Why is it necessary to put in the destination airport elevation for the pressurization system?

Question

Why do pilots put the destination elevation into the flight computer for the pressurization?

Answer 1

At cruise altitude, there is a differential between the cabin pressure and the outside pressure. At landing, you want that differential to be (roughly) zero. In the descent from cruise to landing, that differential decreases toward zero.

(In practice, you actually land with a slight positive differential, which dissipates shortly after weight-on-wheels. So not exactly zero differential at landing, but close to it.)

The schedule for decreasing that differential to zero will be different for an aircraft cruising at FL 370 and descending to land at sea level, than it will for an aircraft starting at the same altitude and descending to land at 5,000' MSL. In the former case, there will still be some differential pressure at 5,000'; in the latter case, it will be down to about zero.

Knowing the landing field elevation allows the pressurization controller to schedule the reduction in differential pressure so that it reaches zero at the right time (altitude) in the descent.

If the aircraft landing at sea level had zero differential at 5,000' (i.e. followed the schedule for landing in, say, Denver), then passengers would feel the full aircraft rate of descent the rest of the way down, which could be a lot greater than you'd prefer -- i.e. 1,500 FPM (typical for a clean 250 knot descent) vs a less ear-popping 300-500 FPM (typical for cabin rate of climb/descent).

On the flip side, if the aircraft landing in Denver used the schedule for the aircraft landing at sea level, it would land with some differential pressure still on the aircraft, which isn't desirable either. Since a pressurized cabin makes it hard or (with enough differential) impossible to open the exits, the aircraft systems will typically vent any remaining cabin pressure shortly after touchdown, meaning that if significant differential exists, the cabin altitude would then CLIMB fairly rapidly -- again, uncomfortable for the passengers and their ears and possibly sinuses. (Alternatively, if the aircraft doesn't vent the pressures, then opening the exits can have some problems.)

The pressurization controller can sense the altitude of the departure airfield and is given both the cruise altitude and the landing altitude, allowing it to schedule the cabin pressure differential going both up and down. Without the landing field elevation, it wouldn't be able to do the latter.

Answer 2

See How does the cabin pressure controller work in a large commercial airliner?

When cruising, the pressure in the plane is greater than outside, and it's not just a fixed amount, nor does it just start pressurization at a particular altitude (like 8000 feet). Instead the pressure system tries to slowly pressurize and depressurize to minimize discomfort.

If the aircraft didn't know the destination altitude, then after landing you might have to either do a fast decompression, or wait longer than you wanted before opening the doors. This way, the the system can plan the equalization at gentle rates and have it complete shortly after the aircraft reaches the target altitude.