Cabin air pressure not constant on descent ?

[jxf63]

And another random question:)

I've had a head cold over the last couple of weeks which has unfortunately coincided with a busy flying period. In all 6 flights I've taken, it seems to me that I've had pressure-related discomfort almost immediately after starting the descent.

Why should there be any change in cabin air pressure between 35000 and 10000 feet ? I thought it was kept constant (8000 or 9000 ft?) once above a certain height?

[QA1]

Just look up 'aircraft differential pressure graph' and all will be revealed.

[Basil]

IIRC, if the cabin pressure was kept at 8000ft until actual height above touchdown of 10,000ft and the aircraft then descended at 2000/3000 feet per minute, your poor old eustachian tubes would be subjected to a high rate of pressure increase and would lock up. With a heavy cold, it could be difficult or impossible to clear them with the Valsalva manoeuvre (https://en.wikipedia.org/wiki/Valsalva_maneuver)

For that reason, reduction in cabin altitude is spread over all of the descent limited, as implied by QA1, by the design operating differential pressure.
This gives the cabin a much lower RoD than the aircraft.

Interesting useless piece of information is that, since the diff press in the cruise is about 8psi, if you look at the size of the doors you can calculate the outward force on them; it is surprisingly large.

[jxf63]

Thanks for the informative replies :)

[PAXboy]

At the beginning of the flight, the cabin pressure usually starts to build during the takeoff roll so that it changes slowly and steadily to allow you to adjust.

[DaveReidUK]

]

In my experience, the only time the cabin pressure will be greater than it is on the ground with the doors open is when you are doing pressurisation checks.

They definitely aren't fun if you've got a cold. :O

[TopBunk]

Dave et al

From my recollection, some aircraft will have a slightly positive pressure for take off to avoid surging.

After lift off, the aircraft will generate a pressure schedule based on projected cruise altitude.. For example if on a short flight and say at FL250 (25,000ft) or below (will vary by type of course) it may maintain Sea Level/Departure level and then in descent schedule the cabin to arrival airport elevation.

As an example, LHR to AMS at FL230, aircraft will remain at sea level throughout.

If the aircraft flies above the level at which it can maintain SLP it will schedule a cabin altitude that maintains max differential pressure.

As an example, a B747-400 at FL350 (35,000ft) will schedule a cabin altitude at max diff of x psi which will result in a cabin altitude of, say, 5,600ft (ie below the 8,000ft maximum). If later on the aircraft climbs higher then max diff pressure will be maintained and the cabin will 'climb' higher. The cabin max of 8,000ft will be achieved only at the certified ceiling. Other types handling may be different.

On descent, a schedule will be started to get the cabin altitude to landing elevation usually changing the cabin altitude at a rate of ~350 fpm, so cabin altitude of 7000ft to sea level takes about 20 minutes. This equalises the rate of change the ears have to endure.

I once had a problem with the rear outflow valve freezing in the almost closed position on a B737 and none of the control modes worked. This resulted in max diff press being maintained, so as we descended so did the cabin to well below sea level. When the temp increased the valve unfroze, which resulted in a VERY quick pressure change equal to an instantaneous climb of several thousand feet! Not fun on the ears, although better on the tubes to have a climb rather than a rapid descent!

[Jet II]

There is a positive pressure higher than ambient at take off and landing to prevent a 'pressure bump' from discomforting the passengers.

http://i97.photobucket.com/albums/l212/sids666/pressure.jpg

[TopBunk]

Thanks Jet II

Just as I was saying, with pictures!

[DaveReidUK]

Fair enough, I won't argue about one-eighth of a PSI.

In my defence, some of my career was spent selling unpressurised commuter aircraft. :O

[Basil]

Like all RAF studes, used to go to 30,000ft unpressurised and come down quickly.
Deffo NOT with a cold!

[SeenItAll]

What I have always wondered about is what happens when the destination airport is higher than than the max cabin altitude -- say UIO at 9000 feet. Does the aircraft decompress the cabin on descent so the inside altitude rises up the outside altitude upon landing? I assume so or door opening would be a pretty dramatic event.

[DaveReidUK]

You may find this discussion helpful:

http://www.pprune.org/tech-log/589374-a320-pressurization-leh-airport.html

[SeenItAll]

Well, one post in the link says that the cabin will be maintained at the destination altitude, while another says that it will be based on what is appropriate for the cruise altitude until on approach for landing -- when cabin altitude will be allowed to rise to match that of the destination.

The second choice seems most logical to me, but is it the correct one?

[DaveReidUK]

I suspect that either could apply, depending on the airline and route.

There are few, if any, commercial airports where maintaining the cabin at that altitude enroute would cause problems for passengers. But equally, pressurization systems can cope with lowering the cabin altitude in the climb and allowing it to increase in the descent if that's the airline's policy.

[WHBM]

Like all RAF studes, used to go to 30,000ft unpressurised and come down quickly.
Deffo NOT with a cold!
But you probably only did it once in a day.

Try flying a skydiving jumpship. Sea level. Max climb to 13,000ft. All out. Max descent (landing before the jumpers). Next stick waiting to go. Max climb to 13,000ft. All out ... repeat all Sunday.

[KayPam]

To talk about this : I was once in a Ryanair flight with a bad cold.
I knew the valsalva maneuver and did it multiple times but it simply did not work !
I had horrible cheek pain (well not the cheek, but the eustach trump inside the hard part of each cheek) throughout descent !

Returning flight : I insisted to give the captain a letter asking him to reduce the cabin descent. I was young but already had some theoretical knowledge at the time and quoted a value that I deemed realistically low (-200fpm) and insisted that pain was very much similar to being shot at in the cheek repeatedly with a nail gun.

No pain on this flight, fortunately

[Basil]

Max descent (landing before the jumpers).
Is that turbine or piston? If the latter, I wonder what effect it has on the TBO - bit like glider tugs. ISTR our tugs modifying their descent to be kinder to the engine.