Discharging Fire Bottles during an Evacuation Boeing FCTM
The evacuation NNC specifies discharge of the engine or APU fire bottles if an engine or APU fire warning light is illuminated. However, evacuation situations can present possibilities regarding the potential for fire that are beyond the scope of the NNC and may not activate an engine or APU fire warning. The crew should consider the following when deciding whether to discharge one or more fire bottles into the engines and/or APU:
• if an engine fire warning light is not illuminated, but a fire indication exists or a fire is reported in or near an engine, discharge both available fire bottles into the affected engine
The question of delay in firing the second bottle is not really applicable as the time taken from the turning of the fire handle, checking the orange discharge light and observing the result is usually more than 30 seconds anyway. In our SOPS, No confirmation is required to discharge a bottle on the ground but as mentioned it if you discharge into a running engine its an opportunity lost.
The amount of time the Halon has to act on the fire is more important than the quantity, and discharging the second bottle too soon may just result in pushing out the Halon present before it has had to to suffocate and start its job.
No need to rush.. take the example of BA!