Originally Posted by Adlerdriver

Jettison issues aside, the problem with main deck suppression isn't an easy one. The main issue is that the crew effectively inhabits the "main deck" along with the cargo. There is no way to seal off the main deck. On the 777F, we can even have a cockpit door due to depressurization airflow/air-load issues on the aircraft structure during a rapid-D. Trying to seal the main deck cargo compartment to allow a suppression agent to be effective (like the lower class-E compartments) isn't feasible.

Fedex has installed an after market main deck FSS. It uses an array of sensors for the main deck positions and a bayonet system which pierces the metal cans and injects an argon based foam or in the case of a pallet, lays down the foam on the pallet. Certainly better the the basic aircraft option (depressurization and main deck airflow control). There was at least one false alarm that didn't pierce the can because the can roof was defective and not rigid enough for the bayonet to puncture. It's also ineffective against a sustained LI battery fire as are all currently available FS agents.

Back to the depressurization thing for a sec. The best option for fighting an in-flight fire is landing withing 15 minutes. The Boeing depressurization for a main deck smoke warning is only going to be used if you're ONLY option is to spend time at altitude due to the proximity of available airports. That portion of the checklist is going to be skipped right over if the aircraft is anywhere over the continental US or anywhere else with easy access to concrete. The other time I'll skip that option is if I know we've got a sustained LI battery involved fire and I'm not within 15-20 min of a runway. Then it's an off airport landing in a remote area or controlled ditch.