hours and years between overhauls

[310]

TCM service letter SIK98-9a lists time between overhauls in hours of operation and also at 12 yrs.

Question: does this apply(mandatory) to part 91 or is it just to part 135/121etc?

Does the statesment on the service letter"technical portions FAA apprived" have a bearing on the answer?

Can someone quote a FAR or AC(non-regulatory?)that sheds light on this?

thanks

 

[310]

oops that was SIL 98-9a

 

[bocefus]

Not mandatory for parts 91, 135, or 121. The Manufacturer recommended overhaul times typically become "mandatory" when listed in the Operations Specifications for that particular air carrier.

 

[IFollowRoads]

This is one of those "Not Mandatory, But Will Be Jammed Up Your Ass By The Attorneys In Civil Court" kind of things.

Pay me now or pay me later. Airplanes aren't cheap. They turn piles of money into smoke and noise. If you can't feed 'em, you don't need 'em.

Do your overhauls by manufacturers recommendations. Comply with service letters. Live life happy. Happy as a hippo.

 

[avbug]

TBO becomes a binding issue for air carrier operations, such as 121 or 135. Part 91 operations are on-condition only, save for 91 subpart K (fractional).

An operator may elect to apply for and receive authorization for any number of different maintenance programs, which can include extended intervals beyond a published TBO for a given component, appliance, powerplant, etc. Generally when such approvals are given, the operator is restricted to the manufacturer-published TBO, and extentions are granted on 100 hour increments...but only to that operator when operating under that specific maintenance program.

Various programs exist for bypassing TBO to one degee or another, such as the MORE program on the PT6A...but the frequency of inspections required, and the associated costs, usually mean it's more cost effective to stick with the published intervals and forget messing around with loopholes. My experience with those programs is that they're just not worth it.

As far as service letters and manufacturer documents giving various overhaul intervals...some are mandatory, some aren't. If it's part of documentation attached to the type certification of the aircraft, such as the type certificte data sheet itself, the manufacturer maintenance manual, or the aircraft flight manual...an approved limitation is binding.

TBO is a recommendation...it doesn't mean the engine will last that long, and it doesn't mean the engine won't go longer. It's a recommendation, and the validity of that recommendation really only applies to the first run on that engine...not subsequent runs on "overhauled" engines...particularly when the manufacturer can't control the nature of the "overhaul" or what goes into the engine.

Many manufacturers have attempted to force customers to undertake various maintenance practices or inspections by making "mandatory service bulletins," or "mandatory service letters." This is a more complex subject that you might think, for a number of reasons, but the bottom line is that the FAA's position for a long time (until some recent posturing that helped cloud the issue a little) has been that if the manufacturer wants to make it mandatory, then the manufacturer needs to petition the FAA to make it an airworthiness directive. Until it becomes an AD, it's not mandatory, even though the manufacturer may call it "mandatory."

Overrunning TBO by callendar or by clock is a good way to run afoul of insurance policies or attorneys, however.

If you're into the second or third run of your engine, in other words if it's been overhauled before, you're into a place where the landscape isn't nearly so cut and dried. An overhaul only means the engine has been inspected and found to be in tolerance...it might be on the ragged edges, but an engine may be overhauled with nothing adjusted or changed...it's overhauled on paper and keeps soldiering on with the same components as before...are you going to get another full TBO run out of it then? Probably not. But consider the engine which has experienced some component changes...the rest are still contiuing as they always did, products of all the original stresses and wear...an overhaul doesn't mean much in that context. That an engine has been overhauled doesn't mean it's as good as new. It means it meets minimum tolerances, period.

Generally speaking, if a manufacturer tells you to do someting by a particular callendar day, or clock time, then you really ought to do it. There's generally a good reason. You may not be aware of the reason, but bear in mind that by the time something becomes regulatory with an AD, it's usually so because it's been written in blood...or might just as well have been. The regulations are seldom crafted or changed until someone dies....compliance with a SB is a lot cheaper and smarter than waiting until enough failures occur that it becomes an AD, before doing the job.

Keep in mind too that timely compliance with published TBO's may mean that you can overhaul for less money. Running the engine until it can't continue any longer (on-condition) may be perfectly legal, and in many cases may be proper too...but work out components may not be repairable or overhaulable, resulting in a more costly repair. Getting it done before it breaks is usually a lot less expensive.