Originally Posted by
A37575
On an allied matter, I understand that Airbus specifies that in event of an engine fire on the ground, the 30 second timing between the firing (if necessary) of the second bottle, does not apply to a fire on the ground but only to an engine fire in the air. I understand the reason being that in the air the second bottle is only meant to be fired after the engine compressor rpm has wound down and thus allows the extinguishing agent to have a greater chance of being effective.
No that's not why.
The extinguishant discharges into the nacelle/pylon/engine casing to put out fires around the engine .
The inside of the engine is meant to be on fire- that's why air, fuel and sparks are brought together in the combustion chambers. Fire is only a problem when it escapes from the engine (e.g. via a cracked combustion chamber) and causes a secondary fire in the fuel system, hydraulics, electrical wiring or even the metal of the airframe. This is why the extinguishant discharges around the engine itself, not into it (where it would have no effect).
The 30 seconds gives time for the first shot to have an effect, extinguish the fire and put out the fiire caption on the flightdeck. If the first shot works the second shot can be saved for later - in case you are unlucky enough for the fire to re-ignite.
On the ground, the 30 seconds does not apply because you don't need to keep one fire bottle in reserve. You blast both bottles into the nacelle, hopefully swamping any fire and if that doesn't work, you evacuate the aeroplane.
Company policy on evacuation varies - I have worked with SOPs where an evacuation was mandatory following an engine fire (even if it had extinguished) and others where it was left to the discretion of the crew. In the former case the company wanted passengers as far away as possible from the seat of fire. In the latter the company wanted to avoid the inevitable injuries that occur during evacuation (and to avoid having shocked passengers milling around amongst moving emergency vehicles).
Slightly off topic, but with reference to the Air France A340 accident in Toronto, do you recall that some of the passengers wandered straight into the path of traffic on a 6 (8?) lane highway after evacuation? In that case they had no choice but to evacuate but it could well be safer to keep pax on board and out of harm's way if at all possible.