Airliner pressurization at high altitude airports

wayne

wayne

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wayne
Do airlines immediately pressurize the plane's cabin leaving an airport at 12,500' to 8,000'?

We left Cusco at 11,000'+ and landed at Juliaca at 12,500'. Does that change what they do?

The upcoming flight is from Juliaca at 12,500' going to Lima, near sea level.
 
I would say when the doors close they start the pressurization.
 
The computer considers takeoff and landing elevations, gross weight, and cruise altitude. It programs a schedule based on that. The rate at which it pressurizes depends on which mode it’s in, e.g. ground, taxi, takeoff, etc. Once the door closes and the engines are running, it will start pressurizing. But is limited based on the mode so that it doesn’t overpressurize.
 
In my airplane, the plane is supposed to pressurize once the doors close. The rate at which the plane regulates is all automatic. We set the higher of the two field elevations into our pressurization panel, sit back, and watch the magic happen. There are times where we have to do an unpressurized takeoff due to an MEL and have to manually pressurize the aircraft.
 
Also depends on takeoff performance. Sometimes you need to go with packs off for max performance, and if your APU is inop for whatever reason, that means unpressurized.
 
I’m not saying there aren’t airplanes that will pressurize on the ground, but I don’t know of any. Outflow valves are generally on a squat switch for this reason. Opening the door of a pressurized airplane.... not good.
I do know of several airplanes that start pressurizing on takeoff roll regulated by thrust lever angle.
 
mtuomi said:
Also depends on takeoff performance. Sometimes you need to go with packs off for max performance, and if your APU is inop for whatever reason, that means unpressurized.
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IIRC, departing SLC in a 727 required packs off. What does the APU have to do with it? In my time at the panel, APUs only ran on the ground.
 
Greg Bockelman said:
Out of curiosity, what does gross weight have to do with pressurization?
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I really don't know. I think it uses gross weight and FMS cruise altitude to determine the target cabin altitude at cruise.
 
Kritchlow said:
I’m not saying there aren’t airplanes that will pressurize on the ground, but I don’t know of any. Outflow valves are generally on a squat switch for this reason. Opening the door of a pressurized airplane.... not good.
I do know of several airplanes that start pressurizing on takeoff roll regulated by thrust lever angle.
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Most pressurize slightly on the ground to seat the plug type doors. Every jet I flew either pressurized once the door closed or on thrust lever advance for takeoff.
 
jonnyjetprop said:
Most pressurize slightly on the ground to seat the plug type doors. Every jet I flew either pressurized once the door closed or on thrust lever advance for takeoff.
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Yes, the thrust lever is what I am familiar with.
 
Interesting. As an airport firefighter, we have always been told that the pressurization activates from the weight on wheels switch. Our training says the plane shouldn't be pressurized on the ground, but the system could fail causing pressurization so be aware of it. I wonder how our training manuals have that so wrong?
 
dtuuri said:
IIRC, departing SLC in a 727 required packs off. What does the APU have to do with it? In my time at the panel, APUs only ran on the ground.
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The 727 apu could only be used on the ground. Most stuff now it’s available in flight. Some smaller jets don’t have the margin within the engine to provide air for anti-ice and pressurization simultaneously during takeoff. So the environmental customers are pushed to the apu so engines only provide anti ice bleed for take off. Once up and going the apu is shutdown and engine runs all. If apu is inop then takeoff and initial climb is unpressurized.
 
midwestpa24 said:
Interesting. As an airport firefighter, we have always been told that the pressurization activates from the weight on wheels switch. Our training says the plane shouldn't be pressurized on the ground, but the system could fail causing pressurization so be aware of it. I wonder how our training manuals have that so wrong?
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I think your manuals are correct.
 
dtuuri said:
IIRC, departing SLC in a 727 required packs off. What does the APU have to do with it? In my time at the panel, APUs only ran on the ground.
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We can run APU bleed up to 15000ft. Most places we can do packs on, but hot/heavy/high we might need bleed from APU, or takeoff unpressurized if APU is inop.
 
EdFred said:
I would say when the doors close they start the pressurization.
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I don’t know of any that pressurize when the doors close. Most everything I’ve flown begins to pressurize when thrust levers are advanced into the takeoff range. After landing, the weight on wheels switch drives the outflow valve to full open. Someone can get hurt opening a door with even a little residual pressurization.

I believe the Airbuses have a red light in the small portal on the passenger door that flashes if the cabin is pressurized.
 
And the next question is, what if you takeoff from an airport at 14,000' or above and you are not pressurized? Do the masks drop?

:eek:
 
At A&P school IIRC, BIG IIRC, we learned about some fatalities on the ground due to a malfunction of the system. Like they could not get the door open? Does that make sense, I can't remember the particulars.
 
alfadog said:
At A&P school IIRC, BIG IIRC, we learned about some fatalities on the ground due to a malfunction of the system. Like they could not get the door open? Does that make sense, I can't remember the particulars.
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Not that long ago in Northern Europe a Gulfstream captain was killed when they left the APU running and closed the door for the walkaround to warm up the cabin. They missed the part about the checklist to verify outflow valve was open. FA was left inside and she was pointing out that her ears were popping. CA returned to investigate, opened the door and boom, killed him instantly.
 
It is pressurized to 1/8 psi IIRC. Not enough to prevent opening a door. At least on the airplanes I have flown.

But this discussion makes me want to go look in the book to see what actually happens.
 
Prior to takeoff, controller set to cruise altitude plus 500 to 1000 feet.
Prior to descent, controller set to destination altitude plus 500 feet.
With door closed, engines running and environmental on, the cabin vsi will show a momentary descent, but not enough to register on the differential indicator or the ears. That is with the safety valve wide open.
 
SkyDog58 said:
And the next question is, what if you takeoff from an airport at 14,000' or above and you are not pressurized? Do the masks drop?

:eek:
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I know our planes have a high altitude switch we can select that reconfigures the altitudes at which you get certain warnings.
 
The 737 manual says that the cabin starts to pressurize on the ground at higher power settings. I take it to mean basically when takeoff power is set.

And it slowly depressurizes after landing while taxiing in.
 
Didn’t American have a F/A get killed several years ago? Seems the cabin had residual pressure and when he opened the door it flew open and jerked him out of the plane. Long way down from an A300.
 
Busflyer said:
Didn’t American have a F/A get killed several years ago? Seems the cabin had residual pressure and when he opened the door it flew open and jerked him out of the plane. Long way down from an A300.
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https://www.sun-sentinel.com/news/fl-xpm-2000-11-22-0011211113-story.html

This might have been what we were told about in A&P school. Would have been catastrophic if there were a cabin fire.

"It seems the airplane was still pressurized for some reason we don't know," Prellezzo said.

On Monday, American Airlines flight 1291 had just taken off for Port au Prince, Haiti, with 121 passengers and nine crew members on board when an emergency light came on in the cockpit, indicating a possible engine fire. The pilot turned the Airbus A300 back.

Once on the ground, the flight attendants tried to open the doors and activate the emergency chutes so passengers could slide off the plane. But passengers said the attendants could not get the doors to open. When Chiu, 34, finally opened the door he was ejected. He was pronounced dead on arrival at Jackson Memorial Hospital.
 
wayne said:
Do airlines immediately pressurize the plane's cabin leaving an airport at 12,500' to 8,000'?

We left Cusco at 11,000'+ and landed at Juliaca at 12,500'. Does that change what they do?

The upcoming flight is from Juliaca at 12,500' going to Lima, near sea level.
Click to expand...

Maybe. Depends on the airplane and climb requirements. What airplane are you taking about? Some, (most?) airliners have some system limitation that requires a trick of the system to allow operations at high alt airports.

The ones I've flown at high alt required a bleeds off takeoff. So, the pacs were being run by the apu, or it's a no pacs no pressurisation takeoff. After takeoff, turn the bleeds from the apu to the engines, and flick on a pac. One at a time for the ears.

Keeping the engines from bleeding air gives them a few more lbs of thrust. The idea is that these extra pounds of thrust will make the difference of making it over that mountain or not if one of the blowers quits. SE performance is the determining factor first, pressurisation system limitations after that.

Make sense?
 
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