"Aviation" wrote in message

So, other than alarms or other signals, there is NOTHING AUTOMATIC
that puts the aircraft into rapid descent.

Correct.


Pilots have to respond to the signals... and initiate the dive MANUALLY.
For well trained pilots this would take, what, only a few seconds at most?
Do they put their oxygen masks on FIRST or start the dive first?

*Always* get the cockpit crew on O2 before anything else.


In some movies, the pilots are often unconscious, slumped over the
controls (wedging the 'steering wheel' / joystick full forward) as
the hero struggles to pull them out of the way and get out of the
dive. It sounds like a pilot, if not alert or in good health, could
actually lose consciousness from hypoxia under these conditions, so
maybe those kinds of movie scenes are ALMOST believable?

Not even remotly close to almost believable. Hollywood would have you
believe that there is an auto-trigger on all aircraft that renders the
engines non-functional, the aircraft uncontrollable, and the crew
unconscious whenever anything stressful occurs in the passenger cabin.
Truth is, if you're cruising at FL350 or higher you've only got about 5
seconds max to get the mask on, but it only takes about one to two seconds
to don the mask and get full O2 flowing. This is why crews train to an
instinctive reaction to get the mask FIRST. A person in average good health
will not have a problem with a few seconds of decompressing atmosphere. The
problem comes in on long flights. Toward the end, your day is already 12 or
more hours long, you're up high for efficiency - maybe 390 or 410 - which
means the cabin altitude is up around 9000 ft or more and everyone is tired.
Reaction time becomes crucial. When you're cruising high, a prudent crew
will ensure that the masks are preset and ready to go, and will don the O2
for a few minutes every little while just to make sure everyone stays alert.



Assuming
there is 3-5 minutes of mask-oxygen and one minute of "holding"
the last breath, they've got 3-6 minutes to get down to breathable
(14,000 ft?) air and then below. For the movie Executive Decision,
they were cruising at 39,000 ft. so they'd have to dive 25,000 ft
to 14,000 ft in 5 minutes, 5,000 ft/minute, average. Doable?

Absolutely. In an emergency descent, the prime (only) consideration is to
*get down safely*. The limiting factor is airspeed buildup. Airplanes are
very "slippery", and will build up speed at an impressive rate when allowed
to run freely downhill. In a rapid descent, you extend all the high drag
devices you can use (flaps, slats, spoilers, Ldg gear) to control airspeed
while you let the craft descend at the maximum vertical rate possible. In
the 747, the limit airspeed is 320 kt, (iirc) and that speed range's not
atypical for other transports, either. Typically, you'll see descent rates
of 10 to 15 thousand feet/minute.



The discussion of the ear problem seems unsettled..... Descending from
25,000+ (39,000) ft at 5,000 ft/min could result in reversible
or IRREVERSIBLE damage depending on a person's ability to
equilibrate REALLY fast.

When you get into emergency actions, you think in terms of minimizing or
prioritizing injuries. A rapid descent may well cause some passengers (or
crew) ear injuries, or even broken bones if they're not belted in. But the
alternative is substantially less desirable.