Cubsrule
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High Altitude Modifications?

Sun Sep 16, 2007 3:31 am

I read in AA's inflight magazine last night that AA has 6 752s specially modified for ops at LPB. What did they do to these birds? Do other aircraft need modifications to operate at LPB or other super-high airports? And, just because I'm curious, what ships are so modified?
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tdscanuck
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RE: High Altitude Modifications?

Sun Sep 16, 2007 7:47 am

Quoting Cubsrule (Thread starter):
What did they do to these birds? Do other aircraft need modifications to operate at LPB or other super-high airports?

The most common high-altitude mod is to change the pressure controllers so that they will let you get the cabin altitude to match with the airport altitude. If you don't, you can't open the doors and, if you did, the oxygen masks would drop.

Tom.
 
KFLLCFII
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RE: High Altitude Modifications?

Sun Sep 16, 2007 8:24 am

Quoting Tdscanuck (Reply 1):

The most common high-altitude mod is to change the pressure controllers so that they will let you get the cabin altitude to match with the airport altitude.

Isn't that what they do anyway?


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Lower right corner: "Land Alt(itude)"
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RE: High Altitude Modifications?

Sun Sep 16, 2007 8:27 am

...Or in the case of the 757:


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Two-thirds of the way down, center-right: "LDG ALT(itude)"

?
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tdscanuck
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RE: High Altitude Modifications?

Sun Sep 16, 2007 11:07 am

Quoting KFLLCFII (Reply 2):
Quoting Tdscanuck (Reply 1):

The most common high-altitude mod is to change the pressure controllers so that they will let you get the cabin altitude to match with the airport altitude.

Isn't that what they do anyway?

Yes, but maximum cabin altitude is supposed to be ~8,000'. The system is not supposed to let it get higher than that. If your landing field is at 10,000' the aircraft doesn't want to get the pressure that low and, if you opened the door without modification, it would think you had an emergency decompression and drop the oxygen masks.

Tom.
 
Cubsrule
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RE: High Altitude Modifications?

Sun Sep 16, 2007 11:42 am

So if I were flying, say, a Q400 to LPB, I could do it without modification?
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Jetlagged
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RE: High Altitude Modifications?

Sun Sep 16, 2007 2:51 pm

Quoting Tdscanuck (Reply 4):
Yes, but maximum cabin altitude is supposed to be ~8,000'. The system is not supposed to let it get higher than that. If your landing field is at 10,000' the aircraft doesn't want to get the pressure that low and, if you opened the door without modification, it would think you had an emergency decompression and drop the oxygen masks.

The masks won't drop at a 10000 ft field because that is triggered by cabin altitude being greater than 14000 feet, not cabin rate. Secondly, when on the ground, the controller moves the outflow valves so as to depressurise the aircraft. Therefore the doors will open, it just might take a while.

IIRC, pressure controllers normally ramp cabin pressure between cruise altitude and landing altitude as the aircraft descends. So as long as correct landing elevation can be selected there should be no problem. There isn't a "not greater than 8000 ft" limit in the controller.

There are mods that are necessary for operation from high altitude airfields. According to this thread it is done by changing the warning and mask dropout altitudes. The controller itself is not affected.
Cabin Pressurisation At High Altitude Airports (by Mozart Feb 26 2007 in Tech Ops)
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tdscanuck
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RE: High Altitude Modifications?

Sun Sep 16, 2007 3:08 pm

Quoting Jetlagged (Reply 6):
The masks won't drop at a 10000 ft field because that is triggered by cabin altitude being greater than 14000 feet, not cabin rate.

Thanks for catching that...using 10,000' was a bad example.

Quoting Jetlagged (Reply 6):
IIRC, pressure controllers normally ramp cabin pressure between cruise altitude and landing altitude as the aircraft descends.

They ramp between the cruise cabin pressure (typically 6500-8000') and landing altitude...the controller would never let the cabin pressure reach cruise altitude. The problem comes up when your landing altitude is higher than the cabin altitude.

Quoting Jetlagged (Reply 6):
There isn't a "not greater than 8000 ft" limit in the controller.

It may not be 8,000' depending on the exact controller, but there is an upper limit.

Quoting Jetlagged (Reply 6):
According to this thread it is done by changing the warning and mask dropout altitudes. The controller itself is not affected.

The controller, at least on the aircraft I'm thinking of, is what determines the warning and mask drop altitudes...changing those means altering the controller.

Tom.
 
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RE: High Altitude Modifications?

Sun Sep 16, 2007 3:35 pm

Quoting Tdscanuck (Reply 7):
They ramp between the cruise cabin pressure (typically 6500-8000') and landing altitude...the controller would never let the cabin pressure reach cruise altitude.

Obviously (or so I thought) I meant the programmed cruise altitude for the cabin.

Quoting Tdscanuck (Reply 7):
The problem comes up when your landing altitude is higher than the cabin altitude.

No problem, it ramps the cabin altitude towards selected landing altitude as the aircraft descends. This can mean the cabin is climbing as the aircraft descends.

Quoting Tdscanuck (Reply 7):
The controller, at least on the aircraft I'm thinking of, is what determines the warning and mask drop altitudes...changing those means altering the controller.

Fair enough. What I meant was the cabin pressure control laws are not much affected, just the warning thresholds.

Quoting Tdscanuck (Reply 7):
It may not be 8,000' depending on the exact controller, but there is an upper limit.

It's usually just below the cabin altitude warning limit. It must be greater than the maximum selectable landing altitude.
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tdscanuck
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RE: High Altitude Modifications?

Tue Sep 18, 2007 1:03 am

Quoting Jetlagged (Reply 8):
Quoting Tdscanuck (Reply 7):
The problem comes up when your landing altitude is higher than the cabin altitude.

No problem, it ramps the cabin altitude towards selected landing altitude as the aircraft descends. This can mean the cabin is climbing as the aircraft descends.

I didn't know that! A.net is great. That would really confuse a person expecting their ears to pop "in" during descent.

Thanks for the clarification.

Tom.
 
redflyer
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RE: High Altitude Modifications?

Mon Oct 08, 2007 12:19 am

This is a fascinating topic. It brings up a question I have. One of my employees took a trip down to CUZ and we were discussing this very topic when he returned last week. One question I have is this: When the aircraft took off, they were already at ~11,000 feet so at what point would the cabin start to pressurize back down to an 8,000 foot level and how fast would this occur? I would think a fully loaded A319 (or any plane for that matter, although I realize the advantage to the A320 series is its high-altitude performance) taking off from 11,000 feet would be somewhat conservative in the use of bleed air siphoned off the engines; at least during the initial climb.
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Viscount724
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RE: High Altitude Modifications?

Tue Oct 09, 2007 5:58 am

Quoting Tdscanuck (Reply 1):
Quoting Cubsrule (Thread starter):
What did they do to these birds? Do other aircraft need modifications to operate at LPB or other super-high airports?

The most common high-altitude mod is to change the pressure controllers so that they will let you get the cabin altitude to match with the airport altitude. If you don't, you can't open the doors and, if you did, the oxygen masks would drop.

I recall reading that masks dropped when the doors were opened on at least a couple of the first Eastern B727 flights to LPB when EA acquired Braniff's South America routes.
 
AAR90
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RE: High Altitude Modifications?

Tue Oct 09, 2007 3:20 pm

Quoting Cubsrule (Thread starter):
What did they do to these birds?

Cabin pressurization controller schedule changed with range increased to 14,000' landing altitude to accomodate the airport altitudes above the normal 8,400' limit. Passenger O2 mask automatic deployment changed to (IIRC) 14,500' cabin altitude. Cockpit O2 supply increased to accomodate increased mandatory pilot use of supplemental O2. All these activated with a simple switch in the cockpit. It has been a very long time since I flew that fleet so I might be off on the numbers.  Wink

Quoting Cubsrule (Thread starter):
Do other aircraft need modifications to operate at LPB or other super-high airports?

Yes. Most airliners' standard configuration is for operations to around 8,000' field elevations, but you can order whatever you want as an option.

Quoting Cubsrule (Thread starter):
And, just because I'm curious, what ships are so modified?

Excluding the ex-TWA planes, ALL AA 752s are "High Altitude" equipped.

Quoting RedFlyer (Reply 10):
When the aircraft took off, they were already at ~11,000 feet so at what point would the cabin start to pressurize back down to an 8,000 foot level and how fast would this occur?

It would depend upon what system is installed and what procedures are used with that system. AA's 757s will hold cabin altitude at the field elevation as the plane climbs (cabin goes no higher). Procedure is to level off and engage ALT HOLD at FL250. After approx. 1 minute in ALT HOLD mode, the cabin pressurization system will "reset" and begin to lower the cabin to normal cruise altitudes (around 8,000' cabin altitude). Nice question though. I flew that fleet for 10 years and never knew that... it is buried in the LPB pages and I never flew "deep south."  Wink
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