Pressurisation

[A340AM]

19.Given the scenario that the expected cruise level of FL310 was selected into the FLT ALT selector during preflight, but subsequently the A/C climbed to FL390, would you expect:
a) A differential pressure of 7.45 psi maintained by the negative pressure relief valve;
b)A differential pressure of 9.1 psi maintained by the positive pressure relief valve;
c)A differential pressure of 8.35 psi maintained by the forward outflow valve;


the ans is b
can anyone please explain this nd the concept of pressurising and depressurising an ac , and examples of situations like these , where u climb higher than expected what would you do , or lower etc

I m asking in terms for the 737Ng , should be the same concept for all aircraft

[mathy]

not sure

9.1 psi is the maximum differential pressure. How your questioner got to 390 instead of 310 isn't specified so I'd elect for the maximum differential pressure and not what you think the programme should do.

As for the rest I'm not sure what you mean at all. Suppose you are at FL390 the outside air pressure is 2.85psi. If the differential pressure is 9.1psi then the cabin pressure is 11.95psi and that according to my calculator is near enough 5,600ft.

Too high a differential pressure is not good for aircraft cycles whichever way you look at it.

Incidentally at 8000ft pressure altitude your cabin gauge should read 10.92psi so if your manual advises an 8.35psi differential for a properly conducted flight with no faults that means the outside pressure can go as low as 2.57psi or 41200 pressure altitude.

[STBYRUD]

The 737 NG pressurisation controllers are pretty simple in their logic, if you select a low flight altitude and climb, the system will try to maintain a constant cabin altitude after climbing. Come on A340AM, please at least make the effort to look in the books, they explain it pretty nicely how the system works and what it aims to achieve (although not in detail what happens if you dont follow procedure and climb 8000ft above the selected FLT Alt). The manual states that deviations from the selected FLT Alt may cause the delta p to vary, descent mode will engage at 0.25 psi differential change, but if you climb above the selected altitude you can expect an ever increasing differential pressure.

[mototopo]

Airbus 321..

Getting a review to the FCOMs, maximum service ceiling is FL390 with a cabin altitude of 8000ft that gives a differential pressure of 8.06PSI.
This differential pressure is mantained by the single outflow valve on the right rear fuselage, controlled by one of the 2 Cabin Pressure Controllers that receive data from ADIRS concerning pressure altitude and FMGS with landing elev. and QNH during takeoff /landing operations.

I would then say that, in automatic mode, pressurization is indipendent from CRZ ALT or FLT ALT inserted during preflight.

Pressurization profile include standard pre-presurization during takeoff to prevent pressure surge at rotation; climb presssurization with pre programmed law that takes into account A/c rate of climb; descent profile mantaining differential pressure of landing elev. +0.1PSI shortly before landing.... and so on.

Hope to be right and helpful!

Seeya!

[Escape Path]

I hold no B737 TR but...

I would then say that, in automatic mode, pressurization is independent from CRZ ALT or FLT ALT inserted during preflight.

How come then, that when one descends before reaching the altitude selected in the controller the "OFF SCHED DESCENT" light comes on?

[STBYRUD]

737NG != Airbus! The bus may be smart, the Boeing is a bit more simplistic. I vote for 9.1 psi, if you dont get into bigger problems earlier because AUTO FAIL might come on if the delta p is higher than 8.65 psi (and the controller doesnt respond correctly, whatever that means in the manual)...

[mototopo]

@Escape path: as far as I remember trhis message has no meaning in Airbus, maybe only Boeing birds have it but don't know. Any Boeing drivers here?

@STBYRUD:

737NG != Airbus!

Not sure about that but I don't know the B73 at all to make comparisons..
Hope not to see an "EXCESS CAB ALT" on the display and I vote for don't let the plane reach such deltaP in real life, but only on a sim ride..

[STBYRUD]

Thats my point, just wanted to say that the airbus is entirely unlike the 737BC - pardon, NG Messages on displays? What? Don't have those

Oh, I drive the Boeing by the way, I assume you guessed that now

[Escape Path]

@Escape path: as far as I remember trhis message has no meaning in Airbus, maybe only Boeing birds have it but don't know. Any Boeing drivers here?

Mototopo,

Yes, I was indeed talking about the 737.

Best,

Escape Path