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Re: Pressurization Of A/c After Landing...  
User currently offlineAircanada From Canada, joined Jul 1999, 148 posts, RR: 0
Posted (10 years 11 months 2 weeks 2 days ago) and read 2238 times:

...specifically AA A300 and the outward explosion of doors due to aircraft pressurization.

I apologize for the long topic, but the first time I posted this it was removed because it wasn't "Descriptive" enough. Gotta cover my bases now! I hate having to retype things.

This is a stupid question, I'm warning you. So you may just want to pass over
it now. However, if you could help, I would greatly appreciate it.

I was reading up in the archives about plug locks on aircraft doors and about
the AA A300 incident where
the cabin remained pressurized after the system was switched over to manual and
a FA was thrown from the
plane and killed because the door was thrown outwards from the pressure.

Most aircraft cabins aren't pressurized to anything near ground level (8000 ft
seems to be the norm). When at cruise, the pressure on the outside of an
aircraft is less than that on the inside. Therefore, a hole in the fuselage
causes everything to be sucked out. When on the ground, you are usually lower
than 8000 ft. Therefore, if you were still pressurized at 8000ft on the ground,
but you were sitting at 3000 ft, if you opened the door, wouldn't it be blown
inwards (into the cabin) as the pressure outside is now greater?

This IS something I should know, but I never really understood
how pressurization systems worked. Any help would be greatly appreciated.

Regards,
Andrew.



6 replies: All unread, jump to last
 
User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 1, posted (10 years 11 months 2 weeks 1 day 21 hours ago) and read 2212 times:

I guess my answer was removed also.

Aircraft can be pressurized on the ground. It is a standard practice for us maintenance folks in order to check the system, find leaks, blow out toilets (back in my pax days). On the ground we pressurize to a pressure below sea-level. In flight as then aircraft climbs the pressure outside the pressure vessel decreases. We increase the flow of air into the vessel and restrict its exit; pressurization. On the ground we just close the outflow valve(s) and all exterior doors and fill the aircraft with air, like a balloon.


User currently offlineFutureualpilot From United States of America, joined May 2000, 2602 posts, RR: 8
Reply 2, posted (10 years 11 months 2 weeks 1 day 21 hours ago) and read 2206 times:

Think of a balloon. If you blow it up at 8000ft., and bring it down to 3000ft. it doesnt get smaller by a significant amount. Im not expert but my guess is the pressure change just isnt enough first of all, and secondly the crew usually de-pressurizes the a/c to the local settings before they land to make it easier on the passenger's ears when they open the door, and to be able to open the doors.


Life is better when you surf.
User currently offlineFDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 34
Reply 3, posted (10 years 11 months 2 weeks 1 day 20 hours ago) and read 2199 times:

>>>When on the ground, you are usually lower
than 8000 ft. Therefore, if you were still pressurized at 8000ft on the ground,
but you were sitting at 3000 ft, if you opened the door, wouldn't it be blown
inwards (into the cabin) as the pressure outside is now greater?
<<<

Aircraft are designed to withstand a positive differential of pressure, meaning a greater pressure inside than outside, like a balloon.

Aircraft weren't designed to withstand continual or great negative differential pressure (higher pressure on the outside than the inside). Because of this, aircraft are equiped with negative pressure relief systems in order to prevent the fuselage from turning into the proverbial squashed beer can. Should the pressure on the outside be greater than the inside, it will quickly equalize.

Now during landing and taxi-in, some aircraft have a small positive differential pressure that though small, carries with it a tremendous amount of potential force because of the large surface area it is bearing down on (this case an entry door).

On most planes, even a small differential pressure will prevent the door from being opened. But on the A300 & A310 the doors (entry and cargo) have a large handle giving a very high mechanical advantage in that a door may be opened with residual differential pressure remaining onboard, with potentially disasterous consequences.

During my stint at Continental during the 1980's, a cargo loader opened a belly door with residual pressurization. The door fleww open leaving this poor soul in tough shape.

Because of this potential problem, A300's are equipped with red residual pressure warning lights and horns on all doors should someone (inside or out) attempt to open any door with pressurization still present.



You're only as good as your last departure.
User currently offlineDc10hound From United States of America, joined Dec 2000, 463 posts, RR: 5
Reply 4, posted (10 years 11 months 2 weeks 1 day 18 hours ago) and read 2166 times:

the AA A300 incident where
the cabin remained pressurized after the system was switched over to manual


And that contributed to the problem. With the system in manual, the aircraft will not depressurize automatically (outflow valves open) before landing or after touchdown. The outflow valves on aircraft 056 where left in the closed position and the airplane built up a positive differential pressure compared to ambient (outside) air pressure. Initially, the FA was not able to open the door. When the Delta P decayed enough, and the door lever was pulled, wham! Everything within 6 feet of that door and the FA was blown out.

A300's are equipped with red residual pressure warning lights and horns on all doors should someone (inside or out) attempt to open any door with pressurization still present

That system has been deactivated on AAL A300s due to false warnings.




"Eagles soar. But weasels never get sucked into jet intakes.."
User currently offlineXXXX10 From United Kingdom, joined Jan 2000, 777 posts, RR: 0
Reply 5, posted (10 years 11 months 2 weeks 1 day 15 hours ago) and read 2135 times:

I was on an A310 the other day

The bottom of the door had a pressure gauge with a notice next to it, it showed that the door should not be opened unless the gauge was showing the correct reading.

Just a guess but if this device was in the AA A300 then perhaps it wasn't looked at or it could have been faulty.

I always thought that the cabin pressure was reduced to equal the landing elevation as part of the descent check-list, does anyone know if this was a factor?


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 6, posted (10 years 11 months 2 weeks 1 day 4 hours ago) and read 2043 times:

It would appear there are some misconceptions regarding the pressurisation of airliners.

Most airliners are slightly pressurised prior to takeoff. The cabin pressure is then decreased gradually as the aircraft climbs, but will always be higher than the outside air pressure. As the cabin pressure reaches 8000' of altitude, it will stop decreasing.

Not having the cabin pressure at sea level reduces the strength required of the pressure hull and thus the structural weight but will not be a problem to most people. The sensitivity to alcohol goes up though (yet another reason not to serve the stuff onboard) and some people with reduced heart or lung capacity are not be able to fly safely.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
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