Prior to my flying career I enjoyed a 16 year career as a professional yacht captain, much of it aboard large Mediterranean based motor-yachts.. On one overnight passage (Corsica to Palma, Majorca) a fire broke out in the engine-room. The recently overhauled fire extinguisher system failed to activate either automatically or manually and the fire quickly got out of control. The acrid smoke from a multitude of burning elements both liquid and solid made breathing pure torture, eyes were next to impossible to keep open. The most memorable aspect of the entire event was how quickly the fire gained control. Every few minutes it had grown exponentially and any plan for a new attempt to gain control of the situation was out of time before we were able to implement it. The vessel was ultimately abandoned and we spent a night and day in the life-raft.
Very many years later I found myself employed as an instructor with Flightsafety Int. I'm a great believer in check-lists and will only modify the manufacturers check-list when no safety issues are involved. It surprised me that the Emergency Check-List for suspected fire on this particular aircraft called for a series of checks to be run, all of which made perfect sense except for the fact that they were called for without any requirement for the crew to commence a descent. My instruction to the candidates was always; Yes, access the check-list but start getting the plane down immediately and safely as possible ... If it turns out to be unnecessary you may have to fill in a report. If it turns out that it WAS necessary... You may just have saved a lot of lives. It can all happen a lot quicker than you think.