If, for whatever reason, the crew need to leave the ac at high altitude then
the crew need to be protected as much as possible from the effects of the
low oxygen/cold environment. The seat supplies supplemental oxygen, but
because the pilot needs to get down to thicker/warmer air as rapidly as
possible, the main chute doesn't open right away. Instead, a smaller chute
called a drogue deploys to stabilize the seat so it doesn't tumble and to
slow the pilot's horizontal velocity. In a near free-fall, he plummets
(still in his seat) until he hits an altitude of, typically, 15-10k feet at
which point he separates from his seat and his main parachute automatically
deploys. If I recall correctly, the time taken to freefall from 55k to 15k
is about 2 mins.



Just hope that you are not flying over an 11,000 ft mountain :-)

Al Minyard