Pressurisation when landing at extreme elevation

[R2112]

A curious question. recently flew from Lima elev. 13M, to Cusco elev. 3310M, courtesy of Star Peru and their fleet of 146´s (very noisy flap retraction after departure as someone pointed out on another thread!) anyway the question: how does the pressurisation system allow for such an extreme landing elevation? I know on (most?) systems you can physically set the landing altitude, and to my knowledge the cabin during cruise is something like 7000ft? so does the system command a gradual reduction in pressure as the aircraft descends until the aircraft and cabin Alt´s meet at the treshold?
Thanks in advance!

[johns7022]

If a plane takes off from see level and wants to land at Lima, the pilot can set the pressurization in the cabin to match landing alt...at such extreme alts like Lima, masks might drop if the the cabin sees an altitude of higher then 10,000 ft - 12500 ft...in that case, the pilot can switch off the 'mask drop' when landing at Lima so that doesn't happen...

[Prober]

I operated from Bogota (8400'amsl) in 757. Make sure APU running for pressurisation if Bleeds Off for T/O (otherwise warnings manifest themselves at 1500', followed by the rubber jungle). When airborne, Cabin Alt starts to decrease in accordance with the auto schedule, to give a cruise Cabin Alt of prob 6000ft. Commencing descent, cabin start to climb. Looks odd if you are staring at the roof. You don't notice it, though, if you would rather watch the shop!

[Pugilistic Animus]

Dial it in:cool:

[muduckace]

Extreme is more like La Paz Bolivia @ amout 14k feet. I understand approach/land procedure is oxy on. They have oxygen standing by in the airport I am told.

Only problem I ever has was out of Quito about 9.3k, for some reason auto-press did not kick in after T/O and we had to go manual, we had masks on and were at 14k cabin by the time we got the environment under control.

As P.A. said dial it in. In any aircraft with auto pressurization it only becomes a big deal if a system fails or the landing alt is dialed in incorrectly. E.G. had a new F/E who dialed in 2k feet for 200 feet. I thought the entry door was broken at the gate, tried the other side (stuck as well) and figured it out.

This has the potential to be dangerous. Most aircraft are not supposed to land with more than 1psi D.P. on the hull for structural reasons I suppose.

[R2112]

If a plane takes off from see level and wants to land at Lima, ..
...the difference is about 13M so there shouldn´t be much of a problem!
Thanks all, I did assume the system was automated enough to just re-adjust the cabin alt upwards as the aircraft descends but I´d never really given it much thought until landing at over 3000M!

[TopBunk]

On the B747-400, in fully automatic mode, the systems know the landing elevation courtesy of the FMCs. Once established in the cruise (so it thinks you will continue to destination) it adjusts the cabin altitude as a function of deltaP and landing elevation.

In a normal flight to a low elevation airport, the cabin altitude will be controlled by deltaP, and be typically below 6000ft for the initial cruise, rising a little as you step climb.

Flying, for example, to Mexico City, the landing elevation will control the schedule and when established in the cruise, the cabin will be scheduled to destination elevation.

This means that the cabin crew end up pushing trolleys around at an enhanced elevation for all the cruise.

As caring pilots we used to manually set the landing elevation to override the FMC until approaching ToD, at which point we would return control to the FMC and an increase of cabin altitude would result in time for touchdown.

This system worked very well.

Then a numpty manager got involved and instructed that the automatic pressurisation schedule must not be over-ridden:=:ugh::mad:

[muduckace]

Flying, for example, to Mexico City, the landing elevation will control the schedule and when established in the cruise, the cabin will be scheduled to destination elevation.




Only airport I have flown into where the air quality in the cabin at altitude was significantly better than on the ground (not the only just the worst), I started tearing up about 2000' agl.


Oh and R2112, I was going to leave that one alone but just about everyone I have spoken to that has not been there thinks that Lima is a high altitude airport. Used to love flying in and out of that layer of fog that persists for a good part of the year. Nothing like climbing out of that muck to see a full moon illuminate a sea of fog stretching into the Andes.

Another nice leg I used to do was Manaus - Lima. Over Lapaz, lake Titikaka and some very rugged terrain, great sight seeing.