Narrative:

Upon leveling off at FL310 we got a cabin altitude annunciation along with the aural warning. The cabin altitude was 9;500 when I looked up. The captain immediately donned has oxygen mask and declared an emergency descent. At the same time; as the pilot flying; I began an emergency descent with idle power; full speed brakes and accelerated to just below barber poll. Once established in the descent I donned my mask. When I looked up the cabin was at 8;500 and descending. The masks in the cabin did not deploy. We got clearance to descend to 10;000 ft and asked for a clearance direct to the airport. Upon reaching 10;000 ft we removed our masks; the captain referenced the QRH; contacted the flight attendants and made a PA. At this point pressurization seemed to be operating normally again. The captain decided that we should hold well east of the airport to burn down fuel to avoid an overweight landing. When we finally descended into the airport; in VMC conditions; we noticed the cabin would was starting to over-pressurize as I added power. The cabin was showing about -500 ft at 3;000 ft. We were descending on a base leg so we decided it would be best to use manual pressurization to slowly bring the cabin up and then simply open up the outflow valve and operate unpressurized. The landing was uneventful and we taxied to the gate. The captain did a great job handling the situation and keeping all parties informed. Also; there was never a time during the flight when I had discomfort in my ears as all pressurization changes where gradual.

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Original NASA ASRS Text

Title: MD80 First Officer experiences a cabin altitude warning upon leveling off at FL310 with the cabin altitude showing 9;500 FT. An emergency is declared and descent initiated to 10;000 FT and the cabin altitude is noted to descend as the aircraft descends. The crew elects to return to the departure airport and the cabin altitude is noted to be below sea level at 3;000 FT. Manual pressurization is used to depressurize for landing.

Narrative: Upon leveling off at FL310 we got a CABIN ALT annunciation along with the aural warning. The cabin altitude was 9;500 when I looked up. The Captain immediately donned has oxygen mask and declared an emergency descent. At the same time; as the pilot flying; I began an emergency descent with idle power; full speed brakes and accelerated to just below barber poll. Once established in the descent I donned my mask. When I looked up the cabin was at 8;500 and descending. The masks in the cabin did NOT deploy. We got clearance to descend to 10;000 FT and asked for a clearance direct to the airport. Upon reaching 10;000 FT we removed our masks; the Captain referenced the QRH; contacted the flight attendants and made a PA. At this point pressurization seemed to be operating normally again. The Captain decided that we should hold well east of the airport to burn down fuel to avoid an overweight landing. When we finally descended into the airport; in VMC conditions; we noticed the cabin would was starting to over-pressurize as I added power. The cabin was showing about -500 FT at 3;000 FT. We were descending on a base leg so we decided it would be best to use manual pressurization to slowly bring the cabin up and then simply open up the outflow valve and operate unpressurized. The landing was uneventful and we taxied to the gate. The Captain did a great job handling the situation and keeping all parties informed. Also; there was never a time during the flight when I had discomfort in my ears as all pressurization changes where gradual.

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.