Originally Posted by
LTP90
Why does it not fire until 50%? I don't understand the mechanics of that.
It can be unexpected reasons. On the 412 pulling the t handles shuts the fuel valve and energises the fire suppression system.
The procedure requires closing the throttle first, I used to think why? It’s slower, there is more likelihood of selecting the wrong engine and the throttles can be awkward, why not just pull the T handle?
Turns out it relates to the particle separator, it will suck all of your suppressant out of the engine bay. When you shut the throttle it closes the system, the time it takes to continue the SOP after closing the throttle is enough to close the particle separator door.
You could of course pull the T handle (which also closes the particle separator) and wait 10seconds which will achieve the same thing but evidently Bell has decided Pilots who’s aircraft are on fire might not be the best at accurately guessing time and might waste their first fire bottle.