Narrative:

Upon level off at FL270 en route eastbound the copilot with some urgency stated that he had to use the restroom. The jumpseat observer stood up, moved the jumpseat out of the aisle and made room for the copilot to exit the cockpit. Instead of putting the jumpseat back into the aisle I told the observer to situation in the copilot's seat. At this time because of our altitude I was putting on my oxygen mask, simultaneously a rapid depressurization of the aircraft occurred. I finished putting my oxygen mask on, established communications, went to manual mode of pressurization and shut the outflow valves, ensured that the seatbelt and no smoking signs were on and that the passenger oxygen was deployed. At this time the cabin altitude was still out of control. Less than one minute after he left the cockpit the copilot returned. The observer told him that he had bumped something. As I initiated a descent the copilot found the discharge valve switch in the dump/ditch position. He returned the switch to normal and control of the pressurization was regained. A check of the passenger was made and there were no injuries. An uneventful return was made to the departure airport. Needless to say this was a freak occurrence, especially when you consider that the valves switch must be depressed and rotated to go to dump ditch. The observer said he bumped it with his left shoulder as he was getting into the copilot's seat. In retrospect it is amazing how task oriented you are when faced with an emergency. The training a pilot receives instills an automatic and methodical reaction, ie, when checking the pressurization panel I made sure that the air valves were open and the packs on. I know I looked at the discharge valve switch but why didn't it register that it was in an abnormal position? Essentially alone with an emergency depressurization is not a situation anyone would like to be in. To prevent this incident from happening again I would suggest that the observer accompany the copilot out of the cockpit. This would ensure that no switches were bumped. The reason for this this is because on this particular aircraft the jumpseat blocks entry into the cockpit when the seat is occupied. It would therefore prevent the pilot from reentering the cockpit in the event of the incapacitation of the other pilot and the observer. Callback conversation with reporter revealed the following: the observer was a pilot from another airline and unfamiliar with our crowded cockpit and moving around in it to get into a seat. The switch is a good design and I doubt if I could duplicate moving it into dump position even if I tried to get my shoulder against it. Just a freak incident.

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Original NASA ASRS Text

Title: FLT CREW OF MLG HAD LOSS OF CABIN PRESSURIZATION AT FL270. FO FOUND DISCHARGE VALVE IN DUMP POSITION. RETURNS SWITCH TO NORMAL POSITION, REGAINS PRESSURE CONTROL.

Narrative: UPON LEVEL OFF AT FL270 ENRTE EBND THE COPLT WITH SOME URGENCY STATED THAT HE HAD TO USE THE RESTROOM. THE JUMPSEAT OBSERVER STOOD UP, MOVED THE JUMPSEAT OUT OF THE AISLE AND MADE ROOM FOR THE COPLT TO EXIT THE COCKPIT. INSTEAD OF PUTTING THE JUMPSEAT BACK INTO THE AISLE I TOLD THE OBSERVER TO SIT IN THE COPLT'S SEAT. AT THIS TIME BECAUSE OF OUR ALT I WAS PUTTING ON MY OXYGEN MASK, SIMULTANEOUSLY A RAPID DEPRESSURIZATION OF THE ACFT OCCURRED. I FINISHED PUTTING MY OXYGEN MASK ON, ESTABLISHED COMS, WENT TO MANUAL MODE OF PRESSURIZATION AND SHUT THE OUTFLOW VALVES, ENSURED THAT THE SEATBELT AND NO SMOKING SIGNS WERE ON AND THAT THE PAX OXYGEN WAS DEPLOYED. AT THIS TIME THE CABIN ALT WAS STILL OUT OF CONTROL. LESS THAN ONE MINUTE AFTER HE LEFT THE COCKPIT THE COPLT RETURNED. THE OBSERVER TOLD HIM THAT HE HAD BUMPED SOMETHING. AS I INITIATED A DSCNT THE COPLT FOUND THE DISCHARGE VALVE SWITCH IN THE DUMP/DITCH POSITION. HE RETURNED THE SWITCH TO NORMAL AND CONTROL OF THE PRESSURIZATION WAS REGAINED. A CHECK OF THE PAX WAS MADE AND THERE WERE NO INJURIES. AN UNEVENTFUL RETURN WAS MADE TO THE DEP ARPT. NEEDLESS TO SAY THIS WAS A FREAK OCCURRENCE, ESPECIALLY WHEN YOU CONSIDER THAT THE VALVES SWITCH MUST BE DEPRESSED AND ROTATED TO GO TO DUMP DITCH. THE OBSERVER SAID HE BUMPED IT WITH HIS LEFT SHOULDER AS HE WAS GETTING INTO THE COPLT'S SEAT. IN RETROSPECT IT IS AMAZING HOW TASK ORIENTED YOU ARE WHEN FACED WITH AN EMER. THE TRAINING A PLT RECEIVES INSTILLS AN AUTOMATIC AND METHODICAL REACTION, IE, WHEN CHECKING THE PRESSURIZATION PANEL I MADE SURE THAT THE AIR VALVES WERE OPEN AND THE PACKS ON. I KNOW I LOOKED AT THE DISCHARGE VALVE SWITCH BUT WHY DIDN'T IT REGISTER THAT IT WAS IN AN ABNORMAL POSITION? ESSENTIALLY ALONE WITH AN EMER DEPRESSURIZATION IS NOT A SITUATION ANYONE WOULD LIKE TO BE IN. TO PREVENT THIS INCIDENT FROM HAPPENING AGAIN I WOULD SUGGEST THAT THE OBSERVER ACCOMPANY THE COPLT OUT OF THE COCKPIT. THIS WOULD ENSURE THAT NO SWITCHES WERE BUMPED. THE REASON FOR THIS THIS IS BECAUSE ON THIS PARTICULAR ACFT THE JUMPSEAT BLOCKS ENTRY INTO THE COCKPIT WHEN THE SEAT IS OCCUPIED. IT WOULD THEREFORE PREVENT THE PLT FROM REENTERING THE COCKPIT IN THE EVENT OF THE INCAPACITATION OF THE OTHER PLT AND THE OBSERVER. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE OBSERVER WAS A PLT FROM ANOTHER AIRLINE AND UNFAMILIAR WITH OUR CROWDED COCKPIT AND MOVING AROUND IN IT TO GET INTO A SEAT. THE SWITCH IS A GOOD DESIGN AND I DOUBT IF I COULD DUPLICATE MOVING IT INTO DUMP POSITION EVEN IF I TRIED TO GET MY SHOULDER AGAINST IT. JUST A FREAK INCIDENT.

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.