Warm (hot) start

[ravenx]

I hear lots of people refer to warm starts on turbine aircraft (particularly 206's). What is one and why is it such a problem.

[skidtoob]

O.K, here goes nothing, and no doubt someone will correct me!

From what I understand, and hot start is when, on start, you get excessive T4.

I have experience of these on the 206, and the ones I have seen are from incorrect fuel modulation, or insufficient NG when cracking the throttle.

If you do not have enough speed, you do not have enough air passing through the engine, therefore giving you an incorrect fuel-air ratio. (Richer than normal)

The problem with this is that with the incorrect ratio the fuel burns hotter, and basically ends up frying the turbine blades (thermal shock) and possibly the combustion can.

I hope this is the kind of thing that you were looking for.

[Hone22]

Hello & welcome Ravenx

As skidtoob outlined, basically a turbine sucks in air, compresses it, adds fuel to it, then ignites it. The resultant rise in temp/pressure then spins windmills which extract the energy and drive the rotors.

The combustion can (chamber) can't withstand the combustion temps inpinging directly on the walls, and excess air from the compressor stage "cradles" the fire within the can stopping the flame from contacting the actual can walls and melting them.

During the start if fuel is added and "lights off" before there is enough airflow to "cradle" the flame, the can walls can melt. Also the windmills downstream can melt.

Basically if the fuel added is more than the "air cooling" component the resultant high internal temps (TOT) will cause damage.

Therefore during starts a minimal N1 (intake air compressor speed) is stated to provide the cooling air. Also a start throttle setting is provided to ensure best air/fuel mix, avoiding a hot start. On some engines the throttle can be manipulated to control TOT. Some have Fadec to control the start and some you have to accept the factory settings of air/fuel mix for the start.

Whichever you have the important thing is, if the TOT gets into the red (danger, hot start area), close the fuel(throttle) BUT.......Keep your finger on the starter. So keeping the cooling airflow through the engine.

. .TOT = turbine outlet temp. .Fadec = Full authority digital engine control

cheers & flysafe. .Hone

[paco]

It's worth mentioning that a hot start can be the result of all sorts of obscure things, including a weak battery.

Phil