One of Jerry Pournelle’s readers discusses smoke on a plane some more:
Many airliners still retain the option to open a window in the cockpit, at least on the ground. That’s because the pilot may have no way to exit the aircraft in the event of a fire preventing the pilot from reaching another exit. Smoke in the cockpit is one of the worst airborne emergencies for 2 reasons. First, the smoke may be so toxic that onset of neurological deficit or blood-oxygen transport problems may be only a matter of seconds. Second, the first indication of an aircraft fire or smoke/fumes in the cockpit is usually someone on board saying “hey do you smell something?”, at which point everyone around immediately takes a deep breath or two, inhaling quite a bit of whatever is in the air, delaying starting the emergency procedure procedures while everyone sits around going “I dunno it smells like a bad air filter, what do you think?”
In military aviation we try to beat these considerations into the brains of our student pilots, but over time a little complacency often sets in.
When airborne depending on the aircraft type, there may be an option to depressurize and “ram-dump” the environmental system, which opens ram air ducts to force outside air into the cockpit/cabin. I’m sure every aircraft will have variations in how this works but the basic idea that there is a switch that immediately shuts off conditioned pressurized air circulation and opens up ram-air from the outside is pretty much standard. It isn’t much different from opening a window.
I do know that my one major smoke/fume in the cockpit incident dropped my blood oxygen level to around 85% in a matter of minutes and resulted in an overnight hospital stay, from only 2 or 3 breaths of the smoke-filled air before I got on 100% oxygen.
Pournelle has had limited experience with cabin smoke but the one incident he went through went much as described:
“Hey do you smell something funny?” Followed by discussion followed by “Let’s get this bird down fast!” Fortunately it was minor, although at 38,000 feet nothing involving blue smoke is really minor.