Narrative:

Initially we were cleared to FL410. After cruising at FL410 for about 1 hour we requested and were cleared to FL450. After leveling off at FL450 our cabin altitude began climbing. We received a low cabin pressure message. The crew donned their oxygen masks and the passenger oxygen masks were automatically deployed. The crew began a descent to a safe altitude in an attempt to regain an acceptable cabin altitude. During the descent; I checked that both bleed air valve switches were in the 'on' position. After verifying that both bleed switches were on; I switched the pressurization control from automatic to manual. These 2 steps were done in an attempt to regain the cabin altitude. After reaching a safe altitude we diverted to the ZZZ airport. After we landed we began a preliminary investigation to determine what could have caused the pressurization problem. We noticed that the left bleed air switch was not in the on position. We also noticed that the bulb in the switch light was not illuminated. The bleed air switch lights use a dark switch concept. In other words when the switch is in the 'on' position there is no 'on' annunciation. A dark indication means the switch is on. If the switch is pushed off an 'off' annunciation appears in the switch light. During our pressurization problem I looked at the bleed switches and saw 2 'dark' switches. I mistakenly assumed both bleed switches were on. In actuality only 1 bleed air switch was on. The one bleed system was not enough to maintain adequate pressurization. There are some things that could have been done; from a procedures point of view; to prevent this incident from occurring. One was to check the bleed air pressure indicators against switch position. I just looked at the switches themselves and saw that they were dark and assumed that they were on. We also found some deficiencies in our normal checklist. On our FL180 check there were no items relating to switch position or bleed air psi checks. All we were checking was our pressurization panel (cabin altitude; cabin differential; cabin rate of climb). We also determined that there are 4 very small bulbs in the bleed air switch light. During a test of the lights it is very difficult to determine how many of the 4 lights are burned out. It's a tedious process.

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Original NASA ASRS Text

Title: GV FLT HAS A PRESSURIZATION PROB AT FL450 AND DIVERTS TO RDU.

Narrative: INITIALLY WE WERE CLRED TO FL410. AFTER CRUISING AT FL410 FOR ABOUT 1 HR WE REQUESTED AND WERE CLRED TO FL450. AFTER LEVELING OFF AT FL450 OUR CABIN ALT BEGAN CLBING. WE RECEIVED A LOW CABIN PRESSURE MESSAGE. THE CREW DONNED THEIR OXYGEN MASKS AND THE PAX OXYGEN MASKS WERE AUTOMATICALLY DEPLOYED. THE CREW BEGAN A DSCNT TO A SAFE ALT IN AN ATTEMPT TO REGAIN AN ACCEPTABLE CABIN ALT. DURING THE DSCNT; I CHKED THAT BOTH BLEED AIR VALVE SWITCHES WERE IN THE 'ON' POS. AFTER VERIFYING THAT BOTH BLEED SWITCHES WERE ON; I SWITCHED THE PRESSURIZATION CTL FROM AUTO TO MANUAL. THESE 2 STEPS WERE DONE IN AN ATTEMPT TO REGAIN THE CABIN ALT. AFTER REACHING A SAFE ALT WE DIVERTED TO THE ZZZ ARPT. AFTER WE LANDED WE BEGAN A PRELIMINARY INVESTIGATION TO DETERMINE WHAT COULD HAVE CAUSED THE PRESSURIZATION PROB. WE NOTICED THAT THE L BLEED AIR SWITCH WAS NOT IN THE ON POS. WE ALSO NOTICED THAT THE BULB IN THE SWITCH LIGHT WAS NOT ILLUMINATED. THE BLEED AIR SWITCH LIGHTS USE A DARK SWITCH CONCEPT. IN OTHER WORDS WHEN THE SWITCH IS IN THE 'ON' POS THERE IS NO 'ON' ANNUNCIATION. A DARK INDICATION MEANS THE SWITCH IS ON. IF THE SWITCH IS PUSHED OFF AN 'OFF' ANNUNCIATION APPEARS IN THE SWITCH LIGHT. DURING OUR PRESSURIZATION PROB I LOOKED AT THE BLEED SWITCHES AND SAW 2 'DARK' SWITCHES. I MISTAKENLY ASSUMED BOTH BLEED SWITCHES WERE ON. IN ACTUALITY ONLY 1 BLEED AIR SWITCH WAS ON. THE ONE BLEED SYS WAS NOT ENOUGH TO MAINTAIN ADEQUATE PRESSURIZATION. THERE ARE SOME THINGS THAT COULD HAVE BEEN DONE; FROM A PROCS POINT OF VIEW; TO PREVENT THIS INCIDENT FROM OCCURRING. ONE WAS TO CHK THE BLEED AIR PRESSURE INDICATORS AGAINST SWITCH POS. I JUST LOOKED AT THE SWITCHES THEMSELVES AND SAW THAT THEY WERE DARK AND ASSUMED THAT THEY WERE ON. WE ALSO FOUND SOME DEFICIENCIES IN OUR NORMAL CHKLIST. ON OUR FL180 CHK THERE WERE NO ITEMS RELATING TO SWITCH POS OR BLEED AIR PSI CHKS. ALL WE WERE CHKING WAS OUR PRESSURIZATION PANEL (CABIN ALT; CABIN DIFFERENTIAL; CABIN RATE OF CLB). WE ALSO DETERMINED THAT THERE ARE 4 VERY SMALL BULBS IN THE BLEED AIR SWITCH LIGHT. DURING A TEST OF THE LIGHTS IT IS VERY DIFFICULT TO DETERMINE HOW MANY OF THE 4 LIGHTS ARE BURNED OUT. IT'S A TEDIOUS PROCESS.

Data retrieved from NASA's ASRS site as of January 2009 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.