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In browsing Trade-a-Plane for my dream (light GA) ride, I often note the TBO (Time Before Overhaul) for an aircraft in relation to its asking price. Obviously, the more time left before an overhaul is due, the greater the value of the airplane compared to one that is near or beyond its overhaul due date (hours).

I've always assumed this was an airworthiness item, meaning the aircraft was illegal to fly if it was overdue for a major overhaul. But now I'm not so sure. I've seen aircraft advertised as having a relatively recent top overhaul while being overdue for a major overhaul and I've seen a couple advertised that were overdue with no recent work done. Is a GA aircraft legal to fly if it's overdue for maintenance, and what are the exact rules for this?

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    $\begingroup$ Aircraft are not illegal to fly after TBO, usually if the compressions are good and oil screens come back clear, flying beyond TBO is perfectly fine. Some people prefer it since a well cared-for engine near TBO is less likely to fail than one that has just been taken apart. TBO is purely manufacturer recommendations, and going beyond is a pretty controversial topic. Unless you are talking about commercial/passenger operations then there may be regulations to abide by. $\endgroup$ – Ron Beyer Apr 26 '16 at 20:41
  • $\begingroup$ In general, for part 91, TBO is no more than a recommendation. I would rather fly an an engine at TBO that has been operated continuously and has been well maintained than fly a 100 hr engine that has been operated very little in the last few years. A good engine can easily go to 3-4000 hrs—or even 6000+ hrs—if flown properly and well maintained. I am in the middle of overhauling a small 6 cylinder at 150 hrs that was operated very little in the last 10 years. It just depends. Another consideration is to merely replace cylinders, which is a cheap, but sometimes effective "overhaul". $\endgroup$ – J Walters Apr 27 '16 at 10:38
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Unless there is a TBO Airworthyness Directive for that particular engine/airframe/serial number, it is not illegal for GA operations to fly beyond TBO. I can't find an example of a TBO AD but I do remember seeing some.

TBO is a manufacturer recommendation for that particular engine, and does not have any legal enforcement with the FAA. An engine that is well cared-for and has good compressions and clear oil screens (along with other positive inspections) is perfectly fine to fly beyond TBO. There are even stories of people flying 1000 hours or more beyond TBO.

Here are some good reads from around the web:

The Savvy Aviator - Debunking TBO
AOPA Engine Overhauls
Aviation Law - Flying Past TBO, Negligence or Smart Economics

In any case, flying beyond TBO is controversial. Some say that an engine near or over TBO is less likely to fail than one that has been recently taken apart, or at least less likely to fail suddenly.

The key aspect though when buying used aircraft is you don't know how the engine was treated in its run up to TBO. Was it shock cooled? Used for training? Operated out of a grass strip? Poor maintenance? Flown rarely? There are way too many conditions to consider when buying an aircraft near TBO to make the judgement of going beyond it safely or not. The only person who can answer that question is an A&P. That is why TBO is tied so closely to the value of an aircraft. Engine overhauls are expensive, running $25k or more.

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  • $\begingroup$ Your last paragraph talks about knowing how the aircraft was flown and maintained. Wouldn't the log book be a pretty good indicator of that? As I understand it, it would show flight frequency, maintenance events, destinations, etc. (Hope this doesn't wander into the realm of "whole new question".) $\endgroup$ – FreeMan Apr 27 '16 at 15:49
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    $\begingroup$ Destinations no, only hours are recorded in airframe, engine, and prop logs. It's not going to tell you if it was training or cross country, easy hours or hard primary training ones. Logs say the maintenance was done, but not the quality of it. Was oil topped off or ran low between changes? Etc... $\endgroup$ – Ron Beyer Apr 27 '16 at 16:28
  • $\begingroup$ Great answer and I learned a lot from the links too. Thanks! $\endgroup$ – PJNoes Apr 27 '16 at 16:33
  • $\begingroup$ @FreeMan An aircraft logbook is only as good as the people who make entries in it. For me, looking at logbooks is good for things like checking for AD and SB compliance, and looking for major repairs, or indicators of problems. When buying an aircraft, nothing beats a thorough inspection done by someone you trust who is familiar with the aircraft type. $\endgroup$ – Juan Jimenez Sep 5 '18 at 8:11

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