Date: 201210
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: MD-83
Operator: Air Carrier
Operating under: Part 121
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: After climbing to FL310 with the cabin pressurization on schedule the aircraft was leveled off and power reduced for cruise flight. Shortly after leveling; a pressure change was noticed by both pilots. Cabin altitude was indicating just above 6;100 FT. The cabin climb indicator displayed a climb rate in excess of 1;000 FT per minute. The QRH was referenced and the First Officer (pilot flying) immediately donned his O2 mask. The procedure was performed with the result of manual pressurization being attempted with the outflow valve in the full forward/closed position. The cabin climb indicator stabilized at a 1;400 FT per minute climb. An immediate request for a descent to 10;000 was requested with Center. In short order a clearance to FL240 was granted and the First Officer began descent. The Captain had now donned his O2 mask as well. The Lead Flight Attendant was contacted and he already perceived the problem and spoke of securing the service carts. Center further cleared us to 10;000 FT and cleared the aircraft to turn direct to the airport as requested. Descending through FL240 the Cabin Altitude master warning illuminated and the cabin altitude indicated above 10;000 FT. At no time was the cabin altitude observed in excess 11;000 FT. During the decent contact with the company via radio was attempted to no avail. Center re-cleared the flight to [to join the arrival to the departure airport]. At 10;000 the manual pressurization managed to keep the cabin at the scheduled altitude. Commercial Radio was contacted again and connected the flight crew with Dispatch and Maintenance Control. Dispatch amended the release. Maintenance Control asked and was told what procedure of the QRH performed. The Lead Flight Attendant was contacted again and informed the pilots that the passengers were doing well. The flight attendants had started the use of a POB in the forward galley as a precautionary measure. An announcement was made to the passengers regarding the problem and our need to return. The arrival and visual approach were otherwise uneventful. An emergency was not declared during this situation. Had the flight crew loitered at FL310 for another three or four minutes it is likely that the passenger O2 masks would have deployed. Centers' swift response to our descent request allowed this abnormal event to resolve itself in an orderly fashion.




Synopsis: MD80 Captain experiences a loss of cabin pressurization at FL310 shortly after level off. QRH procedures; including manual closing of the outflow valve are unsuccessful and an emergency descent is initiated. Flight returns to departure airport. Cabin altitude remains below 11;000 FT and masks do not drop but Flight Attendant's report personally using oxygen bottles on their own initiative.