This is a question for out professional pilots in the forum...
What special precautions or procedures are used for operations in airports at very high altitude?
In particular I am interested in issues regarding the high cabin altitude that the plane will have on the ground.
There are several airports at more than 8000ft, which is the maximum cabin altitude at which the the planes are pressurized.
So, even before closing the door, the plane will be at a cabin altitude that is higher than the maximum.
Then we have a few airports that are above 10000ft. That is the altitude that triggers the cabin altitude warning.
And there are even a couple of airports in China above 14000ft, the altitude at which the O2 masks in the cabin deploy automatically.
So how do operations occur in these conditions? Is the plane pressurized even before take-off? I doubt it, it would be dangerous in case an evacuation is needed since the doors will not open if the plane is pressurized.
Is the cabin altitude alarm disconnected or disregarded?
Are the O2 masks disabled?
Or is there a "high altitude airport" switch that I never heard of?
It is interesting that, in those cases, the plane will climb but the cabin altitude will descend to 8000ft (unlike in normal operations where the cabin altitude also climbs with the plane albeit at a slower climb rate).
I am more aware of other considerations in high-altitude airports operations like:
- Reduced air density (especially since most of the high altitude airports have also temperatures well above the standard ones for those altitudes), which brings 2 consequences:
- Reduced engine thrust available.
- Increased true speed for any given airspeed (including the take-off speeds)
- The combinations of these 2 factors mean a much increased runway needed, since the plane will accelerate at a slower rate but will need to achieve a higher speed (some runways in high altitude airports exceed the 5Km / 16500ft)
- The reduced thrust also means reduced climb rate and gradient, which is mitigated by the use of reduced flap settings to reduce the drag.
- The maximum brake energy speed, and the maximum tire speed (both of which are in ground speed and hence related to true airspeed, not IAS) can become limiting factors for V1 and Vr and for the maximum wight that the plane will be able to operate under those conditions. Contradictory with the previous point, increased flaps settings would be beneficial here but they tend not to be available due to the impact it would have in the climb performance.
However, the effect of the high altitude in the cabin altitude is something I had not thought before, and that I never heard about.
What special precautions or procedures are used for operations in airports at very high altitude?
In particular I am interested in issues regarding the high cabin altitude that the plane will have on the ground.
There are several airports at more than 8000ft, which is the maximum cabin altitude at which the the planes are pressurized.
So, even before closing the door, the plane will be at a cabin altitude that is higher than the maximum.
Then we have a few airports that are above 10000ft. That is the altitude that triggers the cabin altitude warning.
And there are even a couple of airports in China above 14000ft, the altitude at which the O2 masks in the cabin deploy automatically.
So how do operations occur in these conditions? Is the plane pressurized even before take-off? I doubt it, it would be dangerous in case an evacuation is needed since the doors will not open if the plane is pressurized.
Is the cabin altitude alarm disconnected or disregarded?
Are the O2 masks disabled?
Or is there a "high altitude airport" switch that I never heard of?
It is interesting that, in those cases, the plane will climb but the cabin altitude will descend to 8000ft (unlike in normal operations where the cabin altitude also climbs with the plane albeit at a slower climb rate).
I am more aware of other considerations in high-altitude airports operations like:
- Reduced air density (especially since most of the high altitude airports have also temperatures well above the standard ones for those altitudes), which brings 2 consequences:
- Reduced engine thrust available.
- Increased true speed for any given airspeed (including the take-off speeds)
- The combinations of these 2 factors mean a much increased runway needed, since the plane will accelerate at a slower rate but will need to achieve a higher speed (some runways in high altitude airports exceed the 5Km / 16500ft)
- The reduced thrust also means reduced climb rate and gradient, which is mitigated by the use of reduced flap settings to reduce the drag.
- The maximum brake energy speed, and the maximum tire speed (both of which are in ground speed and hence related to true airspeed, not IAS) can become limiting factors for V1 and Vr and for the maximum wight that the plane will be able to operate under those conditions. Contradictory with the previous point, increased flaps settings would be beneficial here but they tend not to be available due to the impact it would have in the climb performance.
However, the effect of the high altitude in the cabin altitude is something I had not thought before, and that I never heard about.
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