Another 737NG pressurization question

[aeropacific]

Hi everyone,

Had an unusual experience recently. 30 minutes after leveling at cruise (about 37K), experienced a pressurization 'bump' and cabin altitude dropped from about 8K to 6K' (with rise in differential). When descending at destination, differential didn't decrease, but cabin altitude wound down to well below zero. No auto fail light or other fault indications. Went to 'Alternate' at 6K' since differential was still above 8.5psi and experienced rapid decompression to normal level for 6K' altitude, and rush of extremely cold air.

Had never seen that before. Ideas?

[ampclamp]

Sounds like the cab press. controller in control had a problem (and failed internally to be recognised as such) and upon switching to alternate, the air con may have had trouble catching up on the rapid change.

[Skyjob]

The change from 8-6000' sounds like the differential increased which results in cabin altitude reducing.
The selection of the Flight Altitude window is paramount, if you set this above 37000 it triggers this situation.
Similar a failure of the selection being read by the controller defaults to the maximum allowed differential, this is a safety precaution. The same precaution maintains full maximum differential throughout flight, thus not reducing in descent. The safety feature is there to enable the crew to complete the flight without the requirement to descend. It does require crew actions to reduce the differential manually before or after landing.

[aeropacific]

Thanks for the info. I figured that the controller must have failed, I was just surprised that it didn't recognize it's own failure (no 'Auto-fail' light).

One more question: What would happen if you landed a 737 with an 8.5 psi differential pressure (the sticker next to the gauge says .125 psi max for takeoff and landing...)

[Goldenrivett]

What would happen if you landed a 737 with an 8.5 psi differential pressure

You would not be able to open the doors in an emergency.

Saudia Flight 163 - Wikipedia, the free encyclopedia

"It took 23 minutes from the engine shutdown until the fuselage was accessed. Saudi reports stated that the crew could not get the plug-type doors to open in time.[7] It is assumed that most passengers and flight attendants were incapacitated during the landing roll or they would not have attempted to open a door on a moving aircraft. It is known that the aircraft remained pressurized during the landing roll as the cabin pressurization system was on standby and the aircraft was found with both pressurization doors almost completely closed. These pressurization doors should have opened completely on touchdown to depressurize the aircraft. The crew were found still in their flight station seats. The source of the fire in the C3 is unknown.[8]"