TBO and rental ops

[Jpilot23]

Was wondering if an aircraft is over its TBO, can you still rent the aircraft out and flight instruct in it. Thanks for the help.

p.s. Sorry if this has been a topic thats been brought up time and time again.

 

[NoPax]

Yes, if its used under part 91 it can be but its not recommended by the manufacturer.

I knew of a Cessna 152 that had over 3200 hours on the engine, that was still running strong, and was used to instruct in.

If its a strong engine with good compressions, it shouldn't be a problem. Have an oil analysis done every 100 hours, and make sure the oil pump/hoses have been overhauled/replaced etc. I'd advise talking to a knowledgeable mechanic about the particular engine you plan to do this with.

Some Part 135 operators that I know about, have actually had the manufacturer approve a particular operation of an engine, so as to increase the 'standard' TBO - Ameriflight's Navajo/Chieftains for example.

 

[groundpointsix]

Check your insurance policy also. There may be a clause regarding opperating past TBO or outside any other mx process.

 

[avbug]

Civil liability should be a concern when operating over TBO. TBO becomes somewhat nebulous when you're past the first run of an engine (ie, new). Has an engine that's been "topped" been overhauled, and is the TBO reset? No. Has an engine that's had a shade tree overhaul be expected to make the full TBO interval again? No. Often an "overhaul" means nothing more than checking parts to see if they're in tolerance and reinstalling them. In fact, in accordance with 14 CFR 43.2(a)(2), a part need only be tested to manufacturer specifications be "overhauled."

The engine may be fine for another thousand hours, or it may not.

The question you need to ask yourself isn't weather getting away with it is lega, but how much of your life, that of your student, that of those on the ground, your reputation, your certificate, and your future, present, and past, you're willing to bet on the matter.

 

[Jpilot23]

thanks for the info. This doesn't concern me directly, it was basically a debate between me and another pilot who is not a CFI. There is a club on our field that is operating a '98 172 that has 2500 hours on the engine. He said they couldn't instruct, i said they can.

Personally if i trusted the mech and if it kept making compressions and ran fine i'd teach in it. I have also done this in the past.

 

[avbug]

Ah, good compressions. The layman's false security blanket. Good compressions sell engines, sell airplanes. Good compressions also mean nothing.

If that's your yardstick for safety, then you're due for a nasty awakening one day. Hopefully it doesn't hurt you too badly.

 

[Jpilot23]

Ah, good compressions. The layman's false security blanket. Good compressions sell engines, sell airplanes. Good compressions also mean nothing.

If that's your yardstick for safety, then you're due for a nasty awakening one day. Hopefully it doesn't hurt you too badly.

Thank you for your concern. Next time i'll watch out for those pesty good compressions. I pray next 100 hr my plane fails.

 

[NoPax]

Ah, good compressions. The layman's false security blanket.

I agree (although I'm just a pilot) Hoses, and accessories have a life expectancy also...is this where you are coming from avbug? If there is more to it I would love to know.

 

[avbug]

No. Pilots and owners bow down and worship compressions as the holy grail of engine life. Mechanics laugh. Pilots also look at oil color as if it means anything, too. Mechanics laugh.

Compression goes up and down with engine temperature, the operator testing it, the test set in use, ring gap alignment, the way one holds one's mouth, and the phase of the moon.

General guidelines exist for replacing a cylinder if compression gets too low (generally less than 75% of the tested pressure), but that's it.

Test an engine over several days, or several times in one day, you'll get different compression readings. Test it with different test sets, and you'll get different readings. Have different personnel test it, and you'll get different readings. Change the metal temperature, and you'll get different readings. And so on, and so on, and so on.

As long as those compressions are good, the airplane must be good, right? No need to mike and magnaflux the crank...leave it all in there and just do compression tests until they get too low to use. No need to do spectrographic engine oil analysis...just check compressions until they get too low to use. The camshaft will last forever. It's the compressions you need to worry about. Don't worry about the oil seals or oil pump...they'll far outlast the compressions. Engines are designed so that nothing will go wrong so long as the compressions are good.

In fact, the true test of the engine's health doesn't require looking beyond the nice shiny paint job. If that's okay, so is everything else.

The accessory drive lasts forever...or at least, until compressions get low. So do the mags, internally. Valves...they never need attention. As long as the compressions are good, the valves will be okay.

I pray next 100 hr my plane fails.

It's not often we have a truly stupid quote to address here, but that was it. Definitely that was it.

Personally if i trusted the mech and if it kept making compressions and ran fine i'd teach in it.

Now that one was ignorant, but not stupid. Not entirely, anyway. Ignorance is bliss.

 

[FlyingToIST]

There is a reason why that TBO is established. Especially in an environment where the engine is getting abused I am not even sure if an engine can make it to the TBO in a very healthy manner.

The other question is you have the 2000 hrs (in the case of the 98 172 you mentioned) as a hard limit established on the engine. If you go beyon it when is the next limit you are going to use? 100 more hours, 200 more hours? until it quits?

We have a PA-38 with 100 hr left on the engine. You can bet that the engine is coming off and is going to an overhaul when the time is due. $16,000 that i will spend on it is much cheaper than people's lives and the lawsuits that we will face.

This is one of those cases of "it's legal, but is it safe?"

 

[avbug]

The other question is you have the 2000 hrs (in the case of the 98 172 you mentioned) as a hard limit established on the engine. If you go beyon it when is the next limit you are going to use? 100 more hours, 200 more hours? until it quits?

Good comments, but remember that TBO isn't a hard limit when operating strictly under Part 91. It's a recommendation. The engine is operated on-condition. Replacing at TBO or sooner isn't a bad thing, but TBO is not a limitation nor a gaurantee. And after the first run, the concept of TBO is nebulous at best.

 

[nosehair]

The question you need to ask yourself isn't weather getting away with it is lega, but how much of your life, that of your student, that of those on the ground, your reputation, your certificate, and your future, present, and past, you're willing to bet on the matter.

...aaaahh, pretty much all of it,...that's how I do every flight.
​

 

[nosehair]

It's the compressions you need to worry about. Don't worry about the oil seals or oil pump...they'll far outlast the compressions. Engines are designed so that nothing will go wrong so long as the compressions are good.

Hot Dang it!! I knew it!...thass what I been thinkin', an' now I know it!
​

 

[USMCmech]

Compresion tests by themsleves mean nothing.

I have seen data from tests of engines that tested only 30/80 yet still produced full rated Hp. OTOH, as lycoming has found out recently, a crackshaft can snap with zero warning.

Re: TBO

TBO is the life expectency of the engine, it is not a recomedtion to do anything at all when the magic numbers roll around. If your engine has reached TBO and is running strong it stands a very good chance of running for quite a bit longer. A freshly overhauled engine (regardless of who did it) is much more likely to fail durring it's first 100 hours.

The average age that an American male need bypass surgery (AKA an overhaul) is 50-55. Does than mean that you should have open heart surgery just because you reach a certian age, of course not! The same logic applys to your engine.

FYI if you live to 61 you are most likely to make it to 90.


I personally know a guy who flys pipeline patrol in a C-172 which has an engine with 7000 hours since overhaul. He flys it almost daily, takes excelent care of it, monitors it closely with oil anylsis, knows EXACTLY how long his takeoff roll should be for any given weather conditions, ect.

I also met a guy who had an engine failure in a Bonanza. He couldn't understand why the engine had failed since it had so little time since new(less than 800hrs I think). The fact that this was a 25 year old plane and had sat untouched for 9 of those didn't seem to concern him. He hadn't had the oil checked for metal when it was changed.

TBO is just a number

Read these articles from avweb.

http://www.avweb.com/news/columns/187037-1.html

http://www.avweb.com/news/columns/188758-1.html

http://www.avweb.com/news/columns/190242-1.html

 

[A Squared]

And after the first run, the concept of TBO is nebulous at best.

And as a side note, a zero-time factory overhaul is beyond nebulous, and into the realm of downright deceptive. Zero time engines are assembled from piles of major components of completely unknown history. your "zero time" engine may have case halves with 15,000 hours and a crank that has 10,000, rods that have 17,000. You don't know, and neither does the factory because the records have been discarded.

 

[NYCPilot]

A freshly overhauled engine (regardless of who did it) is much more likely to fail durring it's first 100 hours.

I think I've heard this before, but what are the general reasons or causes for an engine to fail after an overhaul within the first 100 hrs.

 

[avbug]

And as a side note, a zero-time factory overhaul is beyond nebulous, and into the realm of downright deceptive. Zero time engines are assembled from piles of major components of completely unknown history. your "zero time" engine may have case halves with 15,000 hours and a crank that has 10,000, rods that have 17,000. You don't know, and neither does the factory because the records have been discarded.

True story. Overhaul doesn't mean much...the parts have been inspected and meet tolerance, and that's about it. A factory job may have new parts, but a lot will be reused...most major parts, in fact.

Even a typical "top" overhaul reuses the second most stressed part of the engine...the connecting rods as a matter of course.

Ignorance, for most, is bliss.

Why do engines tend to fail more right off the bat? Most things fail new...they haven't been proven yet. Older equipment, excepting unusual streses or undetected design or manufacturing flaws, tends to be more tried and tested, and whatever is going to break has already broken. Very notable exceptions to that exist, but it's those first few hours out of the shop when thing begin to loosen, parts begin to work against each other that haven't done so before, and stresses can become manifest.

All a new engine says to me is "guess which part is gonna break first?"

 

[pilotmiketx]

There is a reason why that TBO is established. Especially in an environment where the engine is getting abused I am not even sure if an engine can make it to the TBO in a very healthy manner.

The engines that make it to TBO and past are the ones that are run hard and run regularly. I don't expect the engine on my 170 that flies less than 100 hrs/yr to make it anywhere near TBO (nor do I plan on owning it that long.) All of the 135 freight and flight school planes that I flew would make it past TBO. It wasn't magic, they were just flying 100 hours/mo.