Narrative:

During cruise at 26000' the cabin pressure climbed up through 10000', setting off the altitude warning horn. We checked for abnormal lights (door lights, pressurization). Everything looked normal. The first officer put the pressurization to manual and drove the outflow valve closed. The cabin was still climbing, so we got a lower altitude from the center and started a rapid descent. I manually dropped the masks for the passenger. We went through the emergency descent, rapid decompression checklist and leveled off at 10000'. We were then able to pressurize the cabin to 2000'. We talked to the F/a's, who said that everything was fine in the back. The rest of the flight was uneventful. The cabin pressure never exceeded 11000', so we did not have physical problems. I would also like to make 1 comment that is not related to this situation. I would like to see all medium large transport's have a guard over the trim switch for the rudder. I have had jump seat riders accidentally hit it with their foot. This is just common sense to prevent an accident. Callback conversation with reporter revealed the following: reporter states that he has not received any follow up information, and as far as he knows, the specific pressurization problem has not been idented. As to his comment about the medium large transport rudder trim system, he believes that a guard could easily be installed that would prevent occupants of the aircraft observer seats from inadvertently activating aircraft rudder trim.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR MLG FLT CREW INITIATES RAPID DESCENT WITH ATC CLRNC BECAUSE OF INABILITY TO CONTROL CABIN PRESSURIZATION SYSTEM.

Narrative: DURING CRUISE AT 26000' THE CABIN PRESSURE CLBED UP THROUGH 10000', SETTING OFF THE ALT WARNING HORN. WE CHKED FOR ABNORMAL LIGHTS (DOOR LIGHTS, PRESSURIZATION). EVERYTHING LOOKED NORMAL. THE F/O PUT THE PRESSURIZATION TO MANUAL AND DROVE THE OUTFLOW VALVE CLOSED. THE CABIN WAS STILL CLBING, SO WE GOT A LOWER ALT FROM THE CENTER AND STARTED A RAPID DSCNT. I MANUALLY DROPPED THE MASKS FOR THE PAX. WE WENT THROUGH THE EMER DSCNT, RAPID DECOMPRESSION CHKLIST AND LEVELED OFF AT 10000'. WE WERE THEN ABLE TO PRESSURIZE THE CABIN TO 2000'. WE TALKED TO THE F/A'S, WHO SAID THAT EVERYTHING WAS FINE IN THE BACK. THE REST OF THE FLT WAS UNEVENTFUL. THE CABIN PRESSURE NEVER EXCEEDED 11000', SO WE DID NOT HAVE PHYSICAL PROBS. I WOULD ALSO LIKE TO MAKE 1 COMMENT THAT IS NOT RELATED TO THIS SITUATION. I WOULD LIKE TO SEE ALL MLG'S HAVE A GUARD OVER THE TRIM SWITCH FOR THE RUDDER. I HAVE HAD JUMP SEAT RIDERS ACCIDENTALLY HIT IT WITH THEIR FOOT. THIS IS JUST COMMON SENSE TO PREVENT AN ACCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATES THAT HE HAS NOT RECEIVED ANY FOLLOW UP INFO, AND AS FAR AS HE KNOWS, THE SPECIFIC PRESSURIZATION PROB HAS NOT BEEN IDENTED. AS TO HIS COMMENT ABOUT THE MLG RUDDER TRIM SYS, HE BELIEVES THAT A GUARD COULD EASILY BE INSTALLED THAT WOULD PREVENT OCCUPANTS OF THE ACFT OBSERVER SEATS FROM INADVERTENTLY ACTIVATING ACFT RUDDER TRIM.

Data retrieved from NASA's ASRS site as of July 2007 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.