Turbine start

[Foxcow]

I was working the ramp last night and was getting ready to pull a GPU off of a Jetstream 41 when I noticed something strange. When they are starting up, I can see when the computer introduces the fuel and it ignites and the turbine blades turn incandescent. However, as engine speed increase, it seems like the blades cool back down because they are no longer incandescent. I would figure that they would remain incandescent because of the higher temperatures.


Any ideas?

 

[SteveC]

The internal temperatures spike during start because of low air flow. After light-off the turbine starts to spin up, increasing the speed of the compressor section as well. As the compressor blades speed up there is more air flowing through the engine, lowering the internal turbine temperature.