TOGA
Topic Author
Posts: 31
Joined: Wed Jul 18, 2001 9:12 pm

Cabin Pressurisation

Tue Jan 10, 2006 9:07 pm

What happens if the pilot at cruise altitude forgets to set the landing altitude on the pressurisation panel and a decent is commenced ?. Does the differential pressure increase and does the cabin altitude remain as it was at cruise level ?
 
kaddyuk
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Joined: Thu Nov 08, 2001 1:04 am

RE: Cabin Pressurisation

Tue Jan 10, 2006 9:55 pm

IIRC, The pressurisation panel for the most part is kept in the "Auto" position.

There are delta p sensors which will not allow the aircraft to have a negative pressure inside the aircraft. Blow out panels will equalise the pressure if an aircraft descends below the cabin altitude...
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PhilSquares
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RE: Cabin Pressurisation

Tue Jan 10, 2006 10:25 pm

On most "glass" aircraft, both Boeing and Airbus, the landing field elevation is taken from the FMC and the controller uses that to set the descent schedule.
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TOGA
Topic Author
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RE: Cabin Pressurisation

Tue Jan 10, 2006 11:29 pm

Thanks for that guys. I am thinking of older aircraft like the DC9/B727 where there may not be the same level of automation as there is on modern aircraft.
 
SlamClick
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RE: Cabin Pressurisation

Wed Jan 11, 2006 12:18 am

I've been away from the DC-9 for twenty years but I'll take a stab. I don't remember the system well enough to give you the valve operation sequence but the worst that could happen is that you might drive the cabin to the overpressure relief valve setting. 8.06 PSID if I remember correctly. And you might land at that.

I believe there is an auto feature that would then drive the outflow valve full open but, even if you shut the packs off it would be a few minutes before you would be physically able to open the doors.

If you are asking if it would blow up like a balloon, no it woudn't.  Smile
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HAWK21M
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RE: Cabin Pressurisation

Wed Jan 11, 2006 1:12 am

Quoting TOGA (Thread starter):
Does the differential pressure increase and does the cabin altitude remain as it was at cruise level

Upto a particular limit before the Alt/Stby system takes over.Safety Relief Valves & Pressure Equalization valves serve a purpose too.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
jetstar
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RE: Cabin Pressurisation

Wed Jan 11, 2006 11:19 am

Quoting TOGA (Thread starter):
What happens if the pilot at cruise altitude forgets to set the landing altitude on the pressurisation panel and a decent is commenced ?. Does the differential pressure increase and does the cabin altitude remain as it was at cruise level ?

On older jets like the JetStar, we had to manually set the pressurization controller before descent to the field elevation. At FL370, the cabin altitude was about 6500 feet at 8.9 psid and if we were landing at a high elevation airport like Cheyenne WY which is about 6200 feet elevation, we did not have to reset the controller. During descent the cabin differential pressure would drop from 8.9 psid to 0 as we descended and landed. We normally used 500 feet per minute for the cabin rate of climb and descent which made for a very smooth cabin descent or climb.

If we were to forget to reset the controller for landing and were landing at a lower elevation airport, as the airplane passed 6500 feet the outflow valves would open wide and the airplane would then become unpressurized and the cabin would follow the airplane down with the same rate of descent.

Our normal procedure was to set the controller 500 feet below the elevation of the airport we were landing at and when we landed the outflow valves would open and there would be a slight bump up to field elevation and this served to help clear our ears, especially if we made a quick decent to landing.

Flying to out of the way non airline airports with little or no traffic ATC would usually give us a fast descent. One time after passing through FL180 on descent, it was a severe clear day and we had the airport it sight so we canceled IFR. We called the tower at 15,000 feet abeam downwind, we then flew a long downwind and turned a wide base at 8000 feet and landed. We were able to do this because there was no other traffic in the airport control zone. To do this we had 3 engines at idle, one up to 80 percent to maintain enough bleed air for cabin pressurization and the speed brake and approach flaps out so we were dropping like a brick. The field elevation was around 3000 feet and the cabin controller had already brought the cabin to field elevation so there was no problem.
 
sanjet
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RE: Cabin Pressurisation

Wed Jan 11, 2006 11:31 am

Don't quote me on this, but is tehre a system that depressurizes the aircraft upon landing. ex: Outflow valves open wide because of weight on wheels?
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Grbld
Posts: 349
Joined: Tue Dec 06, 2005 9:25 pm

RE: Cabin Pressurisation

Sun Jan 15, 2006 12:18 am

Quoting Sanjet (Reply 7):
Don't quote me on this, but is tehre a system that depressurizes the aircraft upon landing. ex: Outflow valves open wide because of weight on wheels?

That's correct.

On airliners flying today, the cabin will start to descend (increase pressure) when you descend more than a certain amount from your cruise altitude. If you're flying an aircraft that needs manual setting of the landing altitude (737/757/767) and you didn't, it will most likely be set at the altitude of your departure airport, so the cabin will start to descend anyway.

During the descent, you should pick up on this as you set up the aircraft for the approach (and check elevation and descent limit altitudes) and there should be at least one point where you check pressurization during the descent anyway.

Grbld

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