A B733 experienced loss of cabin pressure at FL350, diverted to ZZZ, united states.

Attributes
ACN	521245
Time
Date	200108
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl single value : 35000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zzz.artcc tower : zzz.tower
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 14000 flight time type : 5000
ASRS Report	521245
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : cfi pilot : commercial pilot : instrument pilot : multi engine pilot : atp
Experience	flight time last 90 days : 85 flight time total : 6500 flight time type : 85
ASRS Report	520648
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : alt cabin warning horn other flight crewa other flight crewb
Resolutory Action	flight crew : declared emergency flight crew : overcame equipment problem flight crew : landed as precaution flight crew : diverted to another airport
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

Cruising at 35000 ft, cabin altitude warning horn came on due to excessive cabin altitude. Cabin pressure was climbing and at highest point reached 10000 ft. Shortly after horn the aft rear entry door light illuminated for about 20 seconds, then went out. Cabin continued to climb and we asked ATC for an immediate descent and complied with procedures. Until cabin selector was put in manual, we could not control cabin pressure. Once in manual, cabin began pressurizing. We diverted to ZZZ and landed without incident. Maintenance inspected aircraft and found the outflow valve stuck partially open in the 'automatic' mode. They replaced the valve.

Original NASA ASRS Text

Title: A B733 EXPERIENCED LOSS OF CABIN PRESSURE AT FL350, DIVERTED TO ZZZ, UNITED STATES.

Narrative: CRUISING AT 35000 FT, CABIN ALT WARNING HORN CAME ON DUE TO EXCESSIVE CABIN ALT. CABIN PRESSURE WAS CLBING AND AT HIGHEST POINT REACHED 10000 FT. SHORTLY AFTER HORN THE AFT REAR ENTRY DOOR LIGHT ILLUMINATED FOR ABOUT 20 SECONDS, THEN WENT OUT. CABIN CONTINUED TO CLB AND WE ASKED ATC FOR AN IMMEDIATE DSCNT AND COMPLIED WITH PROCS. UNTIL CABIN SELECTOR WAS PUT IN MANUAL, WE COULD NOT CTL CABIN PRESSURE. ONCE IN MANUAL, CABIN BEGAN PRESSURIZING. WE DIVERTED TO ZZZ AND LANDED WITHOUT INCIDENT. MAINT INSPECTED ACFT AND FOUND THE OUTFLOW VALVE STUCK PARTIALLY OPEN IN THE 'AUTO' MODE. THEY REPLACED THE VALVE.

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.