Considering a TBM700 or Meridian. I am part 91, personal owner / pilot. Any thoughts on flying past TBO and doing on condition maintenance/ HSI, which for me would be 10 yrs out (180hrs/yr)?

We made that decision 200 hours ago on our Caravan. We have owned the airplane since new and it was the first overhaul interval. We decided to do a hot section at a non-Pratt shop. I spoke to several mechanics and shops. Nobody could come up with a reason to overhaul a good running engine. The engine ran great before the hot and it ran great after.

Time will tell whether it was the right financial decision. Certainly cheaper in the short run.

Our Caravan has the -114a, which is a small PT6. There may be different considerations for the TBM or Meridian. I think the TBM may have an additional power turbine. I don’t know if that matters.

If it's my checkbook I'd do the HOTs and fly on. The MORE program on the -21's takes the engine to 8000 hrs with nothing more than paperwork, a few extra inspections and normal HOTs. I think I'd consider that as at least a safe baseline timeframe for a PT6, and even then you're still likely orders of magnitude further from failure than even a brand new piston engine.

On one plane we put the -42s on the MORE program (pt 135) and then decided to overhaul them after we found some FOD damage at an inspection.

We ran them to about 5-5500 hours and had to pay a penalty for going past TBO. Did it work out? The math at the time indicated it was a pay me now or pay me later scenario. For the working plane pt 135 it seems better to just OH them and keep the props turning (not be AOG looking for random parts at HSI).

Pt 91 with low usage (200ish hours per year or less) it might make sense to just run them past. I think on this plane we did this, we put about 500 hrs a year on it. So time flies…

Also since it is a working plane, we put a rental engine on one side and swapped out our engines on the other side. That way we only had to install the rental engine once. The SNs were swapped anyway, so when the second engine comes back, the lower serial will be on the port side.

Very few operators of TBM or Meridian go past TBO. Smart or not is irrelevant it’s just not common. That means when you sell the airplane the buyer is going to deduct the entire overhaul cost. He may deduct even more figuring the overhaul will cost more than average. In the long run with resale considered you won’t save a thing. May as well get it done for yourself.

This is different in the jet world where past TBO is common with JT15 engines. The jet buyer will still consider the value but with less push back if the next buyer is also part 91. On an older jet overhaul of both may be more cost than the value of the airplane AFTER the overhaul. That’s different in TBM and Meridian where the PT6 overhaul is far less than the post overhaul aircraft value.

I guess one strategy would be to run the PT6 beyond overhaul for a thousand or two hours then overhaul in advance of resale. That way you get to sell an airplane with a fresh engine instead if mid life. Will you keep it that long?

Don I know you had a Meridian and post on PMOPA so I’m curious as to personal experiences of who has overhauled?

Most meridians I’ve looked at were all well below TBO. I reviewed logs for a Meridian that was past TBO and it sold before I was able call. Mine is 180 from TBO and I’ll hot it and move on. Everyone I’ve talked to personally is recommending to hot and keep running.

I’ve asked PMOPA to do a survey of the times on the fleet. I’m going to keep pushing.

Pt6A-42s are on king air 200s and are run past TBO regularly so I don’t think it’ll be an issue, but I’m open to specific individuals experiences. I don’t see it makes a lot of sense as the airframe will time out before any life limited parts on the engine.

If you overhaul before the sale, then you will be down that cash as well. Plus all the hours after the overhaul you used before sale.

With a "run out" engine, your hull value is lower which saves money on taxes and insurance.


Possible.

He may also do a ground run check and find the engine is in great shape and not ding you so badly.


I don't think that is true, generally.

Overhaul touches a lot of parts that can run a very long time without much service. Things like gears and bearings.

The hot stuff, turbine wheels and combustor, are looked at during HSI, so those parts are not going to get any less attention.


You are never going to get dollar for dollar on that investment, IMO. Your plane value did not jump the full total of the OH cost.

Also, you raised the buyer's sales taxes doing that. The overhaul can usually be had without sales tax (many states are like that, mine included). So the buyer is better off buying the lower hull value airplane with the run out, paying less sales tax on that, then doing a sales tax free overhaul.

Also, the buyer can fly the airplane some before overhaul to the extent time before HSI and the condition of the engine allow. These are low cost hours.

If it is all the same to the seller, I want to buy the airplane with the run outs and priced appropriately.

The one caveat here are part cycle limits. If expensive parts will cycle out soon or at the next HSI or OH point, that needs to be taken into account. My engines have ~10,000 hours on them, but I'm in a sweet spot where the impeller was replaced at the last OH and the rest of the parts are good for 3 more HSI (1.5 OH) cycles. Someone buying an 8000 hour plane might be facing a $250K impeller cost that I am not. In other words, lower time would be more expensive.

Mike C.

Just curious, what sort of attention might the reduction gear box get? Or is it just run until you get a chip light. Anything more than an oil change?

I'm glad you asked the question!

The fact is that these big Pratt's don't do well past TBO, yes small Pratt's like the -21, -135A, -41/42 are often ran past TBO, the MORE STC allows 8000 hours Part 135 and many engines have made that, but from what I understand they were beyond economic feasibility at that point, so as said above this running past TBO plan is often a pay me now or pay me later scenario.

Here's the part everyone misses, the same metallurgy, quality of materials and components compose the -64/66 as the small Pratts, even though the big engines are producing more power, heat, stress, etc.

BTW, there is no MORE STC for the larger Pratts! That should tell the story right there.

I would never recommend running a big Pratt past TBO, you can do it, probably get 5000 hours on it before it really starts hurting, but you're just increasing the cost of your next overhaul and devaluing your asset.

It's important to understand that these are complex devices, overhauling them means returning the entire engine back to new standards, if you do that it will go another 3500 hours with a HSI and likely nothing else, if you don't, it won't.

We did do a vibration analysis at our hot section, which is supposed (based on my understanding) to give you some insight on gear box health.

Thanks for all the input. I’ve owned about a dozen planes including a TP, so I understand the economic issues on resale.

I am mostly interested in the safety/feasibility/likely ongoing engine Mx costs when flying a Meridian or TBM, with a fresh Hot instead of an overhaul.

From reading many posts on the PMOPA forum, the Meridian fleet is just starting to deal with this question, so broadening the discussion to BT. Haven’t joined the TBM group.

Thanks!

Also depends on your relationship with your ohc shop and location

I have a good relationship with standard aero as Iam a vendor of some items and I was a
Employee in the 80”s

I sold Allison and Pratt ohc and parts .

The tso hoses are spec for Pratt , silver plated nuts and double lock wire holes .

I sell tso hose assys and mfg many custom designed fittings.


GB

For some of those bigger block engines, they didn't need an STC at all, just new ICA instructions:

https://www.ainonline.com/aviation-news ... c-pt6a-60s

"Minden, Nev.-based Maintenance On Reliable Engines (More Co.) received approval from the FAA for its instructions for continued airworthiness to extend the time between overhaul (TBO) for seven Pratt & Whitney Canada PT6A engine models."

"The new engines covered by the instructions are PT6A-60A, -60AG, -61, -65AG, -65AR, -65B and -65R."

It wouldn't surprise me if the engine list expands over time. I think your statement is misleading that there is some sort of issue with making the big block Pratts go past TBO when there's no evidence that is the case.

Indeed, Pratt has issued TBO extension guidance for the -67 engines in Beech 1900s, so even Pratt thinks the big block engines can go past TBO.


The logic here doesn't pencil out.

If you OH at 3500 hours and it is low cost, then most of your parts were reused. Those parts then fly another 3500 hours.

If you fly past TBO, you are flying the same parts. You get to fly them for less than doing a full disassembly and rebuild of the engine just to put the same parts back in.

If you go 1.5 TBO periods (you flew one HSI interval past TBO) and those parts are now worn out, then they would have been worn out had you overhauled at TBO, too. How does going through an overhaul prevent future part wear out? Indeed, in some cases, the overhaul CAUSES wear out. This is true, for example, with props where the blades get dressed down and end up under limits sooner if you overhaul them more.

Your airplane is worth less with a past TBO engine, no doubt, but do you get ALL the money back if you do an OH? I doubt it. So economically, you are still ahead. And a lower hull value translates into lower cost of capital, taxes, and insurance.

This all fits with your general advice of paying more money for things. Sometimes value and cost are not so related as one would hope.


No, that is incorrect. There are new limits and there are overhaul limits, and they are different. An overhaul doesn't bring everything back to "new".

Mike C.

If you elect not to OH I would still overhaul individual components such as the fuel controller and bleed valves. Fuel controllers can and do fail. I had one die on the ground that was around 20 years/3200 hours SOH. I asked the OH shop to tell me what they found when they looked at my core and they said it basically had all sorts of internal pneumatic leaks and corrosion. Remember FCU's use bleed air which can contain moisture and contaminants. If you do the FCU make sure to replace the little plastic splined shaft that connects the FCU to the fuel pump. it's less than $100 but if it breaks you'll have runaway torque. The prop governors don't seem to fail much but they also aren't that expensive to OH so you might want to do those as well just as a precaution. Bleed valves can leak and are relatively cheap to overhaul or at least test and replace the diaphragm. I believe the MORE program requires regular refresh of these components as part of their STC but I could be wrong about that.

As pointed out earlier, the engines on those two planes aren't just two different model numbers, but they come from two different families within the P&W line. This will affect overhaul and HSI costs.

Putting the engine on MORE can save money, even if you're part 91. You can extend the HSI interval if your engines are healthy. MORE doesn't let you avoid HSIs completely. It just allows you to do an inspection to see if an HSI is needed, instead of automatically assuming that one is required at 1800 hours. I used the MORE program to get about 3000 hours between HSIs.

You should assume that the cost of the eventual overhaul will go up with each hour of operation. How much it goes up requires knowledge of the operator's engine management practices as well as a crystal ball.