Narrative:

At FL330 in the vicinity of atl, I was explaining the MD80's pressurization system to an intern jump seat pilot. After the explanation, my first officer continued with 'and you can also control it manually' and switched to manual control. The cabin showed a slight cabin descent at 500 FPM. The first officer then switched to automatic control. The cabin showed a climb of 500 FPM, followed by a rapid climb to over 1500 FPM -- the climb needle was pegged. The first officer immediately went to manual and attempted to manually close the outflow valve, rate of climb still pegged, I asked 'are we losing the cabin?' he replied 'yes.' at that point we all donned oxygen masks, seatbelt sign, and I started an emergency descent. First call to ATC was not received because I forgot to FLIP a switch to mask from microphone. Second was received. I reported out of FL330 because of loss of cabin pressure. I use TCASII to avoid lower traffic, and we received clearance to 15000 ft initially. First officer continued to try to re-establish cabin which he did at around 15000 ft. Cabin oxygen masks did deploy. All checklist items reviewed complete. Diverted to ZZZ. Callback conversation with reporter revealed the following information: the reporter stated the main findings on the actual cause of the pressurization loss were inconclusive. The reporter said the maintenance action taken and the failed components, if any, were not reported. The reporter said while discussing the incident with the FAA the idea of switching from a normal operating system in-flight to a backup system for instruction was not a god idea.

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Original NASA ASRS Text

Title: AN MD80 IN CRUISE FL330 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CABIN PRESSURIZATION. CAUSE UNKNOWN.

Narrative: AT FL330 IN THE VICINITY OF ATL, I WAS EXPLAINING THE MD80'S PRESSURIZATION SYS TO AN INTERN JUMP SEAT PLT. AFTER THE EXPLANATION, MY FO CONTINUED WITH 'AND YOU CAN ALSO CTL IT MANUALLY' AND SWITCHED TO MANUAL CTL. THE CABIN SHOWED A SLIGHT CABIN DSCNT AT 500 FPM. THE FO THEN SWITCHED TO AUTO CTL. THE CABIN SHOWED A CLB OF 500 FPM, FOLLOWED BY A RAPID CLB TO OVER 1500 FPM -- THE CLB NEEDLE WAS PEGGED. THE FO IMMEDIATELY WENT TO MANUAL AND ATTEMPTED TO MANUALLY CLOSE THE OUTFLOW VALVE, RATE OF CLB STILL PEGGED, I ASKED 'ARE WE LOSING THE CABIN?' HE REPLIED 'YES.' AT THAT POINT WE ALL DONNED OXYGEN MASKS, SEATBELT SIGN, AND I STARTED AN EMER DSCNT. FIRST CALL TO ATC WAS NOT RECEIVED BECAUSE I FORGOT TO FLIP A SWITCH TO MASK FROM MIKE. SECOND WAS RECEIVED. I RPTED OUT OF FL330 BECAUSE OF LOSS OF CABIN PRESSURE. I USE TCASII TO AVOID LOWER TFC, AND WE RECEIVED CLRNC TO 15000 FT INITIALLY. FO CONTINUED TO TRY TO RE-ESTABLISH CABIN WHICH HE DID AT AROUND 15000 FT. CABIN OXYGEN MASKS DID DEPLOY. ALL CHKLIST ITEMS REVIEWED COMPLETE. DIVERTED TO ZZZ. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE MAIN FINDINGS ON THE ACTUAL CAUSE OF THE PRESSURIZATION LOSS WERE INCONCLUSIVE. THE RPTR SAID THE MAINT ACTION TAKEN AND THE FAILED COMPONENTS, IF ANY, WERE NOT RPTED. THE RPTR SAID WHILE DISCUSSING THE INCIDENT WITH THE FAA THE IDEA OF SWITCHING FROM A NORMAL OPERATING SYS INFLT TO A BACKUP SYS FOR INSTRUCTION WAS NOT A GOD IDEA.

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.