Narrative:

During cruise at FL430 the cabin altitude began a slow climb. We were alerted to this by a passenger complaining of ears popping. Upon checking the pressurization controller; I noticed the cabin slowly climbing. We switched air data computer controllers for the pressurization and the cabin was still climbing. I went to manual mode and verified the outflow valve was closed; which it was fully. Not knowing yet what was causing this (door seal; window; etc) and watching the cabin slowly climb and not knowing if it was going to continue; I asked ATC for lower altitude. When the controller responded to me; I believe I phrased the request as 'we need lower right now!' we were cleared to a lower altitude in the 30's and eventually descended to FL330. During the descent we noticed that we had stopped the cabin from climbing any further by our descent. While running a checklist and coordinating with the PIC; at some point during the descent I noticed that we had apparently lost the bleed valve on the left engine as there was no air psi output. This explained the slow cabin rise; as the g-iv will not hold a 6000 ft cabin above FL410 with 1 pack. We did not feel at any time that the aircraft was in danger; and we certainly didn't feel the situation called for a full blown emergency descent procedure. It was a slow although unstoppable rise in cabin altitude due to being at FL430. It was a controled situation although we requested lower immediately because at the time we weren't sure what was causing the problem. I didn't want to assume a very slow cabin rise would stay that way. It was after determining that we had lost the left bleed valve for whatever reason that we didn't have a pressurization problem; we had a limitation problem. The g-iv can't maintain a normal cabin on 1 pack above FL410 and we would not have cowl anti-ice on the left engine so we could not continue if the flight would take us into known icing. The WX was VMC the rest of the way; and FL340 was acceptable to us because we had the fuel for a lower altitude. The g-iv QRH didn't really expand upon the situation as if the only reason they could see for not having bleed air from that engine would be if it was shut down. The flight was uneventful. 2 weeks later; the director of operations informs us that the FAA is 'looking into' the flight due to an airspeed or separation issue. This was the first we heard that there was a problem regarding the flight. The controller that was handling us during the event said nothing and the handoff was the normal 'see ya.' we could have taken a turn; a present position hold if we were unable lower straight ahead. We were cleared to our altitude. In hindsight we should have called the center upon landing to let them know what had happened and talked to a supervisor right then and there. At least they would know what had happened and we would know what it is the FAA is 'looking into.' the g-iv has a design gotcha in that there is no light; warning; caution; switch position that indicates failure of the bleed valve. There is a small led readout at the very top corner of the overhead panel indicating the air psi. Unless one flies staring at the readout the entire flight; the first indication that a bleed valve has failed is the rise in cabin altitude. Usually the 'cabin altitude' caution. I feel this is a very important system; especially at FL450 and it would be nice if a caution alerted you to a failure before the other related system are affected. A rising cabin should not be the first indication of this failure; nor should the loss of cowl anti-ice when you are 'in' icing conditions be the first indicator of bleed valve failure. The detection is after the fact and puts the pilots into the situation of solving problems rather than preventing them. I know that we are not the only g-iv that has had this problem. If you fly a g-iv it has happened; or it will one day. I think we will add a check of the bleed psi to our in-flight trend monitoring; it might up the odds of us catching this sooner 'when' it happens again.

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Original NASA ASRS Text

Title: G IV WOULD NOT MAINTAIN CABIN PRESSURIZATION DUE TO A FAILED ENG BLEED VALVE. FLT CREW REQUESTED A LOWER ALT AND CONTINUED FLT WITH ONE PACK.

Narrative: DURING CRUISE AT FL430 THE CABIN ALT BEGAN A SLOW CLB. WE WERE ALERTED TO THIS BY A PAX COMPLAINING OF EARS POPPING. UPON CHKING THE PRESSURIZATION CONTROLLER; I NOTICED THE CABIN SLOWLY CLBING. WE SWITCHED ADC CONTROLLERS FOR THE PRESSURIZATION AND THE CABIN WAS STILL CLBING. I WENT TO MANUAL MODE AND VERIFIED THE OUTFLOW VALVE WAS CLOSED; WHICH IT WAS FULLY. NOT KNOWING YET WHAT WAS CAUSING THIS (DOOR SEAL; WINDOW; ETC) AND WATCHING THE CABIN SLOWLY CLB AND NOT KNOWING IF IT WAS GOING TO CONTINUE; I ASKED ATC FOR LOWER ALT. WHEN THE CTLR RESPONDED TO ME; I BELIEVE I PHRASED THE REQUEST AS 'WE NEED LOWER RIGHT NOW!' WE WERE CLRED TO A LOWER ALT IN THE 30'S AND EVENTUALLY DSNDED TO FL330. DURING THE DSCNT WE NOTICED THAT WE HAD STOPPED THE CABIN FROM CLBING ANY FURTHER BY OUR DSCNT. WHILE RUNNING A CHKLIST AND COORDINATING WITH THE PIC; AT SOME POINT DURING THE DSCNT I NOTICED THAT WE HAD APPARENTLY LOST THE BLEED VALVE ON THE L ENG AS THERE WAS NO AIR PSI OUTPUT. THIS EXPLAINED THE SLOW CABIN RISE; AS THE G-IV WILL NOT HOLD A 6000 FT CABIN ABOVE FL410 WITH 1 PACK. WE DID NOT FEEL AT ANY TIME THAT THE ACFT WAS IN DANGER; AND WE CERTAINLY DIDN'T FEEL THE SITUATION CALLED FOR A FULL BLOWN EMER DSCNT PROC. IT WAS A SLOW ALTHOUGH UNSTOPPABLE RISE IN CABIN ALT DUE TO BEING AT FL430. IT WAS A CTLED SITUATION ALTHOUGH WE REQUESTED LOWER IMMEDIATELY BECAUSE AT THE TIME WE WEREN'T SURE WHAT WAS CAUSING THE PROB. I DIDN'T WANT TO ASSUME A VERY SLOW CABIN RISE WOULD STAY THAT WAY. IT WAS AFTER DETERMINING THAT WE HAD LOST THE L BLEED VALVE FOR WHATEVER REASON THAT WE DIDN'T HAVE A PRESSURIZATION PROB; WE HAD A LIMITATION PROB. THE G-IV CAN'T MAINTAIN A NORMAL CABIN ON 1 PACK ABOVE FL410 AND WE WOULD NOT HAVE COWL ANTI-ICE ON THE L ENG SO WE COULD NOT CONTINUE IF THE FLT WOULD TAKE US INTO KNOWN ICING. THE WX WAS VMC THE REST OF THE WAY; AND FL340 WAS ACCEPTABLE TO US BECAUSE WE HAD THE FUEL FOR A LOWER ALT. THE G-IV QRH DIDN'T REALLY EXPAND UPON THE SITUATION AS IF THE ONLY REASON THEY COULD SEE FOR NOT HAVING BLEED AIR FROM THAT ENG WOULD BE IF IT WAS SHUT DOWN. THE FLT WAS UNEVENTFUL. 2 WKS LATER; THE DIRECTOR OF OPS INFORMS US THAT THE FAA IS 'LOOKING INTO' THE FLT DUE TO AN AIRSPD OR SEPARATION ISSUE. THIS WAS THE FIRST WE HEARD THAT THERE WAS A PROB REGARDING THE FLT. THE CTLR THAT WAS HANDLING US DURING THE EVENT SAID NOTHING AND THE HDOF WAS THE NORMAL 'SEE YA.' WE COULD HAVE TAKEN A TURN; A PRESENT POS HOLD IF WE WERE UNABLE LOWER STRAIGHT AHEAD. WE WERE CLRED TO OUR ALT. IN HINDSIGHT WE SHOULD HAVE CALLED THE CTR UPON LNDG TO LET THEM KNOW WHAT HAD HAPPENED AND TALKED TO A SUPVR RIGHT THEN AND THERE. AT LEAST THEY WOULD KNOW WHAT HAD HAPPENED AND WE WOULD KNOW WHAT IT IS THE FAA IS 'LOOKING INTO.' THE G-IV HAS A DESIGN GOTCHA IN THAT THERE IS NO LIGHT; WARNING; CAUTION; SWITCH POS THAT INDICATES FAILURE OF THE BLEED VALVE. THERE IS A SMALL LED READOUT AT THE VERY TOP CORNER OF THE OVERHEAD PANEL INDICATING THE AIR PSI. UNLESS ONE FLIES STARING AT THE READOUT THE ENTIRE FLT; THE FIRST INDICATION THAT A BLEED VALVE HAS FAILED IS THE RISE IN CABIN ALT. USUALLY THE 'CABIN ALT' CAUTION. I FEEL THIS IS A VERY IMPORTANT SYS; ESPECIALLY AT FL450 AND IT WOULD BE NICE IF A CAUTION ALERTED YOU TO A FAILURE BEFORE THE OTHER RELATED SYS ARE AFFECTED. A RISING CABIN SHOULD NOT BE THE FIRST INDICATION OF THIS FAILURE; NOR SHOULD THE LOSS OF COWL ANTI-ICE WHEN YOU ARE 'IN' ICING CONDITIONS BE THE FIRST INDICATOR OF BLEED VALVE FAILURE. THE DETECTION IS AFTER THE FACT AND PUTS THE PLTS INTO THE SITUATION OF SOLVING PROBS RATHER THAN PREVENTING THEM. I KNOW THAT WE ARE NOT THE ONLY G-IV THAT HAS HAD THIS PROB. IF YOU FLY A G-IV IT HAS HAPPENED; OR IT WILL ONE DAY. I THINK WE WILL ADD A CHK OF THE BLEED PSI TO OUR INFLT TREND MONITORING; IT MIGHT UP THE ODDS OF US CATCHING THIS SOONER 'WHEN' IT HAPPENS AGAIN.

Data retrieved from NASA's ASRS site as of May 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.