A B757 outflow valve failed to regulate pressurization at FL380 so after the EICAS alerted cabin alt...

Narrative:

1:18 after takeoff; northbound on L453 in watrs area; FL380; we had an EICAS warning 'cabin altitude.' cabin altitude was about 11;000 feet and stable; cabin rate of climb/descent was zero. Followed guidance in QRH; attempted to bring down cabin altitude with alternate (auto 2) mode and manual mode with no results. Made 'pan pan pan' call on guard while turning off the track for descent to 10;000 feet. Cabin altitude began to descend as soon as we initiated aircraft descent and within 2-3 minutes was below 10;000 feet. Cabin continued to descend normally during our descent. We leveled off at 10;000 feet and continued southbound to clear the watrs area and return to the departure airport. We did not declare an emergency and passenger oxygen was not deployed. The first officer coordinated with ATC on HF and we had a lot of help from company aircraft in the vicinity with ATC radio relay and ACARS relay to keep dispatch informed. We continued to normal approach and precautionary landing at our departure airport; arriving with about 10;700 pounds of fuel. Mechanics who repaired the aircraft during the night stated that there was a problem with the outflow valve and that it wasn't modulating/closing normally; and wouldn't fully close in any control mode.

Original NASA ASRS Text

Title: A B757 outflow valve failed to regulate pressurization at FL380 so after the EICAS alerted CABIN ALTITUDE the crew declared an emergency; descended to 10;000 feet; and returned to the departure airport.

Narrative: 1:18 after takeoff; northbound on L453 in WATRS area; FL380; we had an EICAS Warning 'Cabin Altitude.' Cabin Altitude was about 11;000 feet and stable; cabin rate of climb/descent was zero. Followed guidance in QRH; attempted to bring down cabin altitude with alternate (Auto 2) mode and Manual mode with no results. Made 'Pan Pan Pan' call on guard while turning off the track for descent to 10;000 feet. Cabin altitude began to descend as soon as we initiated aircraft descent and within 2-3 minutes was below 10;000 feet. Cabin continued to descend normally during our descent. We leveled off at 10;000 feet and continued southbound to clear the WATRS area and return to the departure airport. We did not declare an emergency and passenger oxygen was not deployed. The First Officer coordinated with ATC on HF and we had a lot of help from company aircraft in the vicinity with ATC radio relay and ACARS relay to keep Dispatch informed. We continued to normal approach and precautionary landing at our departure airport; arriving with about 10;700 pounds of fuel. Mechanics who repaired the aircraft during the night stated that there was a problem with the outflow valve and that it wasn't modulating/closing normally; and wouldn't fully close in any control mode.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1046033
Time
Date	201210
Local Time Of Day	1201-1800
Place
Locale Reference	ZNY.ARTCC
State Reference	NY
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Route In Use	Oceanic
Flight Plan	IFR
Component
Aircraft Component	Pressurization Outflow Valve
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical

A B757 outflow valve failed to regulate pressurization at FL380 so after the EICAS alerted cabin altitude the crew declared an emergency; descended to 10;000 feet; and returned to the departure airport.