Was out at the shop getting the Navajo ready for tomorrow and there was a 441 in there on jacks with the mule hooked up (rh cowling off). Looked at the TPE331 and it looks smaller and simpler than a PT-6. My imagination? This old piston driver could just about understand all the components, versus looking at a PT-6 and seeing a bunch of "stuff".

Just me?

There is a lot of subtle complexity that isn't visually obvious.

It is a pretty simple engine in the end. Once running, there are only two pressures that need to be controlled: oil pressure in the prop, and fuel pressure at the fuel nozzles. The prop oil pressure controls blade angle and thus RPM, the fuel pressure controls fuel flow and thus torque.

If you saw inside the fuel controller, you'd be amazed at the complex mechanical computer that it is.

Mike C.

The TPE331-10 engine is an engineering masterpiece.

Another question from a prop dude

1. Is it still being made?
2. What's the cost of HSI & OH?
3. What is the BSFC?
4. What is fuel burn at typical altitudes? Below 10,000ft while being vectored?
5. Why so dang loud?

Loud - TPE prop is directly connected to the engine and the idle has to be high to keep the prop spinning. Pt6 has a fluid coupling between engine and prop - so the engine can loaf quietly at idle

Yes, sort of. The -10 engine is on the MQ-9 Reaper drone, in a pusher configuration. There are a few other new OEM applications and STCs. Not made in high volume. The parts are readily available.


About $230K for OH, $35K for HSI. Typical schedule is 2500 hours HSI, 5000 hours OH.


Around 0.49 lbs/hp/hr. This is directly measured on my plane in cruise flight. This does not include the jet thrust effective HP, which would lower BSFC to about 0.46 lbs/hp/hr.

The TPE331-10 is one of the most efficient small turboprop engines out there. A really good aircraft piston engine, operated LOP, is around 0.38 to 0.40.


I flight plan FL280, 65 GPH. 10,000 ft at 150 KIAS maneuvering is around 60 GPH.


Compressor directly exposed to the front (not through a 180 degree bend in a PT6).

Engine idles at 65% to 78% RPM.

On MU2s, in particular, short exhaust creates a resonance with the ground under the wing. Low wing airplanes don't do this, airplanes with bent exhaust like Commanders and B100 don't do this either.

Mike C.

All in all, I am impressed with this engine and its efficiency.

There has to be some efficiency loss with a Pratt? What is the BSFC on a comparable PT-6 of 750hp?

About 0.65 lbs/shp/hr.

I've never seen a measure of the PT6 jet thrust. Given the exhaust nozzle arrangement, I suspect it is less effective than the straight out TPE331, so the EFSC is probably about 0.61 lbs/eshp/hr.

At $4/gallon, a TPE331-10 will save enough fuel over its TBO cycle to pay for its overhaul as compared to a PT6.

Mike C.

I flew the C441 w/ -10 motors, just stellar! Thinking back, I could easily cruise in the low thirties, sipping 300 PPH / both engines, and pushing thru at 330 KTAS. Easy starts, and post-flight - big thing to reduce engine wear is pull the prop thru a few blades to vent hot gasses as its a direct drive engine. And never start a TPE with a low battery, as she could hot start.

I wasn't paying the bills, so I cannot speak to mtc cost.

Cheers, James

Because if IIRC it's spinning at about 41,000 RPMs!

George