H25 FLT CREW ATTRIBUTED ALT OVERSHOOT TO HYPOXIA RESULTING FROM PRESSURIZATION FAILURE.

Narrative

DURING A NORMAL CLB TO ASSIGNED ALT OF FL230 AND COMING OUT OF FL200; BOTH PLTS CONFIRMED THE CABIN WAS NOT PRESSURIZING. IN NOTIFYING CTR; WE WERE EXPERIENCING PRESSURIZATION PROBS AND WE NEEDED TO DSND IMMEDIATELY AND RETURN TO ZZZ; OUR BASE OF OPS. WE WERE CLRED 'DIRECT ZZZ DSND TO MAINTAIN FL190.' I TOLD THE FO TO TELL THEM WE NEEDED LOWER THAN FL190; WHICH HE DID. WE WERE THEN CLRED TO MAINTAIN 17000 FT. DURING THIS CONVERSATION WITH CTR; THE FO AND MYSELF WERE BUSY WITH DONNING OXYGEN MASKS; COMMUNICATING; AND EXECUTING AN EMER DSCNT TO THE ASSIGNED ALT AND MANEUVERING THE AIRPLANE 180 DEGS TO DIRECT ZZZ. THE HIGHEST ALT I RECALL SEEING ON THE ALTIMETER WAS FL204 WHEN WE STARTED DOWN. IN A HAWKER 400 WITH PWR TO IDLE AND SPD BRAKES FULLY DEPLOYED AND AIRSPD TO THE BARBER POLE; WHICH WAS 300 KTS; IT WILL NOT TAKE VERY LONG TO COME DOWN 3000 FT TO OUR ASSIGNED ALT OF 17000 FT. KEEPING IN MIND; DURING OUR CLB THE CABIN NEVER PRESSURIZED. MY COPLT AND MYSELF WERE BOTH EXPERIENCING HYPOXIA AND THAT UNEASY; LIGHTHEADED FEELING FROM LACK OF OXYGEN. DURING THE EMER DSCNT PROC AND NOW ON OXYGEN BUT NOT FULLY RECOVERED 100% I DSNDED THROUGH 17000 FT AND ARRESTED THE DSCNT AT 15000 FT. CTR ASKED IF WE HAD CONTINUED ON DOWN TO 15000 FT. WE ANSWERED WE HAD AND THAT WE NEEDED TO GET DOWN. AT THIS TIME CTR HANDED US OFF TO PROBABLY ANOTHER CTR FREQ (I REALLY CAN'T RECALL AT THIS TIME) AND EVENTUALLY TO ZZZ APCH. ONCE IN ZZZ APCH CTL'S AIRSPACE WE ADVISED THEM WE NEEDED TO BURN FUEL OFF BEFORE ATTEMPTING A LNDG. THEY VECTORED US TO REMAIN WITHIN 15 MI OF THE VOR AND TO DSND TO 4000 FT; WHICH WE DID. ONCE WE STARTED DSNDING OUT OF 15000 FT; WE WERE AT A MENTAL LEVEL TO TROUBLESHOOT THE SIT. COMPLETING THE CHKLIST; WE WERE DUMBFOUNDED AS TO WHY WE DID NOT PRESSURIZE. ONCE WE WERE WITH APCH AND BROUGHT THEM UP TO SPD OF THE SIT; WE RECYCLED THE MAIN AIR VALVES AND REGAINED CTL OF THE PRESSURIZATION. THE CABIN PRESSURIZED AND WE REQUESTED 10000 FT WHICH WAS APPROVED AND WE TESTED THE ABILITY OF THE CTLR TO MAINTAIN THE CABIN ALT SELECTED WHICH WORKED FAULTLESSLY. WE WERE BOTH COMFORTABLE WITH THE PROB BEING SATISFIED AND CONTINUED ON THE ASSIGNED TRIP WITH NO FURTHER OCCURRENCES.