What Makes an Engine Airworthy?

July 2nd, 2014 by Mike Busch

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.

Mike Busch

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

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The opinions expressed by the bloggers do not reflect AOPA’s position on any topic.

  • Fred geiger

    Great to have this expert advice in the online edition! Hope to see more in the near future. A pilot who has a fuller understanding of the motor keeping us aloft if likely a safer pilot!

  • spfalzo1234

    Hi Mike: Great informative information about engine life and helpful hints used to determine an engine’s actual condition. I’m sure many owners and mechanics will benefit from this article.
    Thanks much.
    Sam Falzone
    A&P, ASEL

  • Lara

    Engines airworthy depends on the performance of the engines.

    • Ira Curtis

      Yes Lara!!! You are absolutely right. I can only add , “It is much better to be rich and healthy then being poor and ill!”

  • FrostedCW3

    Thanks Mie, for a well reasoned, and explained, article.

  • Len

    I wish the A&P who dissuaded a potential buyer from my Grumman had read this before making his determination. Anyway, educational and informative as usual. I enjoy and appreciate every word Mr. Busch writes. Thank you, sir.

  • BAF

    But doesn’t Lycoming have a lower compression limit that requires an overhaul?

    • http://www.savvyaviator.com/ Mike Busch A&P/IA

      Yes and no. On one hand, Lycoming’s guidance still adheres to the old-fashioned 60/80 threshold. On the other hand, Lycoming’s guidance about compression testing is worded in a very fuzzy fashion which does not compel a mechanic to remove a cylinder if it measures less than 60/80, but simply suggests that the mechanic consider doing so. Lycoming does acknowledge that other tools such as borescope inspections can be valuable in assessing cylinder condition, but they don’t go as far as Continental does by requiring a borescope inspection any time a compression test is performed. Bottom line is that Continental’s guidance is far more specific and far more permissive, while Lycoming’s guidance is far fuzzier and far more restrictive.

  • John tate

    Just wondering what Mike’s thoughts are on the AD (2009-16-03) superior Millineum Cylinders that gives a calender time for replacement – regardless of compression, condition or oil consumption. We are going to have to replace a top end in a year on a perfectly smooth running engine.

    • http://www.savvyaviator.com/ Mike Busch A&P/IA

      I consider the Superior cylinder AD 2009-16-03 and its successor AD 2014-05-29 that supersedes it are absolute travesties. We worked hard to persuade the FAA to approve an AMOC that allows these cylinders to remain in service for 17 years rather than the original 12 set forth in those ADs. But the ADs should never have been issued in the first place. There is no unsafe condition with those cylinders, and every enlightened observer is convinced that the safety risk created by all those forced top overhauls will be far greater than any risk presented by the Superior cylinders themselves. In fact, the handful of head-to-barrel separations that prompted the AD in the first place virtually all occurred with a single commercial operator in Alaska that was obviously abusing the hell out of the cylinders, and all occurred within a relatively narrow serial number range. The folks at the FAA Ft. Worth Aircraft Certification Office (who never met a cylinder they felt wasn’t unsafe) decided to use that situation as a pretext to legislate every single Superior investment-cast cylinder out of existence. There is no question that the mandated cure will be vastly worse than the disease. The FAA’s own written policies indicate that these ADs should never have been issued, but the Ft. Worth ACO wasn’t about to let that stop them. This was an example of government at its worst.

      Buy me a beer and I’ll tell you what I really think. 😉

      • John tate

        Thanks Mike,
        After numerous discussions with some knowledgable A&P’s and other Bonanza owners I was unsure if it was an overzealous FAA rule over a few abused engines . . . or a conspiracy theory on Continental vs Superior. Still unclear as to which, but I think I know your thoughts. 1 more voice from a highly experienced IA that thinks this is not only BS . . . but a safety issue. Thanks for your candid reply!!

  • Carl ziegler

    Ok, let’s apply some
    knowledge to common sense. To adhere to Mikes preaching to ignore TBO limits only invites a false sense of security to owners who are flying behind engines that were last overhauled back in the 80’s when the Bee Gees were still hot.
    Sure, TBO busting is ok but why do you think manufactures have calendar year limits in addition to hour limits. Lycoming has some very good information on what they feel are
    annual utilization rates in order to reach recommended TBO’s. (hint, it doesn’t include 20 to 30 year time spans) To the owners out there, #1, you can have good compression with high
    oil consumption. When you start pushing oil limits, its a downward spiral. Contaminated oil , you know , the nice BLACK and BURNT smelling stuff, will just accelerate eng wear. The only resort is frequent oil changes to prolong the inevitable….. dare i say…..one or more cylinder
    replacements. O, i forgot, Mike says that’s dangerous to accomplish.
    #2, Ok pilots, HP indication is indicated by two gauges in the cockpit, MP and RPM. Gradual degradation of engine condition can be masked by the prop control system striving to maintain
    our MAX takeoff setting, (INDICATED rated HP) as long as we are hitting that figure with
    other parameters normal, we feel we are getting rated power. Engine wear, (top
    end, cam wear) is going to show up in decreased climb and lower cruise speeds
    for same settings. Especially climbs when at gross weights.
    For us fixed pitch prop people, when we have trouble meeting static run up minimums its time to start investigating.
    Yes, TBO times are a recommendation, but not to be ignored or tossed aside. As
    Mike does point out, many tools are available to monitor engine health.
    Also, excellent information from manufactures such as Lycoming Service Instruction 1009AW and 1191A and TCM SB03-3. Read them and be a informed and educated owner- operator.

    And yes, a worn or wearing out engine can still be considered Airworthy.

  • Richard

    Hi Mike,

    Another excellent article. I have a Tiger with 1400 since new, which was in 1979. It is consuming oil at about 1 quart per 8 hrs, with most of it on the belly. Boroscope indicates there is no coking on the valve face or seat, the valves seat straight, ie no lateral movement, and I make static power and rated performance, with no metal in the oil filter.

    So, do I do a top now, either OH the first run cylinders, or replacement cylinders, or just continue to monitor. I am struggling since the engine is 35 years old.

    • Carl ziegler

      Assuming your compressions are hanging in there, it sounds like you have a engine thats still going strong and has a good target for TBO. I’ll make a assumption that accessories are in good shape too. 35year old seals will give up way before the engine does. If you have not started, now is good time to start oil analysis to watch for trends. Starting at tbo is rather pointless. One major key to longevity is use and oil change… Lyc says change oil every quarter regardless of hours flown. I do that even if i have only a few hours since last change. The goal is to keep acids out of the case from combustion by products.
      The problem we all face as we approach tbo is what i call “tbo coffin corner” Its a tough call to start replacing cylinders when a couple hundred hrs from TBO, only to have to redo them when you actually do a overhaul. And no, unfortunately a clyinder with 200 hrs still get overhaul treatment in order to legally call it overhauled. If the engine runs strong up to and past tbo, good for you! Trend monitoring needs to start now.
      As for cyinders, with the cost of new so reasonable, we just do new all the time. Second runs should still be repairable, but it can be a crap shoot. Lycoming cylinders tend to start showing cracks as they age with multiple overhauls.

      • Richard

        Hi Carl,

        I try to fly an hour every week. I do change the oil every 4 months no matter how many hrs and cut and inspect the filter. I plan to start oil analysis every other change. I have the money back for the overhaul, so that is not an issue. I guess as long as I am making static RPM and we continue boroscope and the other inspections, I’ll probably just keep on keeping on.

        • Carl ziegler

          Sounds like you have a good ops plan. Amost all engines give lots of warning they are maxing out. I’ll tell you that even borescoping not a catch all,,, i have taken cylinders off for non related issues and the piston rings dropped to the floor. Bores were perfect, and no oil or compression issues. Scopes great for looking at valves. Its just a reminder that no one method is perfect.
          Fly on!

  • Gabriel

    Good insight aside, beware the bad service and costs you might incur from the company Mike gets free advertising for here on this blog, which I think is inappropriate since it’s a tacit endorsement from the organization I pay to be a member of for good advice. When Savvy screws up and it costs you thousands of dollars, they won’t stand up and take any responsibility. Mike may know a lot about fixing planes, but he and his team know nothing about customer service, and they alienate more than a few maintenance shops along the way, too. AOPA members beware.

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