Descent begun when it is discovered that the cabin altitude is climbing uncontrollably.

Attributes
ACN	254592
Time
Date	199310
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : swl
State Reference	MD
Altitude	msl bound lower : 22000 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zdc
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : flight engineer pilot : instrument pilot : atp pilot : cfi
Experience	flight time last 90 days : 150 flight time total : 5000 flight time type : 300
ASRS Report	254592
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : overcame equipment problem other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

After reaching our cruising altitude of 28000 ft the warning horn indicating the aircraft cabin was exceeding 10000 ft went off. We immediately donned our oxygen masks and advised ATC we needed an immediate descent to 14000 ft. By the time we reached 22000 ft the cabin was below 10000 ft and we leveled off to assess the problem. It turned out that the squat switch which switches the pressurization system from ground to flight had failed. The cabin was only above 10000 ft for a few seconds so we did not even have time to deploy the passenger's oxygen masks. The rest of the flight proceeded normally.

Original NASA ASRS Text

Title: DSCNT BEGUN WHEN IT IS DISCOVERED THAT THE CABIN ALT IS CLBING UNCONTROLLABLY.

Narrative: AFTER REACHING OUR CRUISING ALT OF 28000 FT THE WARNING HORN INDICATING THE ACFT CABIN WAS EXCEEDING 10000 FT WENT OFF. WE IMMEDIATELY DONNED OUR OXYGEN MASKS AND ADVISED ATC WE NEEDED AN IMMEDIATE DSCNT TO 14000 FT. BY THE TIME WE REACHED 22000 FT THE CABIN WAS BELOW 10000 FT AND WE LEVELED OFF TO ASSESS THE PROB. IT TURNED OUT THAT THE SQUAT SWITCH WHICH SWITCHES THE PRESSURIZATION SYS FROM GND TO FLT HAD FAILED. THE CABIN WAS ONLY ABOVE 10000 FT FOR A FEW SECONDS SO WE DID NOT EVEN HAVE TIME TO DEPLOY THE PAX'S OXYGEN MASKS. THE REST OF THE FLT PROCEEDED NORMALLY.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.