Here is a cross post from another forum. This is what Eric Barker of Western Skyways had to say about TBO in part 91 aircraft:
"As a large engine shop, we get asked this question multiple times every day. There are some good responses here, but thought I could give some of my opinions on the subject.
So, as an engine shop, of course I should say to overhaul the engine at tbo, no further, ever.
In reality, the answer, as some have posted, is "it depends".
I am not a believer in the "fly it 'till it breaks" propaganda, because that simply increases the cost borne by the current owner who now needs an engine, and in a hurry.
Aside from monetary concerns, it could yield a safety of flight issue and no one wants to be on the other side of the desk trying to explain why you landed in a school yard with a 4,000 hour engine thats 20+ years old when the manufacturer clearly calls out for a 1700 hr tbo or 12 years (example only). The argument that "It's part 91, so TBO is only a recomendation", though accurate, will only carry so much weight in such a scenario.
TBO, in the GA community, is often considered to be a neferious scheme to sell parts by the factories. Not entirely untrue perhaps, but if one was to try and decipher the logic behind TBO's you can certainly find some legitamate reasons for these "Recomendations". So, I submit the following to consider;
1) Power envelope for the basic engine design. Engines operating at lower side of the envelope tend to have a longer TBO, where engines operating at the higher side tend to be shorter, to a point.
2) Calendar age - Main concerns are corrosion AND AD's/MSB's that are applicable only at O/H/splitting engine.
4) Wear trends, specifically the life expectancy for the wear surfaces (IE: bearings, bushings, etc)
5) Oil supply and design - Filter vs Screen for just one example. Think O470U which originally had a 1500 hr TBO, later bumped to 2000 with the introduction an upgraded cylinder and oil system.
6) Cylinder design - Think of the old 421 engine, originally at 1400tbo, later bumped to 1600 with the introduction of a new style cylinder.
7)Magnetism - The longer the engine runs, the more magnetism can be stored, to a point. During the course of an engine overhaul "Demagging" the steel components is normal procedure. Remember that the oil has 3 basic functions- Clean, cool and lubricate. The oil "cleans" the engine by suspending particulate matter in the oil stream until it reaches the filter. Assuming that the particulates are ferrous, and you have a magnatized components, the particles may not make it to the filter thereby causing wear and/or blocking oil passages. Aircraft struck by lightening are required to have certain inspections performed, one being a dissasembly of the engine. The engine is instantly magnatized in this event, along with fried electrical components (alternator, mags, etc). If magnatism wasnt a problem, then we could just change mags, starter and alternator in these events, but it's just not that easy.
8) Aircraft use - IE: Aerial sprayers, though perhaps using a similar engine, have a reduced TBO due to the environment and operating conditions of the engine. The same note applies to Jumpers, and banner towing.
9) Certain TBO's are automatically increased if the engine is operated at 40 hrs per month or more continuously.
Remember that engine prices quoted have a core policy attached to them. Some require the core be less than 20 years old,"undamaged" and as removed (TCM and Lycoming factory) , some require the core to be serviceable for the quote to be good. Others, like us, require the major components be "repairable". The point is, if you "fly it 'till it breaks" your cost is increased, and now you are trying to justify to yourself if those extra 500 hours (example) of use was worth the $16,000 (example) core value. Perhaps the cost per hour makes sense, perhaps it doesnt.
A reasonable rule of thumb, in my opinion, so as not to compromise safety or increase cost is;
If all the tools at our disposal are showing good signs (IE: oil analysis, filter inspections, spin on filter instead of screen, compression tests, borescopes, good routine maintenance, no oil leaks, no vibrations, etc) and the engine is operated regularly for around 100 or more per year THEN you may consider a 10-15% over TBO as a guide. Beyond that I more frequently see "Diminishing Returns" when comparing additional time yielded with the additional costs incurred due to core damage. Realize also that there are relatively few "tools at our disposal" to give indications of bottom end wear.
As they say, Your mileage may vary."
Sorry it's long, I felt it would be beneficial to the thread.
