I’d like to hear from the mechanics here if any of them would be willing to sign off/work on older turbines where the owner is not doing overhauls. My guess is they are few and far between.

I'm not a mechanic so I'd be interested as to hear from one as well.
A hot section inspection will identify any issues with stators, turbine wheels and bearings as well as the plenum, burner can ext. The gear box inspection addresses the bearings and seals in there. That's why a HSI can cost 6 figures easy. At every 100 hour the starter/generator is removed and inspected and brushes replaced as needed and the SOAP sample tells the story of the engine. The fuel nozzles are addresses every 200-400 hours.
So for an overhaul you are talking about replacing or rebuilding a torque sensor, fuel controller, oil pump and fuel pump all of which have been known to fail albeit rarely and addressing all AD's some of which are 20-30 years old depending on the age of the engine.

What is the compelling argument for tearing down a perfectly running and well inspected turbine engine at some arbitrary hour number and spending $250,000? The TPE331's in the AG market run from HSI to HSI to over 8000 hours.

I think it is widely accepted and perfectly within the regs. The MORE program provides a way to do it in compliance with overhaul recommendations, which allows commercial use. I know of many PT-6 airplanes at several shops with engines well past P&W's tbo.

Just look in your logbook. There are plenty of references where the AP/IA did some work and only took responsibility for the work he did.

Agree with most of the above. We have a '76. We wrote the value off over five years and have had it seven now. We do our own M.O.R.E like inspection program. Can't see other than a part 91 owner being interested in purchasing this, but for that owner, it can be run on a much more cost effective basis than overhauling engines and meeting other recommended overhaul requirements. A part 91 operator can have a very nice older bird.
For new, higher value birds, I certainly agree that following the recommended programs will increase value. Overhauls would be over $200,000 a side or more on our bird. That's equal to or in excess of the plane's current value.

One other point, I think the reliability of well inspected turbine engine OVER TBO is exponentially higher than that of a low time piston engine. The damn thing is spinning instead of trying to tear itself apart.

This is an interesting opinion and lightly has merit.

What are the high time failure modes and performance losses to a turbine versus a piston engine?

This is an interesting opinion and lightly has merit.

What are the high time failure modes and performance losses to a turbine versus a piston engine?

High times turbines simply lose power as the hot section wears out. Since that part gets a hot section inspection, that is taken care of at regular intervals.

Unlike a piston, there's no sliding wear like valve guides, piston rings, crank bearings, etc. This means there really isn't a significantly higher chance of catastrophic failure in a turbine that is high time. The turbine engine only needs to do two things to run, turn and inject fuel. It really is amazingly simple.

I'll give an example of how much extra life the engines have. My engines originally from the factory was 1500 HSI and 3000 OH. Later, this was modified to 1800 HSI, 3600 OH. Then it was 1800 HSI, 3600 HSI plus gearbox inspection, then OH at 5400 hours. Then, with a -10 hot section, it went to 2500 HSI and 5000 OH. Now, with certain SBs done, the engine goes to 3500 HSI and 7000 OH. Some plans go to 9000 hours before OH now.

What this means is that the gearbox that was originally to be overhauled at 3000 hours is now going 9000 hours. IT IS THE SAME GEARBOX. So clearly there is a lot of reserve life in it.

I'd have not one iota of worry about a turbine engine over TBO. Indeed, the less you work on them, the better. I'd worry FAR more about an engine at 1 hour SMOH than one just over TBO.

Mike C.

So why do they cost so darn much? Is the manufacturing process and materials 5x more complicated and expensive than a piston engine?

Has anyone ever tried to build a cheap turbo prop (other than these guys: http://www.turbinesolutiongroup.com)

Here is a company trying to build a lower cost turboprop.
https://www.turb.aero/

The one from PBS was around 85K Euros a couple years ago.

Tim

Jim, I actually find piston engines prices less justifiable than turbines. Turbines offer light weight, high power, high torque and high reliability obtained from high rotating speeds, high temps, close tolerances and exotic materials. In addition they tolerate a massive range in ambient temps and can operate for a short period of time on relatively wide tolerance in fuel quality. There's a high degree of technology and constant & ongoing product development.

Piston engines offer high weight, low power, low torque and poor reliability through ancient technology, low speeds, sloppy tolerances and basic materials. In addition they don't tolerate a wide range of ambient temps, they're highly sensitive to fuel quality, they are always trying to overheat and always trying to tear themselves apart. They require constant tuning, constant maintenance and have multiple points of failure. There's been a moratorium on development for ~50 years and things like fuel sources are a constant concern going forward. We joke about them being lawn mower/tractor engines but even those are better these days....

There's ZERO REASON that piston engines should cost as much as they do. There's nothing to them. By this point in history they should have EFI and cost 1/3 of the current prices.

Roughly, yes.

For the temperatures and forces experienced in the engine, you need exotic materials machined to high tolerance. At 41,730 RPM, the forces on the turbine blades are extreme.

Also, the control system is complex. Every seen inside of a mechanical fuel controller? 20 times the complexity of a carburetor.

Mike C.

Liability.

Although the operating principle is simple, just spinning wheels, the realization of a working engine is pretty complex. Specialized materials, precise tolerances and the creation of things we don't think about like air passages in turbine blades to create air films over the blade surfaces so it doesn't burn in operation.

If you look at a single turbine blade from a very modern jet engine, you'll see just how complex they are to make, with lots of tiny passages drilled through extremely tough alloys. Then there are hundreds of them, perfectly made in every engine.

The other thing you fight in building turbine engines is the physics of squeezing air through small passages. Large turbine engines over 500hp, are more reasonable than building small turbines of say, 100hp. Smaller sizes make it more difficult or impossible to implement things like air film cooling, which means you can't operate them as hot, which means they're not as efficient. Fuel specifics are much better on a large turboshaft engine of a few thousand HP, than they are of a 250hp small turboshaft engine.

Reciprocating engines make a lot of sense under 300hp. Turbines start to make sense above 300hp.

I think the expense of current piston engines for aircraft are telling of the industry. You're buying more than an engine, you're buying years and years of support and liability. CMI and Lycoming exist because we pay them enough to operate, supplying parts and service and responsibility for these dinosaurs. There's not much of a market and there's a high burden for every engine in service.

We can't compare them to car engines, where the engineering and other costs are amortized across a couple of orders of magnitude more units.