Starting a turbine engine on the type I operate is easy enough from a control point of view. The engine is selected and the start button pressed. This energises the starter/generator, which starts winding the high pressure compressor and turbine. When Nh is indicated on the engine display, the condition lever is moved up into the start/feather position. This introduces fuel into the engine. This also starts the igniters in the engine.
The starter motor keeps the driving the turbine RPM up to self sustaining speed at which point it will cut out and leave the combustion to drive the engine.
If for any reason the engine doesn't reach self sustaining speed the start is referred to as a 'hung' start and indicated by a low turbine RPM
A 'hot' start on the other hand is where the temperature in the engine rapidly rises. Possibly due to engine damage or unburnt fuel. In the case of unburnt fuel, the start is aborted and the engine cleared of fuel by running the starter motor to pull air through the engine.
Other starting methods include using high pressure air either from the other engine or a ground unit to rotate the turbines.
I've not heard of any real issues with hot starts on short turn arounds on the type I operate, but the Twotter may have issues as I'd imagine it's not a FADEC engine.
I'd guess the main reason for keeping the engine running is to keep the right engines generator online, allowing for a normal start as opposed to battery starting or getting a ground power unit attached to the aircraft. The issue with battery starts is the high load placed on the batteries which on very short trips eg Island hopping may not allow the batteries to fully recharge before the aircraft is shut down which may cause starting problems such as a hung start. Nothing as annoying as being unable to leave a small airfield due to a lack of GPU or spare battery
The larger aircraft get around this through the use of the APU which provides electrical power for the starter motors or possibly bleed air to spin the turbines.
Last edited by Cyclone733; 28th Jan 2009 at 12:14.
Reason: Beaten to posting due to very slow one finger typing and having to relearn how to use a QWERTY keyboard