Negative pressure
Thread Starter
Joined: May 2011
Posts: 31
Likes: 0
From: Seoul, South Korea
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?
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 July 2011 at 00:34.
Joined: Nov 2007
Posts: 180
Likes: 19
From: Who knows where this week.......
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........
Fleet Manager
Joined: Apr 2001
Aviation Qualifications: ATPL
Posts: 7,442
Likes: 307
From: various places .....
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.
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.
Joined: Apr 2004
Posts: 775
Likes: 2
From: near EDDF
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 don´t 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.
Again: It is impossible to get a negative Δ pressure on ground!
(if you don´t 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.
Joined: Mar 2011
Posts: 1,407
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From: engineer at large
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.
Joined: Feb 2010
Posts: 138
Likes: 0
From: Norden
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.
extraction fans from systems like avionic or galley/toilet air extraction.
For in flight IFixPlanes explanation is correct.
Joined: Jul 2011
Posts: 38
Likes: 0
From: in the sky
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,,
Joined: Feb 2004
Posts: 1,410
Likes: 36
From: Australia
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.
extraction fans from systems like avionic or galley/toilet air extraction.
Outflow valves are bigger than equipment cooling outlets, etc
