B757-200
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Training / Qualification
Aircraft Equipment Problem Critical
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; Flight Crew Took Evasive Action; General Declared Emergency; General Maintenance Action
Aircraft
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.
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.