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Commercial ops beyond TBO

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Active member
Mar 1, 2002
I'm pretty sure you can't do commercial operations (specifically flight instruction) in an aircraft with engine beyond manufacturers recommended TBO. Unfortunately I can't find exactly where it says that in the regs. (You guessed it, I need to prove it to someone else).

'little help, anyone?

Thanks in advance

Buy 'em the steak now, 'cuz you're wrong.

Part 91 operations, commercial or otherwise, need only observe any mandatory overhaul limitations. You are probably thinking of 14 CFR 91.403(c), which reads:

"c) No person may operate an aircraft for which a manufacturer's maintenance manual or instructions for continued airworthiness has been issued that contains an airworthiness limitations section unless the mandatory replacement times, inspection intervals, and related procedures specified in that section or alternative inspection intervals and related procedures set forth in an operations specification approved by the Administrator under part 121, 127 or 135 of this chapter or in accordance with an inspection program approved under § 91.409(e) have been complied with."

Under Part 135, yes, TBO is a limitation. However, for part 91 operations, the magic wording is "on condition." TBO is a recommendation, not a limitation. If a component, airframe, engine, etc, meets the tolerances for it's design criteria (in other words, if it works, is in good operable condition, and is within the tolerances set forth by the manufacturer), you're set.

If a life limitation is provided, it must be observed, regardless of the Part under which the aircraft is operated. Many items never make it to TBO. Many can go well in excess of TBO. TBO is a suggestion, rather than gospel.

Before we go any farther and the thread expands into a discussion of the merits of observing TBO, let me ask you: what do you believe TBO is, or represents?
avbug said:

Before we go any farther and the thread expands into a discussion of the merits of observing TBO, let me ask you: what do you believe TBO is, or represents?

Well, I never really researched it. I suppose I've had this notion that a room full of nerdy engineers all wearing pocket protectors and black rimmed circa 1960's eye glasses came up with an estimate of the average useful life of the engine they were designing (assuming proper maintenance & care), and then called it TBO.


PS looks like I'll be eating crow
>>>what do you believe TBO is, or represents?

Along those same lines, what do you think an overhaul is?

A lot of pilots are surprised to learn that an overhaul is merely dissasembling an engine, checking to see that the parts are not worn beyond certain limits, then reassembling it. It is theoreticlly possible to dissasemble a 2000 hour engine, find that all parts are within service limits, and reassemble the parts. What you have now is an engine which has 0 hours Since Major Overhaul (SMOH) and there is absolutely no difference between that engine and a 2000 hour engine which has not been dissasembled. (except for new seals and gaskets)

Many pilots are surprised to learn what a "zero time" engine really is. There's a myth that a zero time engine is almost like a brand new engine. Nothing could be further from the truth. What the factory does is dissasemble a lot of engines and throw the parts in big piles, then when they assemble a "zero time" engine, they just grab parts at random, and reassemble them. You get an engine made from 2 case halves which you have no idea of the total time in service, or what it's history is. They could have 2000 hours, or they could have 12,000 hours All you know is that it met or was repairable to service limits. You have a crankshaft which you have no idea how old it is, or if it had a propstrike. It could have 2000 hours, or it could have 20,000 hours, It might have had multiple roto-tiller episodes. You don't know. All you know is that the crank met or was repaired to service limits. People think that "zero-time" means almost like new, when in reality, the reason it is "zero-time" is that it is such a mish-mash of parts of unknown histories, it is impossible to assign a reasonable total time in service to the engine. So, they call it "zero-time" and fool everyone into thinking that it's almost as good as a new engine. It just ain't so!!!

I'm in the process of rebuilding a small 4 cylinder Lycoming engine for my personal use. It has not yet reached the first TBO. I've owned the engine for a good part of it's life, and I know previous owners, and have a pretty good idea of how the engine has been taken care of and what's happened to it. I know that the crank shaft is in excellent condition, as far as I know there's never been a prop strike, and the bearing journal dimensions and runout were all at new limits. The case is in good condition and I had it reworked at a shop with a good reputation. I know the condition and age of everything that is going back into that engine. I am not overhauling the cylinders. I replaced all 4 with overhauled cylinders 175 hours ago, they're in great shape, I had them checked, cleaned up, and yellow tagged by a cylinder shop. Because I'm not "overhauling" the cylinders at this time, it won't be an "overhaul" of the engine when I get it back together. I will say that I would much rather have my un-overhauled engine, which I know contains good parts, than a "factory zero-time" pig-in-a-poke, and I'll happily and cheerfully run my engine waaaaaay past TBO.

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