Originally Posted by
LTP90
Why does it not fire until 50%? I don't understand the mechanics of that.
50% N1 is the point the engine can self-sustain combustion and run without external assistance. The start sequence also shuts off at 50% N1. So for the extinguisher system to have full effectiveness it's activation is delayed until the N1 is below that self-sustain level.
And another point on the engine shutdown procedure is if the engine switch is turned OFF and the engine still runs or there still is a TOT increase the final measure is to perform a shutdown via the manual twist grip. I believe there is a mx check to verify the manual shutoff rigging is correct.