Negative pressure

[DongJoon Choi]

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?

[FlightPathOBN]

between ZUNZ (9843) and CUZ (10656) I can tell you that they open very, very easily...

[IFixPlanes]

You can not produce negative pressure on ground. :E
...
Hypothetically spoken it would help at plug type entry doors.

[hetfield]

With negative pressure on ground you better leave the doors closed 'cause everybody would fart....
;)

[isaneng]

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........

[FlightPathOBN]

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....

[Aaron-EIWF]

What's this "negative pressure" concept? Can someone explain something non-existent to me please?

[john_tullamarine]

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.

[IFixPlanes]

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) :ugh:

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.

[grounded27]

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.

[isaneng]

Oh yes you can. Admittedly it depends on specific aircraft type, but you can. But you only tend to do it once....!

[glhcarl]

The only way you would get "negative pressure" in the fuselage is if the "negative pressure relief valve" is inoperative!

[FlightPathOBN]

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.

[no-hoper]

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.

[mothergoose1]

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,,

[hetfield]

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.

[NSEU]

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 ;)

[isaneng]

NSEU, sorry, but whilst generally correct, not always true. Type specific, and probably very unlikely on civil aircraft.

[NSEU]

Type specific, and probably very unlikely on civil aircraft.

Actually, I was talking about 747-400's ;)

[grounded27]

I am with NSEU on this one, while a negative pressure may be possible to attain, it is insignificant.

[Golf-Sierra]

Oh yes you can. Admittedly it depends on specific aircraft type, but you can. But you only tend to do it once....!


Could you please tell us exactly what you have in mind?

Golf-Sierra

[john_tullamarine]

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

No different to flying along at 3800ft unpressurised and then setting up normal pressurisation.

Although it will vary between aircraft, due to design pressure differential, there will be a pressurisation schedule for the aircraft such that, for a given pressure height outside the aircraft, X, the cabin altitude will be Y.

As the aircraft climbs from SL/descends from cruise while pressurised, the cabin will climb/descend at a lesser rate according to the pressurisation schedule.

Now, if we start off somewhere other than SL or cruise - unpressurised - and then set up pressurisation, the cabin altitude first will change from the outside pressure equivalent to whatever the schedule calls for at that outside pressure.

Hence, if you takeoff from an above SL aerodrome, but lower than the maximum FL at which the pressurisation can maintain SL cabin altitude, you would expect to see the cabin descend to/towards SL as the initial setup and then climb per the schedule as the aircraft climbs.

There are plenty of atmospheric calculators on the net (the equations you can find in the appropriate textbooks) for you to play with. You can find one example here (http://www.digitaldutch.com/atmoscalc/).

[FlightPathOBN]

In regards to a real time adventure, I would note that the typical cabin pressure, when at 35,000 descending to an airport elevation of 14,300, the system has difficulty adjusting that quickly, and on the tarmack, perhaps it was that not enough time was let to stabilize, or the crew just wasnt used to this pressure differential, at that experience, it was a bit of a shocker when the front door was opened.

[john_tullamarine]

In regards to a real time adventure

Pressurisation schedule monitoring and pilot control is a standard requirement. It should only be a problem requiring extra pilot management if the

(a) system is not operating correctly or if the

(b) aircraft is descending much more rapidly than normal or to an high aerodrome (in which case, the crew should have reset the rate demand to address the problem)

Three end situations -

(a) the aircraft is still pressurised at landing.

The situation arises if the pressurisation system is mishandled. If the aerodrome is above SL then the controller demands must be adjusted to meet the landing diff limit requirements (typically no diff or, for some aircraft, there may be a small residual diff permitted for landing).

Depending on the aircraft, if it lands pressurised, one would normally expect automatic cabin pressurisation dump on touchdown - the occupants would detect that readily via the eardrums.

If the aircraft, for some reason, were still slightly pressurised at the gate, then the doors ought not to be able to be opened. If the crew noticed this and dumped pressure, again the ears would tell the story.

(b) aircraft on schedule at landing - normal situation

(c) aircraft catches the cabin on descent - aircraft becomes unpressurised ahead of schedule expectation and either the crew adjusts aircraft rate of descent or eardrums are uncomfortable

[NSEU]

For reference...

The negative pressure relief valves on the cargo doors on, say, a B747-400, crack open at 0.2~0.4psi. On the 767, it is 0.3~0.5. The 737NG is a little higher with 1.0psi.

[Swedish Steve]

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.

A couple of years ago I was called out to a B757.
During the descent into a sea level airport, at about 8000ft the crew had heard a bang, felt pressure in their ears, looked up and saw the cabin was at the same altitude as the aircraft and followed it down to the ground.
They thought that the bang had been a failure, and they had a hole in the fuselage. After much inspecting and checking and finding nothing, I decided to pressurise the aircraft. Looking up at the pressure control panel the cabin rate of descent knob was set at min! The aircraft had overtaken the cabin in the descent, the bang was the safety valves opening.
Rate knob reset to the pip, and aircraft departed.