Pressurisation systems

[WHBM]

Question from a work colleague. Well you know how it is when you are a weekend PA28 flyer and they think you know it all about B747 systems ....

When you pressurise a commercial aircraft, how do you make the transition ? If say the cabin altitude is determined to be 8,000 feet for the flight, do you go unpressurised up to that height and then just close the valves and maintain the pressure above there ? And the reverse on the way down ? Or is there some more complex way of handling things which starts off nearer the ground and adjusts things more gradually ? Can you end up landing with some pressure still inside which has to be dispersed on taxi ?

[Bealzebub]

In most modern jets the pressurisation is controlled in an automatic mode. This involves a fairly complex interaction between the computerised air data system as well as the takeoff airport elevation, landing field elevation and the rate controller setting. The pressure itself is regulated by an outflow valve that is set to vary the discharge of cabin air from the aircraft. There is usually a standby automatic mode that assumes authority if the primary fails, and also a manual mode that needs to be adjusted by the flightdeck crew if both the automatic systems fail. In addition there are various valves that prevent the cabin overpressurising significantly or having a negative relative pressure.

In the automatic modes, the cabin is scheduled to increase in altitude at a gradual preset rate during the climb and descent. If the maximum permitted differential is reached then the climb or descent rate becomes a function of the aircrafts climb or descent rate whilst still maintaining the maximum differential pressure. When the aircraft levels off the computer (after a delay) compares on the cruising altitude with preselected landing altitude and schedules the higher of these as the cruise cabin altitude.

For takeoff the cabin is slightly pressurised to allow a smoother transistion to pressurised flight, and to better seat the door seals making the cabin quieter.
For landing the cabin is again slightly pressurised, however at touchdown the outflow valve should open and fully depressure the aircraft. Pilots should monitor the sytem after takeoff and landing to ensure that the system is behaving as it should, and correct any anomaly.

Typically a jet will be pressurised to a cruising altitude of between 6000ft and 8000ft during the cruise (depending on final altitude) at maximum allowed differential which in itself will usually be about 8.6 pounds per square inch.

Sorry it is a fairly complicated subject, but I hope that helps.

[TightSlot]

Bealzebub - do I recall something about an on-board fire with a Saudia L1011 - doors could not be opened because cabin was pressurised?

Not in any way queying your excellent (and appreciated) response - just trying to remember if I remember right?

[Bealzebub]

Hi Tightslot,

Yes without refreshing myself on that disaster, I seem to recall that a part of the chain of events involved the aircraft landing with residual pressurisation. These days it is normally a part of the flight deck crews "after landing" checks, that they confirm the aircraft is depressurised.

[wileydog3]

Bealzebub - do I recall something about an on-board fire with a Saudia L1011 - doors could not be opened because cabin was pressurised?
Not in any way queying your excellent (and appreciated) response - just trying to remember if I remember right?


1980.. fire in flight and when they landed, they did not depressurize the airplane resulting in an inability to open the doors. On the old Boeings, you pressurized the airplane on the ground just before takeoff to a pressure of 0.125psi which sounds like very little but it is sufficient to prevent opening of the doors. You depressurize when you roll out. The McDougs (DC-9, MD-80) would pressurize just after getting airborne and on landing.

In automated systems, the pressurization maintains sea level until it reaches a preset limit and then the cabin pressure climbs routinely to a maximum of about 8000ft as previously mentioned. Newer jets are maintaining a lower cabin altitude to higher altitudes which means less fatigue for all aboard and less hypoxia symptoms. Cabins routinely climb at a maximum of about 700fpm and for comfort, descent at a max of about 300fpm to allow pax to clear their ears.

The problem sometimes used to occur when in a rapid descent, the airplane passed the cabin and you were depressurized in flight and going down at 2000fpm. Not a bunch of happy pax....

The newer systems are set and forget... automated