Turbine starting and hot starts

[Dream Land]

Avoiding hot starts.

On the Garett engines if your ground time is short, you can spin the prop, thereby bringing in cool air to cool things down, on the Pt6 I recommend that just after shutdown, you engage the starter and crank the engine for 30 seconds, works great.

[flyhardmo]

on the Pt6 I recommend that just after shutdown, you engage the starter and crank the engine for 30 seconds, works great.

That drains your battery so your next star is a hot start. I wouldn't recommend it unless you have a GPU.

[JammedStab]

Originally Posted by wingisland

I was wondering if someone could explain to me how you start a turbine engine (eg the ever popular PT-6) and how (indeed IF) it differs from starting a turbo fan such as on 737.

Don't know about the modern 737's but there are some similarities between starts on the PT-6(at least older versions) and the old 737's even though one is using an electric starter while the latter uses an air starter.

The similarities are that if you have enough starting power(psi of air pressure or electrical power) engaging the starter and then normally waiting until getting a stable and steady rpm, then and only if you have the required minimum rpm, selecting fuel on and holding the fuel lever until it is certain that there are no exceedences and that the start is normal. As well, releasing the start switch at the appropriate time. Many minor differences though.

[Mecaniquito84]

Technically you are right, but the point here is, again, to follow established procedures. Manufacturer clear indicates to shutoff al others consumers like packs and MINIMUM air pressure for engine start. Should air pressure is below that level, you are not allowed to start engine

[Dream Land]

That drains your battery so your next star is a hot start. I wouldn't recommend it unless you have a GPU.

I am currently operating a TBM 850, we replace batteries every two years, after a typical one hour flight, operating the starter for 30 seconds has no detrimental affect on the start, a typical voltage indication during a start is still 21 volts.

[glendalegoon]

dear original poster

I think you are mistaken in the meaning of a HOT START. A hot start is a bad thing in which the engine gets too hot and may damage the hot section/turbines.

A hot start is not a start of an engine after only a short time of being shut down.

[flyhardmo]

Dreamland

Try doing it on a twin and see where the voltage ends up especially on cold mornings. One of these days you'll have to explain to your pax that you can't go because you have drained the battery unnecessarily cranking over a PT6 after shutdown. Just park the aircraft into wind for shutdown/startup and if you really want to you can motor the engine an extra 5sec after max motoring speed before you introduce fuel. Try it and let me what the difference is in starting ITT/EGT.

I never flew Garretts but an engineer told me the reason for spinning the engine after shut down was to stop the main shaft from flexing once it had cooled down. I'd be happy for some experienced chap/chapette to confirm or deny this.

[barit1]

flyhardmo:

an engineer told me the reason for spinning the engine after shut down was to stop the main shaft from flexing once it had cooled down. I'd be happy for some experienced chap/chapette

Sounds like a phenomenon common on several engines. After an engine is shut down 10 to 60 minutes (depends on engine size & rotor mass), a convection stratification occurs within the case, meaning that the segment of rotor that stopped at 12:00 is a bit hotter than at 6:00.

This makes the rotor shaft bow slightly. After another hour or two, everything cools down and temps equalize.

But if you attempt a start during this bowed-rotor period, it's gonna be out of balance for a few minutes. I've heard of pilots "tickling" the starter while shut down to suck a little cool air through, and to randomize the rotor position.

[A Squared]

barit1, thanks for that explanation, I always understood the "prevents shaft bowing" explanation to mean that if you didn't, the shaft took on a permanent bow due to heat and gravity, which always seemed a little improbable to me your explanation seems a lot more plausible to me

[Chu Chu]

A^2,

Take a flat piece of sheet metal. Heat a spot to orange with an O/A torch. Let it cool. It won't be flat anymore.

That's a lot different than a turbine shaft cooling unevenly; I'd be surprised also if one of those took a permanent set. A back of the envelope calculation might very well show that the maximum stress in that situation is a small percentage of the yield strength. But absent that, I'd probably hesitate to say a permanent set was impossible.

[Dream Land]

Flyhard,

To prevent a warm or hot start on a pt6 installation, you must spin the engine or let it cool on its own, for instance on a twin, you would simply crank one engine after you chock in, then the engine you initially start is used to crank the second engine, not complicated.

On the Garett engine, I'd say with 8000 hours on MU2 / Metro aircraft that I know something about the subject old chap, initially, old engines did have a shaft bow problem that could bind the engine up if not spun after a period of 15 to 20 minutes after shutdown, this is no longer a problem, but a great way to cool the engine if you are doing a quick turn.

Cheers.

[DevX]

Larger two shaft engines are more prone to 'rotor bow' than 3 shaft.
Read P&W, GE vs. R-R, but that's not to say that R-R engines don't also suffer sometimes.
Of course if there's a good blow on the stand it'll help to windmill the engine(s) thus preventing rotor bow starts.