How does an airliner deal with cabin altitude at an airport above 8000 ft?

Question

There are a number of airports above 8000 ft altitude. FAR 25.841 states:

Pressurized cabins and compartments to be occupied must be equipped to provide a cabin pressure altitude of not more than 8,000 feet at the maximum operating altitude

How does an airliner operating from a high airport, say from El Dorado, deal with cabin pressure? Do they actually pressurize the cabin after takeoff and depressurize it after landing?

Note: the answer can be of any jurisdiction though FAA seems relevant here as US airlines operate from there.

Answer 1

Here is the text extracted from Boeing's 777 Flight Crew Operations Manual:

In cabin altitude controller cruise mode, maximum cabin altitude is 8,000 feet. When the takeoff field elevation is higher than 8,000 feet, the cabin descends to the cabin cruise altitude while the airplane is climbing.

When the destination airport elevation is greater than 8,000 feet, cabin altitude controller cruise mode maintains a cabin altitude of 8,000 feet.

(...)

At touchdown, the outflow valves open to depressurize the cabin.

Therefore, when the airplane is on the ground, its cabin altitude is always equal to the ambient altitude. When it is in the air, the maximum is 8000 feet. Whether the cabin altitude will climb to 8000 feet or descend to 8000 feet depends on field elevation.

The reverse happens upon landing. If landing at an airport at 10,400 feet, the cabin altitude is <= 8000 feet until touchdown, then raises to 10,400 feet.

This make sense - if the outside pressure is greater, how do you open the door? (well with enough force you can, but I will leave it to your imagination as to what happens the moment the door is opened).

Answer 2

I am an aerospace engineer, who has worked on the design and certification of pressurized aircraft, so let me try and answer.

The class of regulations that deals with the certification of pressurized aircraft are called 14 CFR 23.841 (General Aviation) and 14 CFR 25.841 (Commercial). Both paragraphs are pretty much identical and in summary say the following (paraphrasing):

All GA and commercial aircraft designed and certified to cruise at altitudes greater than 25000 ft must be pressurized.

This means that aircraft whose maximum altitude (often referred to as "service ceiling") is less than 25000 feet do not have to be pressurized (once you exceed 14000 feet, most people begin to use supplemental oxygen, but above 25000 feet your plane must be pressurized and that's what provides your supplemental oxygen).

Unpressurized GA aircraft routinely operate at altitudes up to 25000 feet. This should answer what pressurization an airliner must provide at high-altitude airports: None! Not unless said airport was above 25000 feet would some pressurization be required (a problem that does not exist on this planet).

What paragraphs 23.841 and 25.841 are saying is that those airplanes that are pressurized, must be capable of providing cabin altitude of no more than 8000 feet at their operating altitudes. So consider two aircraft, one operating at 25001 ft and the other at 45000 ft. Both must provide no less than 8000 feet in the cabin. It shouldn't come as a surprise that the one operating at 45000 ft will require a stouter (i.e. stronger and thus heavier) fuselage as the pressure difference between the cabin and outside is much greater. Many modern commercial airliners provide a 5000 feet cabin altitude and some bizjet manufacturers are already considering sea-level pressure in the cabin, but I digress.

Anyway, I hope this sheds some light on this mystery. Cheers.

Answer 3

The verbiage of the CFR is important (imagine that). The part you quote is really saying the following: if you want to fly an airplane which is equipped with a pressurization system, then, when the airplane is at its maximum operating altitude, the system must be able to keep the cabin pressure at or below 8,000 feet.

Nowhere in the reg you quoted is there anything about maintaining 8,000 feet, and in fact upon landing the dump valves will open and the cabin will depressurize to whatever the ambient pressure is anyway.

I imagine that airline crews would pressurize for passenger comfort until descent, and then gradually bleed away cabin pressure until touchdown. The aircraft will maintain 8000 feet until touchdown and then depressurize. Thanks @Kevin!

Answer 4

In the PC-12, there is a cabin pressurization controller knob (the lower right-hand knob in the image) that allows you to set the actual altitude at which you are taking off at/climbing or descending to/landing at.

Let's say you file for FL220. Before departure, you set the cabin altitude on the dial to 22,000 ft. As the airplane climbs, the engine bleed air pressurizes the cabin until the maximum pressurization differential is reached (shown on the PSI DIFF gauge). The cabin climb rate is shown on the dial in the lower left corner.

When you descend to land, you set the knob to the destination field elevation + 500ft (the lower internal pressure is so you don't blow open the door!)

If the cabin pressure is 5,000 ft, and you're landing at a field that's 8,000 ft, the cabin will climb even as the aircraft descends.

If there is a failure of the pressurization computer, the pilot can adjust the cabin pressure manually using the knob in the upper righthand corner.

It's fairly simple to operate.

Answer 5

I've been waiting for an answer, but none forthcoming - so - this is GUESSWORK :)

The cabin altitude cannot be anything except the ambient altitude when the doors are opened before takeoff and after landing.

For landing, the cabin altitude would therefore be set to the airport altitude. It cannot be anything else since there are valves which automatically prevent negative internal pressure.

For takeoff, my guess is that the cabin altitude would be set to normal according to SOPs for the aircraft and type of flight. If the departure field was at a higher altitude, then this would result in an increase in cabin pressure. Otherwise, the normal reduction in pressure would take place.