As I had hoped some interesting responses.
QAVION: The smoke detectors on the 747-400 I am familiar with aren't quite so numerous. Each cargo hold has two dual loop detectors both of which must sense a fire for a warning to be given. The system on the earlier aircraft had a potential flaw in that if one loop failed the sensor would not reconfigure to single loop operation and as a result if a fire was occuring that particular detector would not signal a fire warning. There are of course two of these detectors in each cargo compartment so the chances of both experiencing a single loop failure on a flight on which you also had a cargo fire are so small as to be insignificant. On the newer aircraft if one loop senses a fire the other loop is automatically checked and if it is serviceable a fault message is generated for the loop detecting the fire. If it fails the test a fire warning is given.
LANCER : If you are waiting for some structural failure to be the first sign that you have an uncontrolled fire in the cargo bay then it will probably be too late for any action to be taken assuming you take the first option in my orignal post and decide to stay high confident that the fire extinguishers have worked.
TINSTAAFL : An interesting suggestion however while you may increase the cabin altitude in order to protect the passengers passenger oxygen would have to be selected on. Would this meet your requirements for an onboard oxidant ?? The checklist I follow for cargo fires requires you to manually select the landing altitude to 8,000ft. This will depressurise the aircraft partially. The benefit of this is that you obviously achieve a reduction in O2 supply to the fire plus a reduced ventilation rate but you don't have to worry about Pax O2 supply. This means you can stay high and as a result have a higher TAS.
CHECKBOARD: A good suggestion re getting feedback from the cabin and feeling the floor. Definitely a good idea. Going on from this idea, as a defence against false alarms, I think checking the cargo compartment temperature would be a worthwhile step also. Not that a low temperature would stop you from running the checklist and discharging the extinguishers but it coupled with the fact that the floor is not hot could be just the information you need to give you confidence in staying high depressurising the cabin to the max landing altitude, such that pax oxygen not required, then proceeding to the nearest airport at your cruising level thus gaining from a higher TAS.
Every situation is different. When in the middle of the Atlantic or the Pacific and you get the fire warning message the first step is to obviously run the checklist. This would in most cases involve firing the extinguishers, reducing the airflow to the cargo compartment and increasing the cabin altitude. Following this after you have talked to ATC and got established direct to the nearest airport the next step I believe should be information gathering from all available sources to establish if the fire is for real and if so how bad is it. Again as mentioned above input from the cabin and checking the cargo temperature can give you invaluable information. If the situation is not improving but shows signs of diminsihing i.e floor getting hotter, smoke appearing my next step would be a descent to 14,000ft and complete depressurisation. By going to this level you should again forgo the need for passenger oxygen although crew oxygen should be used. This futher depressurisation will again achieve the twin blows of reducing O2 supply to the fire and reducing the ventilation rate through the cargo compartment. By descending to this level and not lower you keep to a minimum the TAS decrement from higher altitude cruise. At this level I would also review the ditching checklist accomplishing as much as I could. If feedback still indicates the situation diminishing I would then descend for a ditching. The only modification I would make to this is if feedback indicated a rapidly deteriorating situation I would skip the step at 14,000ft. I believe this plan of action would offer the best compromise between staying high and getting to the airport ASAP but risking a structural failure if the fire is still burning and descending for a ditching and the consequent dangers associated with it.
Comments ? Additional suggestions ?