Narrative:

We were level flight at FL430. (Actually, we were at a block altitude FL410-FL430). We heard a loud 'pop' and rushing air. We donned our oxygen masks, and started a descent. The emergency pressure was on and it took us 1-2 mins to figure out what the problem was. We could not communicate very well with each other or with ATC due to circumstances beyond our control. As we passed through FL404, I heard ATC clear an airline to a lower altitude because of our descent. When I finally contacted ATC, they cleared us to FL350. The problem was an air cycle machine overtemp. We corrected it and continued with no further problems. The problem was caused by high altitude, low airspeed and high power settings while asking the air cycle machine for cooler air. A good lesson was learned with no harm and due to ATC's excellent work we were able to continue. Callback conversation with reporter revealed the following information: the aircraft is certified to operate to a maximum altitude of FL450. Aircraft is equipped with a single air cycle machine or 'pack.' there is no redundancy. When the pack trips, an automatic shutdown occurs. This trip-off cannot be overridden. At this point, the aircraft starts to depressurize. There is an 'emergency pressure valve' which can be opened. This dumps raw engine pneumatic system air directly into the cabin. Pneumatic system temperature is typically 195 degrees C. There is no cockpit temperature indication so the pack temperatures cannot be monitored in the cockpit. The crew is not able to see an overtemp trip-off coming up. Pack trips have been a continuing problem for the aircraft. Cessna has included a warning in the flight manual describing the problem. They don't, however, say what to do about it. The pilot thinks that the air conditioning and pressurization system should operate reliably throughout the operating envelope of the aircraft.

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Original NASA ASRS Text

Title: C560 CREW LOST CABIN PRESSURIZATION AT FL420.

Narrative: WE WERE LEVEL FLT AT FL430. (ACTUALLY, WE WERE AT A BLOCK ALT FL410-FL430). WE HEARD A LOUD 'POP' AND RUSHING AIR. WE DONNED OUR OXYGEN MASKS, AND STARTED A DSCNT. THE EMER PRESSURE WAS ON AND IT TOOK US 1-2 MINS TO FIGURE OUT WHAT THE PROB WAS. WE COULD NOT COMMUNICATE VERY WELL WITH EACH OTHER OR WITH ATC DUE TO CIRCUMSTANCES BEYOND OUR CTL. AS WE PASSED THROUGH FL404, I HEARD ATC CLR AN AIRLINE TO A LOWER ALT BECAUSE OF OUR DSCNT. WHEN I FINALLY CONTACTED ATC, THEY CLRED US TO FL350. THE PROB WAS AN AIR CYCLE MACHINE OVERTEMP. WE CORRECTED IT AND CONTINUED WITH NO FURTHER PROBS. THE PROB WAS CAUSED BY HIGH ALT, LOW AIRSPD AND HIGH PWR SETTINGS WHILE ASKING THE AIR CYCLE MACHINE FOR COOLER AIR. A GOOD LESSON WAS LEARNED WITH NO HARM AND DUE TO ATC'S EXCELLENT WORK WE WERE ABLE TO CONTINUE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE ACFT IS CERTIFIED TO OPERATE TO A MAX ALT OF FL450. ACFT IS EQUIPPED WITH A SINGLE AIR CYCLE MACHINE OR 'PACK.' THERE IS NO REDUNDANCY. WHEN THE PACK TRIPS, AN AUTOMATIC SHUTDOWN OCCURS. THIS TRIP-OFF CANNOT BE OVERRIDDEN. AT THIS POINT, THE ACFT STARTS TO DEPRESSURIZE. THERE IS AN 'EMER PRESSURE VALVE' WHICH CAN BE OPENED. THIS DUMPS RAW ENG PNEUMATIC SYS AIR DIRECTLY INTO THE CABIN. PNEUMATIC SYS TEMP IS TYPICALLY 195 DEGS C. THERE IS NO COCKPIT TEMP INDICATION SO THE PACK TEMPS CANNOT BE MONITORED IN THE COCKPIT. THE CREW IS NOT ABLE TO SEE AN OVERTEMP TRIP-OFF COMING UP. PACK TRIPS HAVE BEEN A CONTINUING PROB FOR THE ACFT. CESSNA HAS INCLUDED A WARNING IN THE FLT MANUAL DESCRIBING THE PROB. THEY DON'T, HOWEVER, SAY WHAT TO DO ABOUT IT. THE PLT THINKS THAT THE AIR CONDITIONING AND PRESSURIZATION SYS SHOULD OPERATE RELIABLY THROUGHOUT THE OPERATING ENVELOPE OF THE ACFT.

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.