Narrative:

We were at FL340; at night; in and out of clouds with anti-ice on. I had just asked ATC for a left deviation to avoid a cell and we were told to stand by. There was a sudden sensation of depressurization in our ears. There were no associated warnings; sounds or other sensations. We saw both blue pressurization lights (stby on and transfer-lockout) illuminated. I noted the cabin altitude somewhere above 8;000 ft and the cabin rate of climb indicating near the peg of 1;500 FPM. The outflow valve appeared to be already very near the full forward position; but the cabin was continuing to climb.I requested lower with ATC and selected 'manual' on the cabin altitude control lever and attempted to close the outflow valve manually. After pushing down to unlock the cabin altitude control wheel; I attempted to roll it forward with significant force. It moved very little if at all. Either the outflow valve was already at the full closed position or it was jammed somewhere near the closed position. In any case; the cabin pressure was not controllable in either auto or manual.the captain made a quick call to the number one flight attendant; telling her that the cabin was losing pressure and to have all the flight attendants don their oxygen masks. About that same time; the red cabin altitude light illuminated along with the associated aural warning. We immediately accomplished the red box items and donned our oxygen masks. I double checked my oxygen supply and demand switches in the on and 100% positions. I double checked that the cabin altitude control lever was in the manual (down) position and that the outflow valve was as far forward as I could get it and manually locked.I declared an emergency and told ATC we were beginning our descent and coming left for weather. The captain smoothly transitioned the aircraft into an emergency descent with the throttles at idle and speed brakes extended. I continued to talk and listen to ATC to coordinate turns away from weather and clearances to lower altitudes before reaching them. Over the intercom; the captain and I concurred that our priority was to continue the descent and get the aircraft headed back to the east; away from weather and the higher terrain to the west. The captain kept the weather display on his side and I switched to the terrain mode. We told ATC we wished to divert to ZZZ; which was about 50 miles to the east.when we passed through about FL220; I noted that the cabin rate of climb had passed through zero and that the cabin altitude had reached its peak altitude of something slightly less than 15;000 ft and that the cabin oxygen on light had come on. I then noted the cabin rate of climb had now switched to an abrupt decrease in cabin altitude (near the peg). I was now able to manually control the outflow valve to a more open position to control the rate of descent in the cabin. I attempted to target about a 1;000 FPM descent to keep the altitude coming down; but not so fast as to damage anyone's eardrums. Somewhere near 13;000 ft MSL; I told the captain that the cabin was now below 10;000 ft. He removed his mask and passed the control of the aircraft to me telling me to keep talking to ATC and get us to ZZZ.he immediately called back to the flight attendants to assess the situation and learned everyone was on oxygen and doing fine. I took my oxygen mask off and worked with ATC to continue our descent and set us up for a visual approach at ZZZ while continuing to manually descend the cab toward ZZZ's field elevation until the aircraft was below 10;000 ft MSL. The captain coordinated more with the flight attendants; telling them that they could remove their masks and expect a normal landing and normal taxi to the gate. He also went back over checklists to make sure we hadn't skipped anything critical.after parking; deplaning; and debriefing maintenance; we were told there was a spare aircraft at ZZZ that dispatch was arranging for us to take as soon as luggage could be transferred. I felt fine. The captain said he felt fine. My impression was that the flight attendants all preferred that we go rather than stay in ZZZ. Every passenger re-boarded the new aircraft and dispatch sent a release. The flight to our destination was uneventful.

Related reports:

Google
 

Original NASA ASRS Text

Title: While operating with anti-ice on at FL340; the flight crew of an MD80 was unable to control pressurization and initiated an emergency descent and diverted to a nearby airport. As they passed through FL220; control was regained. The maximum cabin altitude reached was 15;000 FT.

Narrative: We were at FL340; at night; in and out of clouds with anti-ice on. I had just asked ATC for a left deviation to avoid a cell and we were told to stand by. There was a sudden sensation of depressurization in our ears. There were no associated warnings; sounds or other sensations. We saw both blue pressurization lights (STBY ON and TRANSFER-LOCKOUT) illuminated. I noted the cabin altitude somewhere above 8;000 FT and the cabin rate of climb indicating near the peg of 1;500 FPM. The outflow valve appeared to be already very near the full forward position; but the cabin was continuing to climb.I requested lower with ATC and selected 'manual' on the cabin altitude control lever and attempted to close the outflow valve manually. After pushing down to unlock the cabin altitude control wheel; I attempted to roll it forward with significant force. It moved very little if at all. Either the outflow valve was already at the full closed position or it was jammed somewhere near the closed position. In any case; the cabin pressure was not controllable in either auto or manual.The Captain made a quick call to the number one Flight Attendant; telling her that the cabin was losing pressure and to have all the flight attendants don their oxygen masks. About that same time; the red CABIN ALT light illuminated along with the associated aural warning. We immediately accomplished the red box items and donned our oxygen masks. I double checked my oxygen supply and demand switches in the ON and 100% positions. I double checked that the cabin altitude control lever was in the manual (down) position and that the outflow valve was as far forward as I could get it and manually locked.I declared an emergency and told ATC we were beginning our descent and coming left for weather. The Captain smoothly transitioned the aircraft into an emergency descent with the throttles at idle and speed brakes extended. I continued to talk and listen to ATC to coordinate turns away from weather and clearances to lower altitudes before reaching them. Over the intercom; the Captain and I concurred that our priority was to continue the descent and get the aircraft headed back to the east; away from weather and the higher terrain to the west. The Captain kept the weather display on his side and I switched to the terrain mode. We told ATC we wished to divert to ZZZ; which was about 50 miles to the east.When we passed through about FL220; I noted that the cabin rate of climb had passed through zero and that the cabin altitude had reached its peak altitude of something slightly less than 15;000 FT and that the CABIN OXYGEN ON light had come on. I then noted the cabin rate of climb had now switched to an abrupt decrease in cabin altitude (near the peg). I was now able to manually control the outflow valve to a more open position to control the rate of descent in the cabin. I attempted to target about a 1;000 FPM descent to keep the altitude coming down; but not so fast as to damage anyone's eardrums. Somewhere near 13;000 FT MSL; I told the Captain that the cabin was now below 10;000 FT. He removed his mask and passed the control of the aircraft to me telling me to keep talking to ATC and get us to ZZZ.He immediately called back to the flight attendants to assess the situation and learned everyone was on oxygen and doing fine. I took my oxygen mask off and worked with ATC to continue our descent and set us up for a visual approach at ZZZ while continuing to manually descend the cab toward ZZZ's field elevation until the aircraft was below 10;000 FT MSL. The Captain coordinated more with the flight attendants; telling them that they could remove their masks and expect a normal landing and normal taxi to the gate. He also went back over checklists to make sure we hadn't skipped anything critical.After parking; deplaning; and debriefing Maintenance; we were told there was a spare aircraft at ZZZ that Dispatch was arranging for us to take as soon as luggage could be transferred. I felt fine. The Captain said he felt fine. My impression was that the flight attendants all preferred that we go rather than stay in ZZZ. Every passenger re-boarded the new aircraft and Dispatch sent a release. The flight to our destination was uneventful.

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.