Air carrier large transport experiences pressurization problems in climb.

Attributes
ACN	180835
Time
Date	199106
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : sac
State Reference	CA
Altitude	msl bound lower : 25000 msl bound upper : 32000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : den
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	climbout : intermediate altitude
Route In Use	enroute airway : j32
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 150 flight time total : 10800 flight time type : 1300
ASRS Report	180835
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : overcame equipment problem flight crew : declared emergency other
Consequence	Other
Supplementary
Primary Problem	Aircraft

Narrative:

Departed sfo climbing through FL320 on way up to FL330 en route to msp. Cabin altitude horn sounded indicating cabin had exceeded 10000'. We notified center and requested lower. After second officer was unable to correct situation using abnormal procedures, we declared emergency and initiated descent. Cabin climb rate was arrested descending through FL250 and was descended through 10000' with the aircraft descending through FL200. Center requested we level at 16000' and we complied since cabin altitude was stabilized below 10000'. Cockpit crew donned masks immediately after horn had sounded. Passenger masks did not drop as cabin altitude never reached 14000'. We returned to sfo and maintenance determined that equipment cooling outflow valve failed to close at appropriate time during climb (2.2 diff pressure), causing excess air to be dumped overboard and subsequent cabin climb above selected altitude. Cabin rate of climb was so gradual, about 400-500 FPM, that it went unnoticed when it exceeded selected altitude. Since the first indication of a problem was the cabin altitude horn, only extreme vigilance of cabin pressure gauges could have prevented this occurrence. Nothing could have prevented the malfunction.

Original NASA ASRS Text

Title: ACR LGT EXPERIENCES PRESSURIZATION PROBLEMS IN CLIMB.

Narrative: DEPARTED SFO CLBING THROUGH FL320 ON WAY UP TO FL330 ENRTE TO MSP. CABIN ALT HORN SOUNDED INDICATING CABIN HAD EXCEEDED 10000'. WE NOTIFIED CENTER AND REQUESTED LOWER. AFTER S/O WAS UNABLE TO CORRECT SITUATION USING ABNORMAL PROCS, WE DECLARED EMER AND INITIATED DSNT. CABIN CLB RATE WAS ARRESTED DSNDING THROUGH FL250 AND WAS DSNDED THROUGH 10000' WITH THE ACFT DSNDING THROUGH FL200. CENTER REQUESTED WE LEVEL AT 16000' AND WE COMPLIED SINCE CABIN ALT WAS STABILIZED BELOW 10000'. COCKPIT CREW DONNED MASKS IMMEDIATELY AFTER HORN HAD SOUNDED. PAX MASKS DID NOT DROP AS CABIN ALT NEVER REACHED 14000'. WE RETURNED TO SFO AND MAINT DETERMINED THAT EQUIP COOLING OUTFLOW VALVE FAILED TO CLOSE AT APPROPRIATE TIME DURING CLB (2.2 DIFF PRESSURE), CAUSING EXCESS AIR TO BE DUMPED OVERBOARD AND SUBSEQUENT CABIN CLB ABOVE SELECTED ALT. CABIN RATE OF CLB WAS SO GRADUAL, ABOUT 400-500 FPM, THAT IT WENT UNNOTICED WHEN IT EXCEEDED SELECTED ALT. SINCE THE FIRST INDICATION OF A PROB WAS THE CABIN ALT HORN, ONLY EXTREME VIGILANCE OF CABIN PRESSURE GAUGES COULD HAVE PREVENTED THIS OCCURRENCE. NOTHING COULD HAVE PREVENTED THE MALFUNCTION.

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.