737-800

[AdamC]

Hi Guys,

Can anybody tell me why the 738 (Ryanair especially) cabin seems to pressurise on the ground? - I noticed this in Stansted whilst at the hold for RWY23, that my ears "popped" and found it strange as it's never happened on any other aircraft before.

Cheers,
Adam.

[I Just Drive]

I think that when the thrust levers go past a certain point, the cabin starts to pressurise. This is to a, stop doors etc.. rattling and b, give a smooth transition pressurisation wise from ground to air. The holds for 23 at STN are up a steep hill and require a load of power to get going, this was probably what you felt. Totally normal. But im sure I will shortly be corrected if im wrong.

[Mr @ Spotty M]

All B737 models pressurise on the ground before take-off.

[CaptainSandL]

My guess is that it would be an increase in airflow in the cabin gaspers as thrust is increased and the duct pressure rises. You can certainly hear this effect on the flightdeck. It probably happens more on the 737-800 than the smaller versions because they are heavier and may require more thrust to start moving when taxying. On the other hand their engines are proportionally larger so maybe the FR guys are bit heavier on the throttle ;)

[sudden Winds]

right, all of them do, in facto most jet transport a/c do as well. The reasons have been previously explained...
There 2 pressurizations systems available from the Classics on. these are the analog and digital. the analog requires the pilot to move a switch in the pressurization mode selector to a position called FLT right after starting the engines. This closes the outflow valve (not completely) and allows about .1 psi of diff pressure inside the a/c (.125 is the limit) if that´s the case then no matter what the pilot does with the thurst levers, the a/c is already pressurized.
The digital system requires the engines to exceed certain thrust percentage. (n1 > 60% & n2 > 89% after a 1.5 sec time delay 737 CL). At this point the airplane pressurizes the same as if you had positioned the previously mentioned switch to FLT. Such power settings are normally transitioned only when takeoff is initiated. I doubt that such high thurst setting is required to get a plane moving on an uphill, but the NG could start pressurizing at a lower power setting, thus making the uphill theory more likely.
Regards,
SW.

[IFixPlanes]

http://img522.imageshack.us/img522/5909/b737ngpressurizationfliur4.th.jpg (http://img522.imageshack.us/my.php?image=b737ngpressurizationfliur4.jpg)

[flyboyike]

The CRJ does the same thing.