Engine Overhauls – Arbitrary or Necessary?

June 17, 2009 by Bruce Landsberg

When should engines be overhauled? Some believe that the engine manufacturer’s recommendation are loose guidelines and that your mileage may vary greatly. Others adhere to them as gospel and treat them as mandatory. Times typically run between 1,500 and 2,500 hours and there’s usually a calendar factor of about 12 years so the guidance is for whatever comes first. Piston aircraft engines are extremely reliable when properly maintained, when flown regularly and correctly. You can see some important qualifiers creeping into the discussion. Some of the most outspoken advocates on both sides of the argument are extremely knowledgeable which means we best look deeper.

Several charity medical transport groups are beginning to require volunteer pilots to certify that their engines are below the manufacturer’s Time Between Overhaul (TBO) recommendations. There’s been some predictably negative reaction but the groups may have a point and here’s why. Under the FARs, flights for hire must have engines below TBO and while the charity flights are certainly not in that category, the groups are concerned that they kind of look like it to the FAA and NTSB. The groups are acting as a facilitator or broker between pilot and a trusting passengers, who likely know nothing about aviation safety. It’s quite a different relationship than when we just take a friend flying. Despite liability disclaimers and efforts to put a firewall between the group and the pilot, there’s a perception and an expectation of a higher level of care.

Reliable statistics are a great starting point for any safety discussion and in turbine aircraft, which are renowned for reliability, the manufacturer’s overhaul guidance is typically followed to the letter but there’s great emphasis on tracking. With piston engine aircraft, there just isn’t much to go on. For example, in the 2008 Nall Report there were 87 accidents attributed to powerplant malfunctions. What’s confounding is that we often don’t get the total time on the engine and prop.

We also don’t know how many parts and pieces are caught on the verge of failure and at what point in their life span. I know from personal experience that brand new engines are some of the quirkiest. It’s also true that organizations have no desire to be publicly called out, no matter how remote the risk, if there’s an appearance that they didn’t take every reasonable precaution to prevent a mishap.

We’d like to hear your thoughts pro and con.

Bruce Landsberg
Senior Safety Advisor, Air Safety Institute

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  • Mark

    I know of a flight school that got over 3500 hours on their 172 and another that put similar hours on an Archer. As I understand both were under rigorus maintenance and had any sub-par parts repaired or replaced in a timely manner. My own aircraft is currently over TBO by about 100 hours but I am watching the vital signs carefully and am quite confident in it and am prepared to have it overhauled when it does need it.

  • Jack Williams

    The IO470K in my Debonair is 380 hours past TBO and 22 years since REMAN. That’s an average of 85 hours a year. Frequent use, maintenance and oil changes certainly help. I have also used auto fuel since REMAN and that doesn’t seem to hurt the longivity.
    When I flew for the airlines we often ran R2800 engines to 3600 hours or more. When I flew the same engines in the military on the same type of airframe we generally got 700-800 hours. The airline airplanes got 9-10 hours PER DAY. The military airplanes got the same for each trans-pac trip but only one trip per month.
    The IO47K has a TBO of 1500 hours. The IO550 which evolved from the 470 now has a TBO of 2000 hours in some models.

  • David Viglierchio

    Mike Busch has quite a bit to say about this subject backed up by quite a bit of research. I went to all of his seminars at Oshkosh last year and came away with a much better understanding of the potential life of my motor. In fact I sold the backup I had purchased thinking I was coming to an imminent overhaul. Am still running strong with good oil analysis, compressions and borescope checks.
    AOPA knows him well:

  • niel

    The Callendar Factor is the wild card ! A guy has a new engine, flys an hour a month, keeps his plane in a hanger . In 12 years he has flown the plane 150 hours and the guy thinks he has a relatively new engine. The engine has life cycled though as to the Callenday Factor. The same guy has 5 cars in a similar situation ,the difference is he just pulls over to the side of the road when the fire goes out in the car. This guy was kind of like me!!!!! live and learn.

  • Ed

    I have a 0470 in a Cessna 182Q and just had the engine overhauled to factory new specs. It had only 1000 hrs of the 1500 TBO ( of which I put 300 hrs). Even though the compressions were very good it was discovered in the annual there was oil in one cylinder. This led to a visual of the cam shaft which had worn past the hardened metal and the discovery that cylinders had been over bored. The log book showed that the last re-man was done only to legal limits, also 16 yrs had passed since overhaul. So I would say that it is not only a matter of TBO but also a matter of time and use as well as the type of overhaul last done.

  • Don

    Interesting views. I was involved in a Cherroke that was approaching TBO with no apparent problems. TBO was performed. Shortly afterward the OH, the plane had a forced landing owing to a crankshaft failure.

    I previously owned a “67 Mooney wit a partner. We OHed the engine because we thought it was time. That led to more problems than we had before the OH.

    I bought a ’64 Mooney M20E, with a high time engine in 1997. I got a good price because of the engine. My plan was to do the OH soon. However, after recalling the two previous scenarios (above), I began thinking… “the enemy of good is perfect!” I have flown the ’64 Mooney (50-100 hours per year) for 11 years now. I have replaced a couple of jugs (one for a crack and another for low compression). It is no guaranty but I rely on good maintenance and my EDM to provide early warning to engine problems. I track oil consumption and it has not changed.

    There are two theories on fixing things. 1. “If it ain’t broke don’t fix it.” 2. “If it ain’t broke fix it until it is.” In my case, I am going with the former. I hope I don’t regret writing this.

  • John Burnett

    Kind of like Dan says. I was the VP of a used airplane dealership. We saw engines that were at or over TBO that were in great shape. We traded for a SE airplane with an engine that had only 85 hours since overhaul. I flew the airplane back from Wyoming. The day after I returned, I flew it to show to a client and just after takeoff the engine exploded.

    A buddy of mine bought a Piper Pacer in Canada with an engine that was within 100 hours of it’s TBO. That was about 15 years and 800 hours ago. little engine is still running strong.

    There’s a lot of history to wise old sayings

  • Gennaro Avolio

    In the first reply Mark states he knows of a flight school having gotten 3500 hrs on their 172. Apparently Mr. Lansberg simplified the FARs in his statement that under FARs flights for hire must have engines below TBO. If some flight schools are not required to have engines below TBO, should charity medical flights have a different standard?
    You would think by now we would have data all over the place, but with no time being reported in reports, all we can do is go by our previous experience. This is not a scientifically valid method. Does anyone have any good ideas on how to obtain valid data on engine longevity?

  • Vern

    Engines are like people. Some die at an early age, and some seem to live forever. Some appear strong and die and others appear frail and seem to go on and on. If something goes wrong while flying, I cannot pull over and call a tow truck. What you choose to do with yourself and your family is between you and them, and as for me, I am not willing to take that chance. I do not fly any further from dry land without a flotation device than I can swim.

  • Scott

    Calendar rule aside, I have no plans to OH my engine at an arbitrary point (i.e. 12 years). The engine runs well, oil consumption is negligible, compressions are excellent. I have the oil and filter changed every 25-30 hours, the engine is borescoped and an oil analysis is performed at annual. No problems, no OH. I plan to fly up to TBO and then decide.

  • Jim

    I had my engine OH done because I could not know the specific wear and tear suffered without having it taken apart. It took months of research to find two shops that seemed to have impeccable reputations. They also charged more than any one else. As it turned out, my engine had no damage or major wear problems. It looks like a gamble to me. Sometimes the engine manufacturer provides bad parts, like crankshafts and cylinders. Put one on those on in an OH, and you really end up in a fix. I feel lucky, as I now have 700 hours and 10 years on the OH with no problems.

  • John

    My own engine is rather old, having been out of production since 1943. It’s recommended TBO is 700 hours. At 2700 hours I encountered some low compression readings. I tore the engine down. All nine cylinders were in the center of the range for new parts! I replaced rings and one exhaust valve guide and valve. The lower end looked so nice I didn’t believe it and I have overhauled a lot of engines. Of course, when my engine was built they didn’t pay any attention to TBO’s.

    Personally, I believe the TBO is a fallacy based on attempts to sell engines and avoid liability lawsuits. I overhaul “on condition” and check the condition regularly. I recommend oil analysis ALL the time. Without a baseline for the engine, the oil analysis data can be misleading. Any change in readings that lasts for more than one oil change is suspect and will prompt more investigation. Borescopes are wonderful things and tell you much about the condition of the working parts of the engine. A quick check of valve lift, especially the valves on a Lycoming four where two valves operate off the same cam lobe, can give you a heads up if your camshaft is starting to wear abnormally.
    There are a lot of things you can periodically check to get a good reading of current condition.

    Overhauling an engine arbitrarily will put a reliable workhorse back in the “infant mortality” stage for several years.

    The best thing you can do for engne longevity ir run it regularly for at least an hour at a time, keep good clean oil in it, and be reasonable with cruise power settings.

    On my personal aircraft I purposely limit my maximum power to the maximum continuous RPM and avoid using the five minute power option. I normally cruise at the highest manifold and lowest RPM that will give me 50 percent power. In my engine that is 1850 RPM at 22 inches of MP.

  • http://rose.net herb ludgewait

    NICE JOB GUYS. It’s apparent you responders know a lot more on this subject than i do. I was going to recommend Mike Busch’s articles on Reliability Centered Maintenance, but see you are on top of it. Hope the non-responding pilots are as astute as you. Please spread the word.

  • Bob Thorson

    While many good points are brought up in the comments I would like to suggest that the authors are bold since they are on the ground with the money in the bank. If you do single engine overwater, flights at night, IFR or any combination of the three the flight risk goes up considerably. If the authors are flying at the low rates reported their proficiency in forced landings may not rise to the occasion. If you are willing to take those risks then that is your choice. Please don’t come back with litigation after your engine fails and you have your hospital bills, aircraft damage, ground damage and maybe several suits against you. What would a prudent guy do? Put away money for an overhaul when recommended by the manufacturer and maintain your engine as suggested. The folks that built it know best….why argue with the engineers? Or buy a ballastic chute!!! LOL

  • http://www.ipilot.com Bill

    I recently posed a question regarding taking an engine beyond recommended TBO on the community message board on ipilot.com and it created a livey and thought povoking discussion much like is occuring here. You can find the thread here http://ipilot.com/yaf/Default.aspx?g=posts&t=21202

    It also references some of the excellent articles written by Mike Busch.

  • Jeffrey J. Sacks

    What does the AOPA Insurance Carrier for one say about coverage when private aircraft engine goes over TBO ? Are there technical requirements that need to be supplied to insurance carrier to continue beyond TBO or is going beyond TBO a no-go with carrier ?


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  • Lebor Thorvaaldski

    I don’t know how you can have an intelligent discussion of this subject without a thorough analysis of the data manufacturers use to set TBO’s. Seems to be some big secret, doesn’t it? Before we start mandating that charitable groups force compliance with TBO’s, maybe we should force manufacturers to prove that TBO’s mean something. Better yet, the TBO limit should be set by an organization independent of manufacturers, as the latter have an obvious conflct of interest.

    We need actual data on the risk introduced by overhauling an engine that is currently running fine. Finally, we need better diagnostics. For example, compression testing is widely known to be a poor guage of health, yet many people rely on these numbers.

  • SM

    Aviation has become a game of CYA and blame someone else. Unfortunately we, the pilots, pay the price in this game. TBO’s, Service Bulletins, AD’s and regulations are ways of shifting the blame to someone else. When we climb into an airplane we are PIC and are “responsible for and the final authority” for that flight. We take the risk. Wether you fly a low time engine or one over TBO, if you are not prepared for an emergancy, either one can bite you. When it happens you the pilot are alone. The lawyers, insurance companies, FAA and the last A&P(IA) to do your annual are not there. They just figure out who is paying and who is making money afterwards.

  • Dean A. Fawley

    I find it interesting that in your email article, August 7, 2009, you have an entry about YET another AD issued for cylinders on Continental Engines. It really is dissheartening that an owner should tear down a perfect running engine in the interest of safety and then to have another costly AD issued for that newly overhauled engine.
    The Coast Guard Auxilliary is also mandating the TBO rule. My IA says there is no reason to crack open my engine. We have had several instances of “bad” overhauls in our unit. My IA suggested to me that I just ignore the TBO and use the overhaul money to fly more, outside of the Coast Guard Auxilliary. The Coast Guard does not pay enough for me to manage an expensive AD.

  • Rodger Tracy

    In 1996 I bought a C-205 with a friend. After a couple of years we were forced to do an overhaul due to jet-fuel contamination of our fuel – unknown to us and many others. We had a high end overhaul done at a very very reputable shop – and paid more for it. Within the first 100 hours we had a major oil leak and the engine had to be pulled and repaired because of a missing seal done during the overhaul. Fortunately, the problem was not catastrophic – just messy. As a result I do not trust even the best of OH’s, FREMAN’s, or even new engines.

    In 2002 I bought a 1970 C-206 with a fairly high time FREMAN Gold Seal engine that had been very well cared for. I am now over TBO and log every quart of oil I add. I have an Air Wolf filter that gets changed with the oil every 35 hours. I do an oil analysis and cut the filter open every 35 hours. At annual we do a borescope and compression. I have averaged 6 hours/quart over the seven years I have owned the aircraft, and the oil useage has gradually gotten better over time – it started at 4 hours/quart and my last oil change is up to 7 hours/quart. I am a CPA member and have studied Mike Busch’s recommendations carefully. I fly the engine very conservatively and carefully. I have not had a single engine problem of any kind in the seven years I have owned the aircraft. I will continue to fly the aircraft and do what I have been doing. I feel MUCH safer with this engine which is now 29 years since FREMAN and just beyond TBO of 1,700 hours on the IO-520F than I would with either a factory new or FREMAN. Mike Bush’s studies indicate your greatest danger of catasrophic failure is during the first 500 hours. I have been sharing the aircraft with a missionary aviation pilot friend who is also my mechanic who has flown the aircraft all across the US with no problems. We have changed out serveral accessories over these seven years at the first indication of any problem. The aircraft has a Sportsman STOL kit, BAS shoulder harnesses, and many other saftey improvements in case I have to land it off field. My airline pilot – flight instructor son puts me through the ringer every two years on my four hour bi-annual.

  • Don Lovett

    I replaced the engine in my 1966 Cherokee 180 with a new one in 1979. About 1650 hours on it now. I never use the primer to start it. Change the oil frequently (hourwise, at least). Do not abuse the engine. Slow descents. Shallow climbs. Installed a 4 cylinder EGT over 20 years ago. The compression at the most recent annual was not lower than 74 pounds in any cylinder. At its best, it is so smooth the Turn and Bank needle looks to be glued in place. Get 8 hours per quart. As long as oil and filter analysis do not indicate to the contrary, I will fly her just like I always have. I too believe in not fi\xiing that which isn’t broken.

  • Thomas Olsen

    The FAA allows air carriers who have the appropriate programs to extend overhauls beyond the required TBO, but they must have appropriate monitoring programs, and they generally build up to the increased TBO’s in increments.

    I don’t know how that translates to General Aviation Operations, but even during the days of the large, round piston engines, there were some carriers who were allowed to increase the overhaul interval beyond the manufacturer’s recommended TBO because they had a program to watch the engines to keep them on the wing as long as possible.

  • Bob Davidson

    The engine in my 81 172P was torn down and inspectedat 1500 hrs that engine is now at 2000 hr TBO. I have been told not to due the overhaul but do 50 hr oil tests and keep flying until metal show up then do the OV. What’s your take?

  • winterhawk

    TBO is a concept that the airlines and military moved away from years ago and went to ‘condition’ maintenance, so why does GA cling to such an outmoded concept? It appears that TBO, like some ADs, are based on secret data and likely a lawyer’s opinion. Add that one can replace all but the engine data tag with new parts and it is still considered a repair unless the work is done by the manufacturer and the term “overhaul’ takes on a certain level of silliness in search of a counterfeit data tag, so lets move away from both the term and the concept and learn to speak of ‘IRAN’, which is Inspect and Repair As Necessary. This is condition driven maintenance and it avoids euthanizing good engines and repairs bad ones while avoiding both needless infant mortality of new parts and maintenance induced failures. But it requires data collection and analysis.

    I suggest learning to collect your own data using such tools as a data-recording engine monitor, oil filter inspection and oil analysis. When a component fails, ask why? Inspect those things that can be inspected and try to find pass/fail criteria. Get educated and dabble in the black art of analyzing the data: Advanced Pilot Seminars from GAMI and Savvy Owner Seminars from Mike Busch are on line, inexpensive, data driven and teach what you need to know or where to look for answers. Join a type club and look for data-driven experiences. Aviation Consumer also has a wealth of information on their web site. When all else fails, google it. You’ll be amazed at the discussions going on out there. Most blogs are opinions, but a few are data driven and very good.

    Think about the things that can kill you and proceed accordingly: The lack of spark, fuel, air and oil are going to ruin your day, so magneto, fuel, air and oil system conditions are critical to long-term happiness. Unfortunately, many owners lump them into TBO when they should be inspected and maintained on condition.

    Constant speed propellers are a special case in my opinion. Their TBO is based on worse case, but If the aircraft is hangared, oil changed often and prop inspected and dynamically balanced, an overhaul is a waste of money. Instead, ask for an ‘inspect and reseal,’ which gives experts the option of fixing what’s broke and reusing what’s serviceable. Asking for an overhaul means they have to follow the overhaul manual, which mandates replacing parts and grinding blades regardless of condition. It’s important to know that there are only so many grinds in a blade before it is scrap. If it isn’t necessary why do it?

    There are some things that have not improved in 40 years and we need to look for alternatives. Dry vacuum pumps fail randomly and insidiously. Why do we keep using them? Lycoming exhaust valves stick so often they have an SB called the valve wobble test, yet Continentals rarely suffer from stuck valves. Why can’t Lycoming fix that? I might add that a stuck valve might have resulted in a broken follower or damaged the cam, but there’s no SB for that. Why? Likewise, magneto ignition in an era of breakerless ignition is a crime, but the options are few. Why is that? Engine and airframe manufacturers set high cylinder temp limits that are murder on metals and oil. Aluminum alloys like cylinder heads can only withstand so many heat cycles, and higher temps translate directly into shorter cylinder life. Why not design cooling systems so the cylinders run cooler so they last longer?
    Finally, there is lean of peak operation: LOP is easy using GAMI injectors on a properly instrumented engine, but carburetted and engines can do it too, which results in cleaner burning, cooler running engines. GAMI offers irrefutable data on it’s benefits, and Continental has slowly warmed to the idea. Unfortunately Lycoming continues to resist the concept. Go with the data!