Yes, I am a 777FO, need to update profile.
Correct, 777 is different (if I remember the -11 right). If you get a main cargo deck fire, there is no fire suppression system (bottle). There is the overhead system that will attempt to put out a fire in a can by penetrating it and injecting suppressant. But it will not address pallet loaded cargo or a non-can related fire. The only means of fighting those kind of fires is oxygen starvation (the procedure) or taking a hand held into the back (prohibited).
I agree, you can dive the aircraft to sea level in 5-6 minutes. I’ve added more time in for the reaction, decision, and get ready to put the airplane in the water time. In a controlled environment where you knew the warning was coming, you could obviously shave a few minutes off. I’m just trying to present a real world time frame.
But it doesn’t matter how fast you can dive to sea level—that is not the procedure! The procedure is to depressurize and get to FL250, which you might as well do slowly.
So, back to my disconnect between procedure and time available. In my scenario, the 777 is more than an hour from land when it presents a “Main Cargo Fire”warning. Sitting in the cockpit, you don’t know if it’s a false warning, a Li battery fed fire, a small smoldering fire or one that is soon to be out of control. The book says do not go back and look. Two of those scenarios could be survivable by using the procedure. Two of those scenarios have you possibly going uncontrollable before you can get to sea level if you follow the procedure. The problem is, you don’t know what you have, and the clock is ticking....
If you were to descend to sea level immediately (while preparing to ditch) and then investigate (wait for smoke, look for failing systems, hell-blow off the book and send someone into the back to investigate) you could determine which of the 4 scenarios you have. If there is no fire, you can climb back up and divert. If there is a Li battery fire or one that is out of control, you are in an infinitely better position than you would be at FL250.
I present this scenario for discussion, like I said, I’m no expert. When you take the history of inflight fires and the fact that you don’t know what is going on in the back of the airplane, I’d sure hate to be at FL250 when the clock expires....