseal typically costs half what a full overhaul costs (with 95% of the benefit). If the prop has electrothermal deicers installed, then the reseal typically costs only a third of what a full overhaul costs. For example, we just got quotes on a Cessna P210 propeller with deicers and the quotes were $4,400 for the full overhaul and $1,800 for the reseal.

  5. I think you are missing a critical part of the analysis here — correcting for amount of population expected to fall within a certain bucket. You need to divide by the bucket proportions of the population to correct for that. For example, if 90% of the population is in bucket 1 to 4, you would expect that bucket to be large in raw count due to the sheer number in that part of the population.

    • I’m not missing it, Ian. It’s just that no denominator is available to be able to calculate failure rates. Nobody (not even Continental or Lycoming) has a clue about what percentage of the fleet is fling at various numbers of hours on their engines (and if Continental or Lycoming did know, they wouldn’t tell us). I once tried to do a study to get some data on this by reaching out to the two leading oil analysis labs, figuring that man owners send in oil samples at every oil change and provide information on their TSMOH when they do. The labs looked at their databases and concluded that the information they received on engineTSMOH was too sparse and to unreliable to provide any useful results. The FAA doesn’t collect this data in their triennial survey. The engine overhaul shops don’t keep track of it. To the best of my knowledge, there is no reliable source for such data. Which is too bad.

      So all we have is failure data as plotted by Dr. Ulrich. We don’t have failure RATE data, which would require a denominator to calculate. But the failure data alone tells us clearly that (as I said in the blog bost) “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.” We do not need a denominator to make that assertion.

      We would need a denominator to support a statement like “an engine is no more likely to fail at TBO+500 than it is at TBO-500” or one like “the risk of failure at 50 SMOH is much higher than the risk of failure at TBO+50” so I am careful never to make assertions like those (although personally I believe them to be true).

  6. I have had my Cherokee 235 for a few decades, Airplane has over 9,000 hours on it. it uses a Lycoming O54B4B5 engine. First engine 3,300 hrs without an overhaul, I did change a couple jugs due to burned exhaust valves. Second engine 3,700 hrs, with no overhaul, had changed one jug due to exhaust valve issue, but then changed out the engine at 3.700 hrs only because Chevron gave me another one for free. They put jet fuel in the local airport gas tanks, Third engine changed at 1600 hours because it was making metal and we could not find source without a complete teardown, so I just put in another zero time rebuilt. (In every case I have put in zero time rebuilt engines) Now on the fourth engine with 1700 hours and I have no intention of doing anything until needed. But I have replaced one jug due to a burned exhaust valve.
    So if you read all this and have a Lycoming O540 I don’t see why in the world you would overhaul at 2,000 hours unless there was an existing problem.

  7. As an example, an abusive one, that supports Mike’s contention. I assisted with a teardown of a Lycoming O360 on a C180. The engine was making heat in climb and I said that the bearings must be worn. The owner said he bought the plane with a fresh overhaul at 2000 TT and it was now 4000TT. .
    When we took it apart we saw that the bearings were worn all the way into the metal, and it was plain that the engine had only been topped at 2000 hours and misdescribed to the buyer.
    The point is – the bottom end made it to 4000 since new with no catastrophic failure – very impressive.

  8. thanks, nice article. I was going to ask the failure rate question, but it’s covered. Unfortunately our last engine showed signs of pitting/corrosion at 1500h. So we’re hoping to have this debate with the current engine 🙂

  9. We’ve got a ’77 Cardinal FG that’s on its 850th tach hour SMOH (in 1987!) and it probably only sees 40-50 hours of annual time. A little bit of light reading in the Cardinal Flyers owners group digest has revealed some opinions that RCM entails more risk if the engine isn’t frequently used.

    That makes intuitive sense – but looking at the recent history:
    – we’ve replaced one hairline cracked cylinder, broken in the new one over ~100 hours, and nothing is different (abnormal) about the plane’s operation before or since
    – continuing annual Blackstone oil analysis has returned very healthy assessments
    – we have replaced flexible fuel lines in the last 3 years
    – general operation since I acquired the aircraft in 2008 has been unwaveringly consistent
    – i’m unsure about magneto hours; need to look into that pronto

    No pressure from our A&P to overhaul; in fact I think he’s in your camp. I’m inclined to continue to operate this Cardinal with no thoughts toward an overhaul unless the upcoming annual reveals significant trouble. Consistent with your thinking, Mike, or am I overlooking an obvious >gotcha<?

  10. Two points. First, while there is no way to calculate accident rate, and I am no statistician, it seems to me that over the 5 year period of the study it would be unreasonable to assume that there were more “new” engines than “old” engines. It does seem more likely to have a problem early on in an engine’s life. Along those lines, what is the fascination now with “new” anyway? What about “proven” and “reliable”? I feel much safer in my ’77 Warrior with ~7,000 airframe hours that has been through a lot – and proven that it can take it – than I would in a new carbon fiber Cirrus or LSA. Now, I do think I’d feel a little faster in the Cirrus…

  11. Well cared for engines can absolutely outlast TBO – especially if flown a lot (vs. sitting in hangars for months on end between flights). TBO doesn’t take frequency of flight into account. Taking good preventative maintenance measures and addressing things before they are urgent is great, but don’t hire Mike’s team to help because when they do nothing about it after repeated efforts to get it done (when you could do it yourself with a shop in the meantime) it will be your wallet that suffers, because Mike and his guys won’t take responsibility. I have had a maintenance shop make a mistake and they took responsibility and gave a credit for the emergency repair that resulted, kudos to them for that. Too bad Savvy doesn’t stand by its service.

  12. so if an 0-200 was overhauled 14 years ago and only has 660 smoh, what would be the implications of putting 300 hours on it in a year. Would the change in pace hurt the engine

  13. William Joseph James Roberts

    April 18, 2015 at 11:27 am

    Part of the reason the TBO issue isn’t going to be settled anytime soon, is because a higher TBO would result in less money for manufacturers, and eventually lawyers.

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