FLT CREW OF CL550 EXPERIENCED RAPID DECOMPRESSION. DURING EMER DSCNT SQUAWKED HIJACKING XPONDER CODE VICE EMER CODE.

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WHILE DSNDING TO FL220; I NOTICED THE CABIN CLBING AT 500 FPM; AS DID THE FO; WHO WAS FLYING THE ACFT FROM THE L SEAT. NO MORE THAN 10 SECONDS AFTER THE PROB OCCURRED; THE CABIN RAPIDLY DEPRESSURIZED. WE BOTH DONNED OUR OXYGEN MASKS AND THE FO BEGAN AN EMER DSCNT. I SELECTED THE OXYGEN MIKE SELECTOR TO WHAT I THOUGHT WAS THE 'MIKE OXYGEN MASK' POS AND ATTEMPTED UNSUCCESSFULLY TO DECLARE AN EMER WITH ATC. FINALLY; I LIFTED THE MASK AND USED A HAND MIKE TO DECLARE THE EMER. SEEING MY USE OF THE HAND MIKE; THE FO ESTABLISHED COMS WITH ATC AND REAFFIRMED THE EMER. HE THEN POINTED TO THE XPONDER. INADVERTENTLY; I HAD SELECTED THE HIJACKING SQUAWK CODE. HE POINTED AGAIN; AND I RESET THE XPONDER TO EMER. WE COMPLETED OUR CHKLISTS AND PROCS; DSNDED TO 10000 FT AND CANCELED THE EMER. I WENT BACK TO THE CABIN; DESCRIBED THE SIT TO OUR ONLY PAX; WHO HAD FOLLOWED MY INSTRUCTIONS AND CORRECTLY USED HIS OXYGEN MASK. WE THEN COMPLETED A NORMAL LNDG IN GREENSBORO. THIS WAS MY SECOND RAPID DEPRESSURIZATION IN THIS ACFT IN APPROX 2 WKS. ON THIS OCCASION; I RUSHED THROUGH MY PROCS; CONTRIBUTING TO MY ERROR. SHOULD I EXPERIENCE A SUBSEQUENT PROB OF THIS NATURE; I WILL KNOW TO TAKE MY TIME AND ACCOMPLISH THESE PROCS ACCURATELY. ALTHOUGH I FELT EMBARRASSED BY THESE ERRORS; I LEARNED A VERY VALUABLE LESSON THAT WILL SERVE ME WELL IN THE FUTURE. WE TAXIED THE ACFT TO A SVC CTR TO REPAIR THE ACFT. NO CONFLICTS OCCURRED; AND ATC MADE NO REF TO OUR BRIEF BUT INCORRECT XPONDER CODE ERROR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE SUBSEQUENTLY INVESTIGATED THE MAINT HISTORY OF THIS ACFT AND LEARNED THAT IN THE SPACE OF A FEW WEEKS; IT HAD EXPERIENCED 4 RAPID DECOMPRESSIONS. THE FIRST 2 INSTANCES MAINT MADE IDENTICAL FIXES; REPLACING THE PRESSURIZATION CTLR. THE 3RD TIME; A MANUFACTURER AUTH REPAIR CTR DETERMINED THE FAULT WAS LOOSE CLAMPS AND RESULTING LEAKAGE WITH VARYING DIFFERENTIAL PRESSURES. THE ACFT WAS FINALLY TAKEN OTS AFTER THE 4TH EPISODE AND THE FAULT DETERMINED TO BE A FAULTY NOSE GEAR STRUT SWITCH WHICH WAS GIVING FALSE 'ON THE GND' INDICATIONS TO THE PRESSURE CTLR WHILE AIRBORNE. THIS OPENED THE OUTFLOW VALVES AND PRESSURIZATION WAS LOST. RPTR DISCUSSED THE PROBS ASSOCIATED WITH OPERATING MULTIPLE CONFIGNS OF THE SAME ACFT TYPE AND THE ACCOMPANYING DIFFICULTY IN RETAINING EFFECTIVE RECALL OF EMER PROCS UNIQUE TO THE INDIVIDUAL ACFT ON WHICH HE IS FLYING ON A GIVEN DAY.