Narrative:

Aircraft, part 135 charter with two passenger, in IMC conditions, had been at FL390 for approximately 30 mins. Time was xa:45 on fri-04. Radar and radio contact with ZZZ center. At the above stated time, I noticed that there was a problem with our pressurization system and we could not control it. The pressure differential was into the red zone, so we went to the checklist. While going through the checklist, the problem was getting worse. I, PNF, asked for an immediate descent to a lower altitude. No response from ATC. I asked again and looked up and saw our altitude was at FL387 and decreasing. While all of this was going on, I was able to manually take control of the pressurization system. ATC informed us, after the PF executed an emergency maneuver (with traffic in sight), to dscnd to a safer altitude. We controled the pressurization system, leveled off at FL310 and continued to our destination. We called ZZZ at xb:30. This problem was subtle, only indication on EICAS was a small pressurization differential number going from green 8.8 psi to amber 8.9 psi to red at 9.0-9.2 psi. Had to search through checklist to find correct page, even though there was a red indication (on pressurization differential) there was no alerting signals and no master warning/master caution. The maintenance facility stated at 10 psi differential, there would be structural failure. Callback conversation with reporter revealed the following information: the reporter stated that maintenance replaced the automatic pressure controller and tested ok. When the reporter was asked if the over pressure relief was replaced or tested, the reporter answered, 'no, but might be a good idea.'

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Original NASA ASRS Text

Title: A G200 IN CRUISE AT FL390 MADE AN EMER DSCNT DUE TO CABIN PRESSURE INCREASING SLOWLY FROM GREEN INDICATION 8.8 PSI, TO AMBER 8.9, AND THEN TO RED AT 9.0-9.2 PSI. THEY WERE FINALLY ABLE TO CTL CABIN PRESSURE MANUALLY AT FL310.

Narrative: ACFT, PART 135 CHARTER WITH TWO PAX, IN IMC CONDITIONS, HAD BEEN AT FL390 FOR APPROX 30 MINS. TIME WAS XA:45 ON FRI-04. RADAR AND RADIO CONTACT WITH ZZZ CTR. AT THE ABOVE STATED TIME, I NOTICED THAT THERE WAS A PROB WITH OUR PRESSURIZATION SYSTEM AND WE COULD NOT CTL IT. THE PRESSURE DIFFERENTIAL WAS INTO THE RED ZONE, SO WE WENT TO THE CHKLIST. WHILE GOING THROUGH THE CHKLIST, THE PROB WAS GETTING WORSE. I, PNF, ASKED FOR AN IMMEDIATE DSCNT TO A LOWER ALT. NO RESPONSE FROM ATC. I ASKED AGAIN AND LOOKED UP AND SAW OUR ALT WAS AT FL387 AND DECREASING. WHILE ALL OF THIS WAS GOING ON, I WAS ABLE TO MANUALLY TAKE CTL OF THE PRESSURIZATION SYSTEM. ATC INFORMED US, AFTER THE PF EXECUTED AN EMER MANEUVER (WITH TFC IN SIGHT), TO DSCND TO A SAFER ALT. WE CTLED THE PRESSURIZATION SYSTEM, LEVELED OFF AT FL310 AND CONTINUED TO OUR DEST. WE CALLED ZZZ AT XB:30. THIS PROB WAS SUBTLE, ONLY INDICATION ON EICAS WAS A SMALL PRESSURIZATION DIFFERENTIAL NUMBER GOING FROM GREEN 8.8 PSI TO AMBER 8.9 PSI TO RED AT 9.0-9.2 PSI. HAD TO SEARCH THROUGH CHKLIST TO FIND CORRECT PAGE, EVEN THOUGH THERE WAS A RED INDICATION (ON PRESSURIZATION DIFFERENTIAL) THERE WAS NO ALERTING SIGNALS AND NO MASTER WARNING/MASTER CAUTION. THE MAINT FACILITY STATED AT 10 PSI DIFFERENTIAL, THERE WOULD BE STRUCTURAL FAILURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT MAINT REPLACED THE AUTOMATIC PRESSURE CTLR AND TESTED OK. WHEN THE RPTR WAS ASKED IF THE OVER PRESSURE RELIEF WAS REPLACED OR TESTED, THE RPTR ANSWERED, 'NO, BUT MIGHT BE A GOOD 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.