I came across this statement in a book - "During a descent, the cabin pressure in a pressurized aircraft usually climbs at as lesser rate than ambient pressure".
Could someone explain this statement in simple words?
$\begingroup$
$\endgroup$
1
-
$\begingroup$ There's a graph of possible cabin and outside pressure for a flight on this answer. aviation.stackexchange.com/a/26345/3159 $\endgroup$– BowlOfRedNov 21, 2019 at 16:33
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
During descent, the rate of change of pressure inside the cabin of a pressurized aircraft is lower than outside (ambient).
Inside the aircraft, under active control the cabin pressure increases from equivalent ~8000ft above sea level to ground level. In the meantime, the aircraft descends from cruising altitude which is typically much higher, also to ground level.
Assuming the pressurization and descent start and end roughly at roughly the same time, the cabin is necessarily pressurized slower than the increase of air pressure outside in absolute terms (e.g. psi per minute) assuming a steady pressurization profile .
Corner cases arise when the airport is at an elevation above 8000ft or so since the cabin will be further depressurized during descent. So while the relation holds in absolute magnitude , they will be of opposite sign. (until there is an airport 24,000 ft or so....)
$\begingroup$
$\endgroup$
The pressurization system uses a controller that manages pressure changes for the least impact on the passenger. This means avoiding sudden changes and keeping the rate of change as low as possible, generally between 100 and 300 fpm, and never more than 500 fpm (about the most most people will tolerate without complaint) while the airplane is going 2000-3000 fpm.
One important issue the controller is designed to avoid is the "pressure bump" right after liftoff. This is from the system reacting to the ambient pressure drop of the initial climb (the input/output response loop is quite long) and there can be a bit of a response overshoot that pops peoples' ears, generating complaints.
To avoid this, the system will slowly pre-pressurize the cabin to a few hundred feet below the current elevation before takeoff, too slowly for most people to notice, to get the system settled into its regulation mode and have a bit of pressure margin that allows the system to smooth out its own reaction to the sudden drop in pressure during the initial climb.
