The reason for the descent is due to the rapid drop in temperature in the cabin as a result of shuting off the trim air. The procedure also shuts down packs 2&3 which supply air to the main deck and lower cargo lobes. Obviously the aircraft will become very cold very quickly.
Remaining at cruise altitude can buy you time, lower cabin pressure remains an option where as at 250 and below it is not, that additional reduction in pressure and the available oxygen for the fire, put another way you have more time and a lower probability of flashover.
Colder temperatures would also delay the time for heat to transfer that would otherwise ignite combustable materials near the smoldering components that have previously ignited. Example, the pallet next to the one that caught fire. the lack of oxygen, colder temperatures, and distance (albeit 5-10cm) could make it more difficult for radiant heat to become a secondary ignition source and allow the fire to spread. I will grant you that this may be a very small benefit given the heat soaked nature of cargo relative to the ambient temperatures at altitude.
Depriving the fire of oxygen accomplishes everything that Halon, CO2 or other fire fighting gasses do. Once you remove the oxygen time is on your side. anybody that has use halon knows how quickly a fire will re-ignite once the gas disperses. It stops the fire and the secondary ignition sources but it does not remove the primary ignition source or hotspots that can cause re-ignition.
An aircraft kept at altitude retains the benefit of oxygen deprivation, increased time to reduce the heat from the primary ignition source and possibly time to locate and put the fire out.
Last edited by Deltabravowhiskey; 4th October 2010 at 23:10.