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Negative pressure

Old 27th Jul 2011, 01:10
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Negative pressure

Hi all,

It is hard to open the entry door on ground with positive pressure.

How about negative pressure?

If negative pressure exsists on GRD, how does it affect door operation?

Easy?

Last edited by DongJoon Choi; 29th Jul 2011 at 00:34.
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Old 27th Jul 2011, 01:35
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between ZUNZ (9843) and CUZ (10656) I can tell you that they open very, very easily...
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Old 27th Jul 2011, 03:57
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You can not produce negative pressure on ground.
...
Hypothetically spoken it would help at plug type entry doors.
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Old 27th Jul 2011, 10:57
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With negative pressure on ground you better leave the doors closed 'cause everybody would fart....
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Old 27th Jul 2011, 13:05
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Hmm, IFix, guessing that your smiley means that, like me, you know you can get negative diff on the ground. And yes, boy do the doors/hatches open easily. Still, doesn't take long to get them back on the runners. Bugger........
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Old 28th Jul 2011, 23:06
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Its when they give you tanks of oxygen and coca leaves that you start to wonder....

cabin press at 5K atm local atm at 10K....
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Old 29th Jul 2011, 00:08
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What's this "negative pressure" concept? Can someone explain something non-existent to me please?
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Old 29th Jul 2011, 03:20
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What's this "negative pressure" concept

Wordiology at work.

Think pressure differential for pressure hull work.

In the usual state, we have slightly higher absolute pressure inside compared to outside - giving lower cabin altitudes than the actual outside altitude. Structurally fine as round vessels work fine with hoop tension. Guarding against too much of a good thing, we have various pressure relief mechanisms so that the pressure differential doesn't get too high. We might refer to this situation as being a positive pressure differential.

Structurally, the reverse differential (higher absolute pressure outside) is not good due to structural instability. To guard against this there are various pressure relief mechanisms so that the pressure differential doesn't get too high. We might refer to this situation as being a negative pressure differential.

In both cases there is no negative pressure, per se - a bit hard conceptualising the air somehow sucking against the skin. The pressures in each space are quite normal and positive, if you like. However, the differential can either be trying to explode (positive diff) or implode (negative diff) the structure of the pressure vessel.

Typically we see the positive diff situation with gas bottles generally and aeroplanes - submarines are the usual example of negative diff structures.
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Old 29th Jul 2011, 06:08
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The wording "negative pressure" in conjunction to cabin pressure mean that the pressure difference between inside and outside the aircraft. Normally the cabin pressure is higher than outside the A/C.

Again: It is impossible to get a negative Δ pressure on ground!
(if you donīt get this, try to understand how a door seal work)

You can get in this situation only if you descent in a very high rate for a longer time.
In this case the aircraft altitude can pass the cabin altitude.
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Old 29th Jul 2011, 11:40
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You can not put negative pressure on an aircraft and even with .5 psi DP most doors are just about impossible to open or have mechanical devices / warnings installed to prevent opening the door.
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Old 29th Jul 2011, 20:51
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Oh yes you can. Admittedly it depends on specific aircraft type, but you can. But you only tend to do it once....!
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Old 29th Jul 2011, 22:26
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The only way you would get "negative pressure" in the fuselage is if the "negative pressure relief valve" is inoperative!
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Old 30th Jul 2011, 00:49
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Airflow into the fuselage is approximately constant, and pressure is maintained by varying the opening of the "Out Flow Valve" (OFV). In the event the OFV should fail closed, at least two Positive Pressure Relief Valves (PPRV) and at least one Negative Pressure Relief Valve (NPRV) are provided to protect the fuselage from over and under pressurization.
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Old 30th Jul 2011, 05:10
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On ground,with packs off,all doors closed you can get negativ pressure via electrical
extraction fans from systems like avionic or galley/toilet air extraction.
For in flight IFixPlanes explanation is correct.
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Old 31st Jul 2011, 14:13
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since we aare all talking about pressure,,i have a question too,,when we takeoff from high elevation airports such as at 3800amsl,it is often that the cabin rate descends on climb,why is this so,all i know is that the cabin tries to exhibit the conditions as at sea level,and to do that it must descend,,any help please,,
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Old 31st Jul 2011, 14:18
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If your preselected Cruising Level is somewhere around 240 or lower and your Destination Elevation is lower than your Dep airport Elevation, normal system behavior.
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Old 2nd Aug 2011, 01:54
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On ground,with packs off,all doors closed you can get negativ pressure via electrical
extraction fans from systems like avionic or galley/toilet air extraction.
I disagree. Air going via extraction fans will be balanced by air coming in from outflow valves and other vents such as Forward Overboard Valves. Even if these were sealed, you would still get air leaking past door seals, negative pressure relief valves, etc.

Outflow valves are bigger than equipment cooling outlets, etc
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Old 3rd Aug 2011, 09:36
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NSEU, sorry, but whilst generally correct, not always true. Type specific, and probably very unlikely on civil aircraft.
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Old 4th Aug 2011, 07:03
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Type specific, and probably very unlikely on civil aircraft.
Actually, I was talking about 747-400's
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Old 4th Aug 2011, 15:02
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I am with NSEU on this one, while a negative pressure may be possible to attain, it is insignificant.
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