View Full Version : Pressurisation Systems

4th May 2004, 20:07

I`m currently studying the pressurisation system and I have a lack of understanding the two different modes of operation:
Auto(1&2) and manual

When do you select either of these two and what is the difference?

Why are there two different modes for the auto mode(1&2)?

Furthermore, why is the Cabin ROC controlled in the manual mode?

And finally, I`m not quiet sure if I got the point about the landing elevation selector.
Is it right, that the system increases the pressure during decent until reaching this predetermined value?

I appreciate any kind of help, thx


5th May 2004, 04:36
Hi subsidence,

You will have to provide a bit more info to get a good response.What aircraft is the system fitted to?

As an example,
A B737NG pressurization control can be automatic or manual.
There are two digital cbain pressure controllers(cpc`s).Each CPC has its own systems interface and outflow valve motor system.This gives the AUTO mode of control a Dual redundant architecture.Only one CPC controls the outflow valve at any time.The other CPC is a backup.The active controller changes for every flight or when there is an AUTOFAIL event.
The manual control mode overrides and bypasses the two CPC`s.The manual control system has its own valve motor system.This gives the pressurization control system a triple redundant architecture.

Late model B737 Classic`s are similar.
Early model B737 Classic`s use a very different analog systems.

Regards DDG

5th May 2004, 06:06
Since you used the terms AUTO 1 and AUTO 2, I am going to ass-u-me that you are using the B757/B767 aircraft as your model for study.

The B757 uses to Digital Cabin Pressure Controllers, Auto 1 and Auto 2. The Outflow Valve has 3 motors on it:
One controlled by Auto 1
One controlled by Auto 2
One controlled Manually by the flight crew using overhead panel

The Cabin pressure controllers automatically control cabin pressure based on Current altitude, the planned Altitude (Cruise or Landing field altitude) and the actual cabin pressure.

The B757, unlike the B737 series (and I belive the MD80) DOES NOT have automatic switching during air/ground mode transition. So the first crew of the day usually chooses which controller will start in control for the first flight of the day using a very complicated formula. On Odd nunbered days, Auto 1 will start in control and be switched by the flightcrew on each subsequent flight. On even numbered days, Auto 2 will start in control.
The controller not in command, will remain in standby waiting for a failure of the controller in command. If a failure shall occur, then control will switch automatically.

If both controller fail, then the crew can manually control the outflow valve to maintain cabin pressure.
Furthermore, why is the Cabin ROC controlled in the manual mode?
The Rate of Climb is not controlled by the ROC knob in manual mode. The knob is for setting the ROC in Auto Mode.

And finally, I`m not quiet sure if I got the point about the landing elevation selector.
Setting the Landing Elevation allows the Auto controllers to schedule cabin pressure for descent and landing.

Is it right, that the system increases the pressure during decent until reaching this predetermined value?
Huh?? Cabin Differential Pressure & Cabin Altitudes should decrease with descent.

7th May 2004, 04:32
Good answer B73567AMT, but the decrease in cabin altitude during descent corresponds to an increase in cabin pressure. Do please remember that increases in cabin altitude amount to the same thing as decreased cabin pressure.

So, subsidence's assumption about landing elevation selection is correct. The system does increase the cabin pressure on a schedule that brings the cabin pressure to equal the ground pressure at landing altitude.

"Pressurisation" is a widely used misnomer. Cabin Pressure Control systems do not "pressurise" the cabin, they control the de-pressurisation by reducing the rate at which the pressure of the air in the cabin decreases during climb, until the minimum pressure that corresponds to maximum cabin altitude is achieved. And vice-versa. Thus, during flight, although cabin pressure will be higher than the outside air pressure, aircraft cabin pressure remains lower than ground level pressure.


7th May 2004, 06:11
During a descent, the Cabin Pressure Controllers do not increase the cabin diff pressure.

Even if the Landing Altitude is higher than the Takeoff field altitude, the schedule will just alter the cabin depressurization schedule. I hope that makes sense. Diff Press rates are not higher at lower altitudes.

I have a good diagram that could illustrate this, hopefully you guys can see it.


7th May 2004, 07:44
B73567AMT, your graph shows both cabin and aircraft altitude against time. This is the usual way this subject is depicted and it causes a lot of misunderstanding. It is acceptable to depict the operation in terms of altitudes when considering normal operation but study of possible failure modes must be looked at from the perspective of absolute and differential pressures. Both cabin pressure and outside pressure decrease as the aircraft climbs, but cabin pressure is controlled to reduce at a slower rate than the rate of decrease in outside pressure, causing an increase in cabin differential pressure just as you say.

During descent the cabin differential pressure (the difference between cabin pressure and outside pressure) decreases again - so, just as subsidence assumed, the aircraft cabin pressure increases according to the schedule until it reaches the nominal ground level pressure at the selected landing altitude.

The key that subsidence is looking for is that the controller computes a suitable descent schedule based on the difference between outside pressure at the start of the descent mode and the assumed outside pressure at the selected landing elevation - the area shown in your graph as proportional control.

A complicating factor in the system shown in your graph is that during ground operation the cabin pressure is ramped up to be controlled at a pressure equivalent to 200 feet below runway elevation prior to take-off and ramped down from a pressure equivalent to 300 feet below runway elevation at landing. This is to prevent ear-popping changes of cabin pressure due to sudden operation of the outflow valve at lift-off and touch-down.

7th May 2004, 09:19
thank you very much, that helped me a lot!

Flight Detent
7th May 2004, 11:20
Hi all,
Am I correct in recognizing that graph as from a B737NG!


gas path
7th May 2004, 13:14
Heres one that went wrong:ooh:
Thankfully no one hurt!
'spose this could go in the skin patches thread as well:E

7th May 2004, 13:34

Recall a starlifter having one of its wing tanks blown up because someone forgot to open a vent during refuelling, but how the hell did this happen? (Assuming it has something to do with pressurisation) :hmm:

7th May 2004, 15:59
Blacksheep...I read you now.
yes that is from the B737 AMF.
But pressurization schedules are very much alike.

gas path
7th May 2004, 17:10
It was a mechanic doing a pressurisation run after maintenance he was on his own and using a 'home made' guage! Unfortunately said guage didn't have a max stop and he apparently did not notice when it went past the stop :ooh: Twice!:ooh: The resulting failure also blew the rear hatch some 70 yds. over a blast fence.

Actually this could also go into the 'outsourcing maint' thread on R and N :suspect: