Narrative:

At FL390 as we were approaching our destination and I asked the first officer to turn off the recirculation fans. He complied. Shortly thereafter the first officer observed the cabin altitude beginning to rise. The recirculation fans were promptly turned back on. Immediately; the first officer worked to control the gradual rise in cabin altitude with little or no success. The cabin altitude light illuminated just as we both donned our oxygen masks. The status message ovbd exh val open was displayed. I stated 'my aircraft' and the first officer stated 'your aircraft' as he continued to work to stop the ascent of the cabin altitude. Using the QRH procedures we both agreed to contact ATC to immediately request a lower altitude. They said standby. About 15 to 20 seconds later they said 'unable'. We whole heartedly agreed to declare an emergency; get 15 miles off track to the north and descend. We squawked 7700 and descended to FL310 then on down to FL270. The cabin altitude stopped ascending and eventually stabilized around 9;400 ft. We continued to monitor the pressurization and once ATC verified us we canceled our emergency. This event occurred for a couple of reasons. The first reason is because of my own lack of recall of what happens on the boeing 757 when the left recirculation fan is turned off. I completed my initial training a few months prior and had been flying predominantly the boeing 767 internationally. I had observed the turning off of the recirculation fans during the 9 plus hour flights to and from those destinations. Upon my inquiry as to why this was done I was informed the cabin received a fresh dose of air. By my asking the first officer to turn the recirculation fans off this initiated the gradual loss of pressurization and eventual need to declare an emergency to descend to a lower altitude; since ATC would not approve lower. However; since we kept this same aircraft the next two days we observed in the logbook that maintenance had discovered a low pressure sensor equipment cooling fault which may have affected the rise in cabin pressure above 10;000 ft. I believe that aircraft should have held the cabin pressure below 10;000 ft even with the left recirculation fan off. I know though I will never turn those fans off unless instructed to do so.

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Original NASA ASRS Text

Title: A B757-200's cabin altitude climbed above 10;000 FT after the crew turned off both Recirculation Fans in flight causing the overboard exhaust valve to open. An emergency was declared and crew descended to FL270 where the cabin altitude stabilized.

Narrative: At FL390 as we were approaching our destination and I asked the First Officer to turn off the recirculation fans. He complied. Shortly thereafter the First Officer observed the cabin altitude beginning to rise. The recirculation fans were promptly turned back on. Immediately; the First Officer worked to control the gradual rise in cabin altitude with little or no success. The CABIN ALTITUDE light illuminated just as we both donned our oxygen masks. The status message OVBD EXH VAL OPEN was displayed. I stated 'My Aircraft' and the First Officer stated 'Your aircraft' as he continued to work to stop the ascent of the cabin altitude. Using the QRH procedures we both agreed to contact ATC to immediately request a lower altitude. They said standby. About 15 to 20 seconds later they said 'unable'. We whole heartedly agreed to declare an emergency; get 15 miles off track to the north and descend. We squawked 7700 and descended to FL310 then on down to FL270. The cabin altitude stopped ascending and eventually stabilized around 9;400 FT. We continued to monitor the pressurization and once ATC verified us we canceled our emergency. This event occurred for a couple of reasons. The first reason is because of my own lack of recall of what happens on the Boeing 757 when the left recirculation fan is turned off. I completed my initial training a few months prior and had been flying predominantly the Boeing 767 internationally. I had observed the turning off of the recirculation fans during the 9 plus hour flights to and from those destinations. Upon my inquiry as to why this was done I was informed the cabin received a fresh dose of air. By my asking the First Officer to turn the recirculation fans off this initiated the gradual loss of pressurization and eventual need to declare an emergency to descend to a lower altitude; since ATC would not approve lower. However; since we kept this same aircraft the next two days we observed in the logbook that Maintenance had discovered a low pressure sensor equipment cooling fault which may have affected the rise in cabin pressure above 10;000 FT. I believe that aircraft should have held the cabin pressure below 10;000 FT even with the left recirculation fan off. I know though I will never turn those fans off unless instructed to do so.

Data retrieved from NASA's ASRS site as of April 2012 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.