Narrative:

Departed dfw with left pack inoperative. On takeoff, left engine bleed off light appeared with associated EICAS. MEL said this might occur and should go away when power is reduced. This did not happen, but normal duct pressure was present on the gauge. Accomplished abnormal. When left bleed air switch was pressed off, duct pressure went to zero. When turned back on, light remained on, but duct pressure returned to normal. Continued on with flight, with all other indications normal -- cabin was in normal schedule. At FL350, first officer went back to check out a problem with the first class lavatory. Ride started to become moderate chop. I asked ATC for FL280, which was reported smooth. I used flight level change, bringing power to idle to do a quick descent through chop. Duct pressure for both engines stayed around 30-35 psi. Around FL310, cabin altitude warning horn and associated messages and lights appeared. I was on oxygen at the time, with first officer gone, started emergency descent and called for the first officer over the PA. An emergency was declared. When warning started, cabin altitude was slightly over 10000 ft. First officer came up as soon as I called for him. He immediately went on oxygen, assessed the problem and started right in. The crew coordination kicked right in. He flew the airplane when needed, handled the pressurization, keeping the cabin to no higher than 11500 ft by going to manual. The first officer brought the cabin back down to normal schedule, while I descended to 10000 ft. Flight attendants were notified of the problem, that we were returning to dfw, no emergency evacuate/evacuation was needed. During the descent, we say the status messages of cabin automatic #1, cabin automatic #2. Callback conversation with reporter revealed the following information: the reporter stated the loss of cabin pressure was caused by a failed outflow valve. Maintenance replaced the outflow valve and pressurization system #1 and #2 operated normally.

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Original NASA ASRS Text

Title: A B757-200 IN CRUISE AT FL350 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CABIN PRESSURE CAUSED BY A FAILED OUTFLOW VALVE.

Narrative: DEPARTED DFW WITH L PACK INOP. ON TKOF, L ENG BLEED OFF LIGHT APPEARED WITH ASSOCIATED EICAS. MEL SAID THIS MIGHT OCCUR AND SHOULD GO AWAY WHEN PWR IS REDUCED. THIS DID NOT HAPPEN, BUT NORMAL DUCT PRESSURE WAS PRESENT ON THE GAUGE. ACCOMPLISHED ABNORMAL. WHEN L BLEED AIR SWITCH WAS PRESSED OFF, DUCT PRESSURE WENT TO ZERO. WHEN TURNED BACK ON, LIGHT REMAINED ON, BUT DUCT PRESSURE RETURNED TO NORMAL. CONTINUED ON WITH FLT, WITH ALL OTHER INDICATIONS NORMAL -- CABIN WAS IN NORMAL SCHEDULE. AT FL350, FO WENT BACK TO CHK OUT A PROB WITH THE FIRST CLASS LAVATORY. RIDE STARTED TO BECOME MODERATE CHOP. I ASKED ATC FOR FL280, WHICH WAS RPTED SMOOTH. I USED FLT LEVEL CHANGE, BRINGING PWR TO IDLE TO DO A QUICK DSCNT THROUGH CHOP. DUCT PRESSURE FOR BOTH ENGS STAYED AROUND 30-35 PSI. AROUND FL310, CABIN ALT WARNING HORN AND ASSOCIATED MESSAGES AND LIGHTS APPEARED. I WAS ON OXYGEN AT THE TIME, WITH FO GONE, STARTED EMER DSCNT AND CALLED FOR THE FO OVER THE PA. AN EMER WAS DECLARED. WHEN WARNING STARTED, CABIN ALT WAS SLIGHTLY OVER 10000 FT. FO CAME UP AS SOON AS I CALLED FOR HIM. HE IMMEDIATELY WENT ON OXYGEN, ASSESSED THE PROB AND STARTED RIGHT IN. THE CREW COORD KICKED RIGHT IN. HE FLEW THE AIRPLANE WHEN NEEDED, HANDLED THE PRESSURIZATION, KEEPING THE CABIN TO NO HIGHER THAN 11500 FT BY GOING TO MANUAL. THE FO BROUGHT THE CABIN BACK DOWN TO NORMAL SCHEDULE, WHILE I DSNDED TO 10000 FT. FLT ATTENDANTS WERE NOTIFIED OF THE PROB, THAT WE WERE RETURNING TO DFW, NO EMER EVAC WAS NEEDED. DURING THE DSCNT, WE SAY THE STATUS MESSAGES OF CABIN AUTO #1, CABIN AUTO #2. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LOSS OF CABIN PRESSURE WAS CAUSED BY A FAILED OUTFLOW VALVE. MAINT REPLACED THE OUTFLOW VALVE AND PRESSURIZATION SYS #1 AND #2 OPERATED 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.