Had a KA pilot describe a situation, cannot remember if it was a prebuy or checking the health of the engine but I have to ask...

Is is possible to "split" the PT6 while still mounted on the airframe and inspect the inside of the engine?

Pilot had a traveling PT6 repair guy, one who primarily whose expertise was Ag planes, come and split the PT6 on the airframe. I remember the cost was not that astronomical, four figures a side, but it provided a better look at the insides of the engine, as I recall.

Is it feasible to split a PT6?

Does a split provide a better view or can one see all you need with a borescope?

You can absolutely pull the hot section off the airplane without removing the entire engine. Signature Technicair will do this for $4000 per side (off and back on, including rigging, etc). With the hot section off you can see everything but at that point you might as well send the parts off for inspection.

My right side HSI was just done on site, on the airframe by a traveling crew. The left required more significant work, and it was removed and crated.

Interesting point: the 400 hr inspection required by the M.O.R.E. program used to require a split at the C flange, but they recently received FAA approval to use a borescope after demonstrating that borescope inspections were not missing major issues that were later found during the splits.

Correct me if I'm wrong, but also if you split the C flange, you can be in a position that you'll "become aware" of unairworthy items that must be fixed prior to to reassembly.

Did you advise them against that and they did it anyway, or were you supportive of the purchase?

The fact such purchases are being done at the paid consultant level, that is, by buyers who hire someone like Chip to help them, indicates past TBO is becoming more common and accepted.

Mike C.

Did you advise them against that and they did it anyway, or were you supportive of the purchase?

The fact such purchases are being done at the paid consultant level, that is, by buyers who hire someone like Chip to help them, indicates past TBO is becoming more common and accepted.

Mike C.

Out of curiosity, when did the FAA put it in writing? And what exactly did they say? I had always thought it was aircraft owners reading between the lines.

Out of curiosity, when did the FAA put it in writing? And what exactly did they say? I had always thought it was aircraft owners reading between the lines.

That’s literally the point of an inspection.

If it’s not airworthy, I want to know about it. Hiding my head under the sheets, sticking my fingers in my ears and singing “LALALA I CAN’T HEAR YOU” doesn’t make my engine safer.

The reason the notice was canceled is because the language was transferred to the omnibus order 8900.1 and not presented as a separate document.

See this note in the Notice:

"6. Disposition. We will incorporate the information in this notice into FAA Order 8900.1 before this notice expires. Direct your questions or comments concerning this notice to the Aircraft Maintenance Division (AFS-300) at 202-267-1675."

Note that the Notice is about clarifying a policy, not changing it. Nothing changed when it was published, flying past TBO was legal before it was published. The Notice was to give explicit guidance to FAA inspectors on the subject.

Inspections are required, overhauls are not. That's the way it has always been for part 91 operators.

Mike C.

Ok, let’s go over the difference between an overhaul and an inspection again with an oversimplified example. Let’s say you have a widget in your engine. It starts life at 1.0” in diameter brand new. It’s acceptable to install a used one into an overhauled engine if it’s 0.95” or greater. It’s airworthy if it’s 0.8” or greater. Sure, my numbers are off, but they work for this discussion.

Also, for purposes of this discussion, let’s set aside maintenance-induced failure. It’s like ignoring friction in physics 101 - it makes understanding the problem a bit easier.

You open the engine for inspection. You find your widget is worn to 0.8001” in diameter. You can reuse the widget and close the engine back up. An inspection only provides an engine that is airworthy TODAY. It might wear down to unairworthiness on the next engine start. Any reasonable owner working with a reasonable shop will have a discussion about the expected life of parts and expected life of the engine. An owner may choose to use the worn part, or replace it, as long as it’s within service limits.

Now let’s say you open a different engine for overhaul. The shop finds the widget worn to 0.94999”. By their standards, they have to replace this perfectly airworthy part. Why? The purpose of an overhaul is not just to provide an airworthy engine, but to provide an engine with an expected lifespan close to the lifespan of a new engine. Obviously, an overhaul is going to cost much more - and it’s not just the labor. You pay more, and you get more, but you will likely replace thousands of dollars of parts that are legally airworthy and have years of life remaining.

Now, if you want to just do hot section inspections over and over without overhauls on part 91, you’re legally ok. However, there are some parts of the engine that an HSI never looks at. If there’s a moving part in your engine that you never inspect, someday it will fail on you.

So can you do a “full inspection” and not overhaul? Can you IRAN an engine without a full overhaul? Legally, yes. But I don’t know any shops that are doing that. Maybe I’m just not clued into all the options out there. But that’s the option I would take if I could find it.

In real life, every time work is done, there is a chance of a mistake. And it isn't just the mechanic who can do something wrong (like the jet airliner which lost all oil due to missing O-rings). It can be a bad batch of new parts, like the crankshaft and cylinder ADs that hit some piston models.


In your example, 75% of the useful life of the part was thrown away (presuming even wear rates).

And it isn't just wear limits, it can be cycle limits.

For example, the JT15D impeller that was original in my engines has a 9000 cycle life limit. You can't legally fly it 9001 cycles. At the third overhaul of my engines, that impeller had about 6000 cycles on it. Great there are 3000 cycles left for the next 3500 hours, that's about our average hours to cycle ratio, so put it back in.

Nope, Pratt (who did the overhauls) said the remaining cycle life on any part reused has to be over 3500, for a 1 cycle per hour average. So at that third overhaul, my impellers got replaced (with 12,000 cycle ones, no less). This was major $$$ and threw away 1/3rd of the impeller life. Had they simply HSI at that time, they would have been able to use more of the impeller life before trashing it.

I am the benefactor of that policy since I have young impellers and my next overhaul won't need to replace them. But it was costly to the prior owner. My higher time engines will actually be cheaper to maintain over the next 5000 hours versus a lower time engine facing an impeller change.


That's not really the standard. The objective of the overhaul limits is to provide an engine that will have enough life left to make it to the next overhaul without going below serviceable limits. There is no "make it last as long as new" part to an overhaul, all you need to do it achieve the overhaul limits. Some overhauls may be to new limits, but they don't have to be, and that's also not the same as actually being "new".


That's true of everything on the airplane, like, say, the wing spar. The question is if the failure can be detected before safety of flight is compromised, and just how long does it take to get to failure. For the simple turbine engine, those "cool section" parts last a very long time.


Lots of shops do that for piston engines and props. I suspect it is also done for turbine engines, too. They can get repaired without triggering a full overhaul.

Mike C.

In real life, every time work is done, there is a chance of a mistake. And it isn't just the mechanic who can do something wrong (like the jet airliner which lost all oil due to missing O-rings). It can be a bad batch of new parts, like the crankshaft and cylinder ADs that hit some piston models.


In your example, 75% of the useful life of the part was thrown away (presuming even wear rates).

And it isn't just wear limits, it can be cycle limits.

For example, the JT15D impeller that was original in my engines has a 9000 cycle life limit. You can't legally fly it 9001 cycles. At the third overhaul of my engines, that impeller had about 6000 cycles on it. Great there are 3000 cycles left for the next 3500 hours, that's about our average hours to cycle ratio, so put it back in.

Nope, Pratt (who did the overhauls) said the remaining cycle life on any part reused has to be over 3500, for a 1 cycle per hour average. So at that third overhaul, my impellers got replaced (with 12,000 cycle ones, no less). This was major $$$ and threw away 1/3rd of the impeller life. Had they simply HSI at that time, they would have been able to use more of the impeller life before trashing it.

I am the benefactor of that policy since I have young impellers and my next overhaul won't need to replace them. But it was costly to the prior owner. My higher time engines will actually be cheaper to maintain over the next 5000 hours versus a lower time engine facing an impeller change.


That's not really the standard. The objective of the overhaul limits is to provide an engine that will have enough life left to make it to the next overhaul without going below serviceable limits. There is no "make it last as long as new" part to an overhaul, all you need to do it achieve the overhaul limits. Some overhauls may be to new limits, but they don't have to be, and that's also not the same as actually being "new".


That's true of everything on the airplane, like, say, the wing spar. The question is if the failure can be detected before safety of flight is compromised, and just how long does it take to get to failure. For the simple turbine engine, those "cool section" parts last a very long time.


Lots of shops do that for piston engines and props. I suspect it is also done for turbine engines, too. They can get repaired without triggering a full overhaul.

Mike C.