As a matter of interest, does firing the extinguisher damage the engine? I assume at the least it prevents it from restarting until some fairly major clean-up has been performed? Is this a factor in the decision-making on the ground ? I guess the owners would not be happy to see their expensive aeroplane with 2 f'd-up engines plus a f'd-up APU without very good reason. Or is it just an inert gas that disappears? In the air can you ever re-start an engine after firing its extinguisher?
So many questions......
What if questions get a little technical in this forum.
Engine extinguishers are external to the spinning bits inside the engine. They are designed to work between the outside of the engine and inside the cowling around the engine. Restart afterwards is a function of getting air pressure to the spinning bits in the engine and the time it takes to do this vs where the aircraft is on-the-ground or in-flight. In a high workload environment you manage the aircraft first