mozart
Posts: 2020
Joined: Thu Aug 14, 2003 12:21 am

Cabin Pressurisation At High Altitude Airports

Tue Feb 27, 2007 3:13 am

I am wondering how this works...

I think I know that when planes are parked on the ground, the "cabin alttitude" is equal to the "outside altitude". Then, when lifting off, cabin altitude goes up, but never as high as the "real outside altitude" (this is the point of the pressurized cabin, right?).

I also think I know that most modern planes have oxygen masks drop down when cabin altitude exceeds 10,000 ft.

Now what happens in the case of airliners flying to airports that are at altitudes higher than 10,000 ft (for instance, La Paz, Cuzco)? When arriving into these airports, cabin altitude will slowly near "real outside altitude", thus higher than 10,000 ft. Does that mean that when approaching a high altitude airport cabin pressure actually goes up, from its "artificial" pressurized level to the actual level of the airport? And since it will exceed 10,000 ft, will oxygen masks drop down? Or will people simply fade?

How does it concretely work in La Paz? I know that pilots wear oxygen masks, but what about pax? Masks, or risk to fade?

Thanks for any insights
 
CoolGuy
Posts: 366
Joined: Tue Jan 18, 2005 8:13 am

RE: Cabin Pressurisation At High Altitude Airports

Tue Feb 27, 2007 4:48 am

Quoting Mozart (Thread starter):
How does it concretely work in La Paz? I know that pilots wear oxygen masks, but what about pax? Masks, or risk to fade?

Is that really true about pilots to/from La Paz?
 
cdekoe
Posts: 53
Joined: Thu Nov 30, 2006 1:14 am

RE: Cabin Pressurisation At High Altitude Airports

Tue Feb 27, 2007 4:55 am

All modern CPC systems that I know will regulate Cabin Pressure to or from Field Altitude with a variable pressure rate.
I.e. taking off from a field above 8000 ft will trigger a greater than normal pressure rate to bring cabin altitude down to 8000 ft within a prescribed time-limit (usually this is around 10 to 15 minutes).
In no circumstances will masks drop due to an automatically scheduled cabin pressure over 8000 ft.

In the La Paz example, the cabin altitude would reach field elevation about 10 minutes before landing.
The impact on the pax is not any greater than when they actually exit the aircraft...  Wink

Regards.
We can't solve problems by using the same kind of thinking we used when we created them.
 
broke
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Joined: Wed Apr 24, 2002 8:04 pm

RE: Cabin Pressurisation At High Altitude Airports

Tue Feb 27, 2007 6:26 am

The cabin altitude warning horn, generally, will sound when the cabin altitude exceeds 10,000'. The passenger oxygen masks will generally deploy when the cabin altitude exceeds 14,000'.
For aircraft operating into high altitude airports such as La Paz, Bolivia; these settings are higher to avoid unnecessary warnings and unnecessary mask deployments, especially when the ambient barometric pressure is low.
 
AirWillie6475
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Joined: Thu Jan 27, 2005 1:45 pm

RE: Cabin Pressurisation At High Altitude Airports

Tue Feb 27, 2007 1:31 pm

Quoting Mozart (Thread starter):
Now what happens in the case of airliners flying to airports that are at altitudes higher than 10,000 ft (for instance, La Paz, Cuzco)? When arriving into these airports, cabin altitude will slowly near "real outside altitude", thus higher than 10,000 ft. Does that mean that when approaching a high altitude airport cabin pressure actually goes up, from its "artificial" pressurized level to the actual level of the airport? And since it will exceed 10,000 ft, will oxygen masks drop down? Or will people simply fade?

Not that many commercial airports that are higher than 10K. The cabin altitude is set manually to the airport elevation and the cabin is depressurized to match the elevation, in the case of La Paz, 13K What I'm not certain is if pilots bring the altitude to 13K right from the beginning of the flight or during the decent. I was talking to an airline pilot and they said usually if they are going to an airport that is higher than the cruise cabin altitude(6K cabin 8K airport) they just manually bring the cabin up to the airport elevation. But this was for an 8K foot high airport, don't know what they would do for a 13K one.

Quoting Mozart (Thread starter):
How does it concretely work in La Paz? I know that pilots wear oxygen masks, but what about pax? Masks, or risk to fade?

No risk of fading below 15K as long as you aren't moving around so it's not a major problem. Even for pilots, they only wear the oxygen because over 12K the mental capacity to do work is reduced, however for pax sitting in the back it's not a problem. I personally went up to 12K unpressurized in a light twin once. I did feel a slight headache, similar to what you feel at sea level if you exhale continuously without breathing in however I never felt uncomfortable.
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Cabin Pressurisation At High Altitude Airports

Tue Feb 27, 2007 2:56 pm

Quoting Mozart (Thread starter):
Does that mean that when approaching a high altitude airport cabin pressure actually goes up, from its "artificial" pressurized level to the actual level of the airport? And since it will exceed 10,000 ft, will oxygen masks drop down? Or will people simply fade?

Yes... cabin altitude will INCREASE during the descent so as to be at airport elevation prior to landing.
No... pax oxygen will NOT deploy automatically. That is because on those airplanes that are qualified to La Paz (and other high altitude airports) have system modifications that reset the alarms & systems to activate at higher than normal cabin altitudes. Been a long time since I flew those birds, but IIRC 14,000' cabin altitude sounds the alarm and 14,500' cabin altitude causes automatic pax O2 mask deployment (or somewhere close to those numbers).

Quote:
How does it concretely work in La Paz? I know that pilots wear oxygen masks, but what about pax? Masks, or risk to fade?

IIRC, at AA the pilots are required to be on supplemental O2 prior to starting descent until after acft is parked at the gate. F/A's and pax.... no noticable change (unless someone forgets to flip the switch to the modified cabin altitude warning system).  Wink
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