Commercial operations in high-altitude airports

[Gabriel]

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.

[ATLcrew]

Or is there a "high altitude airport" switch that I never heard of?

Yes.

[Gabriel]

Yes.

Thanks, then it is simpler than I thought. A little more detail would be nice, though. Let me guess...
- The switch increases the triggering threshold for the cabin altitude warning and for the pax masks deployment.
- The pilots need to don their masks and use O2 during taxi, take off and the first part of the climb, as well as during the latest stages of the descent/approach and landing, since they will be operating at a cabin altitude of more than 10000ft.

[3WE]

Crap-shoveling Aggie comments in red.

This is a question for out professional pilots in the forum...

In particular I am interested in issues regarding the high cabin altitude that the plane will have on the ground.-- Interesting area that I never thought much about.

...Blah Blah...explanation...examples...Blah blah blah...

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.

Umm...actually, most of the time EVEN AT 'LOW ALTITUDES' the plane is pressurized before takeoff*...ATL crew will have to cite the type-specific procedure, but it seems to happen very shortly after engine start (or even door closure?). I guess you have leaky ears, because I usually can detect it...and yes, that means you aren't popping the emergency exits until 1. The pilot depressurizes, 2. The power shuts off, or 3. The "cabin-pressure-vessel" is compromised.

There are also an array of type-specific procedures to depressurize the cabin right before landing (again, your ears suck, and mine can detect this if I pay attention). I recall my J-31 buds twisting a knob at about 50 ft (Pressure vents from inner ear), and even my now-10-year-old DC-9 series MSFS planes (yes MSFS) show the pressure dump port open on touchdown. I THINK this is done to 'enable' doors and exits. I also think the Value Jet DC-9-30-pronged-gear-near-total-disaster may have been due to cowboy improvisation to depressurize the plane...(popped the spoilers).

I THINK the plane is pressurized before takeoff as a method to test the pressurization system. There may be other reasons, but I am an agronoengineer, not aero.

Oh, and one other thing- while your ears my be leaky, I think you can often detect the pressurization in the flow of the little air-vents (although that's harder as those systems go through big off and on cycles with ground-sourced AC vs, APU and I even think DC-9 series planes take it ALL for start ups...)


...Blah Blah...more examples...more explanations...Blah Blah Blah...

However, the effect of the high altitude in the cabin altitude is something I had not thought before, and that I never heard about.

Yes, I concur, all those system intricacies where the plane is way high where you probably have at least some dose of comfort pressurization (or hypoxia-prevention pressurization for your really high airports)...Interesting stuff.

...and yes, I saw the checklist with all the cool-but-cryptic acronyms on it.

[Gabriel]

Ok, so high altitude = fewer molecules of air per unit of volume. This means that the plane will need more speed to have enough molecules of air passing around the wings to be able to take off and fly. That also means that there are fewer molecules of oxygen to be combined with fuel, so the engines produce less power. The combination of those means way much runway is needed to achieve a higher speed with less power. Also, because of the less power and higher speed, the climb slope will be impacted negatively: a plane will not be able to climb anywhere close to as steep as it would at sea level. Because of that, we want to reduce drag to recover some of the lost climb slope, so the take-off is done with the flaps not so extended as in a normal take-off at sea level. This, in turn, means that even more speed is needed to lift off. All this means that the speed at lift off will be much faster than in a normal take-off, and then there are two things that mast be analyzed cautiously: 1) There is only so much energy that the brakes can absorb before they get too hot and fail. What happens if you need to abort the take-off at such a high speed? ans 2) The tires have a speed limit after which they can fail, and we don't want that. Many times planes operating is very high airports are fitted with special high-speed tires (and with high speed I mean higher than normal which is already high).

Here you have: zero acronyms.

Now, about the pressurization, I know that at least in some planes the outflow valve opens automatically to full with WOW (that is weight on wheels), so it opens at touchdown (and you can feel it if the pressure was not equalized just before that) and will remain open until the plane lifts off again. The air you feel from the AC can be recirculated in a good part, and the rest, if the outflow valve is open, any air that you inject into the plane will displace the same volume out of the outflow valve. SO feeling that air is being "injected" is not the same than the plane being pressurized.

[3WE]

1. ***so the engines produce less power***.

2. ***SO feeling that air is being "injected" is not the same than the plane being pressurized.***

1. I have been a parlour-talking, ass-hat, forumuite for many years and many forums. I distinctly remember some ATP (Nav5, VNav, Dummy Pilot) saying that due to a number of factors and limitations, turbofans do not necessarily 'lose power' until well up into the flight levels.

Yeah, sure our light plane engines lose power and in lots of theoretical ways ALL engines lose power due to there being less 02 available in the combustion area, but the airliners "we" ride have these crazy sophisticated computer controlling thingies that tweak all sorts of stuff, AND aero engineers are always thinking about stress and temperatures, and blah blah blah.

Given all of that, I believe 5Nav or NavV, that for practical purposes and perhaps some sort of flat rating type of thing, a turbofan engine may lose very little or no power at what a 172-jock would consider a high altitude.

2. Agroengineer will now talk MORE out of his ass about what he only knows high level details.

The inner ear is something of a closed vessel. There is a Eustacian tube connecting the ear to the throat. The inner ear fairly easily dissipates and equalizes pressure when the outside pressure falls. However, the Eustacian tube tends to clog...AND the inner ear often senses when the outside pressure rises.

If one has decent ear sensitivity, they can sense a minor increase in cabin pressure upon door closure/engine start/something as a plane pulls away from the gate at low altitude.

The air flowing from overhead vents is affected by many mechanisms. However, it is plausible parlour speculation that when the pilot 'pressurizes' the plane as you leave the gate, it might be manifested in a change in airflow. If this happens at the same time as your ear-popping, I bet MORE THAN ONE BEER that the airflow change is being affected by the pressurization at this particular time.

Cabin airflow is also affected by power changes, AC source changes, and a host of other reasons- to assume that airflow change = pressurization is bad cause-effect reasoning.

To assume that your ears pop + airflow changes at the same time = pressurization change...there's some chance of that being cause-effect.

Comprende? (Perhaps not- I am not sure if you are familiar with the inner ear feelings of a descending plane/increasing pressure...speaking of cause-effect...it tends to make infants cry )