Originally Posted by Escape Path
...Anybody knows the reason?
The power lever on an aircraft equipped with a PT6A-27 engine sets the speed of the gas generator (compressor portion) of the engine. As long as the power lever is not moved, torque will change if altitude is changed (decreasing with climb and increasing with descent), but gas generator speed (Ng) will always remain the same.
The fuel control unit adjusts fuel flow (Wf) in order to maintain whatever Ng set point that the pilot has selected using the power levers. As the aircraft climbs, fuel flow is reduced, and as the aircraft descends, fuel flow is increased - again, this assumes that the power lever is not moved.
I can only guess that the reduction or increase in fuel flow is more or less proportionate to the reduction or increase in air density, and for that reason, T5 does not change significantly.
Michael