A CESSNA 550 CITATION DURING DSCNT FROM FL350 AND PASSING THROUGH FL270 CREW NOTED CAB CLIMBING. DURING ABNORMAL CHKLIST THE CAB RAPIDLY DEPRESSURIZED. DECLARED AN EMER AND DIVERTED.

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DURING DSCNT FROM FL350 TO FL260 AND PASSING THROUGH FL270; I NOTICED THAT THE CABIN WAS CLIMBING AT APPROX 500 FPM WITH A CABIN ALT OF APPROX 5000 FT. NORMAL CAB ALT SHOULD HAVE BEEN APPROX 3000 FT. THE CAPT AND I DISCUSSED THE FACT THAT THIS PARTICULAR ACFT HAD EXPERIENCED A RAPID DEPRESSURIZATION WITH SIMILAR INITIAL SYMPTOMS APPROX 1 MONTH AGO. THE CAPT AND I FURTHER DISCUSSED CHANGING THE PRESSURIZATION SOURCE; AND PULLED OUT THE EMER/ABNORMAL CHKLIST TO SEEK ADDITIONAL GUIDANCE. WE ASKED THE CENTER CTLR FOR A LOWER ALT AND WERE TOLD TO EXPECT ONE SHORTLY. THE EMER ABNORMAL CHKLIST CONTAINED NO INSIGHTFUL INFO AND WE WERE PREPARING TO REAFFIRM OUR REQUEST FOR A LOWER ALT WHEN THE ACFT RAPIDLY DEPRESSURIZED. WE DONNED OUR OXYGEN MASKS; DECLARED AN EMER; INITIATED AN EMER DSCNT AND ENSURED THAT OUR PASSENGERS WERE BREATHING FROM THEIR OXYGEN MASKS. WE COMPLIED WITH ALL APPLICABLE EMER AND ABNORMAL PROCS AND CHKLISTS AS WE DESCENDED TO 10000 FT MSL. WE THEN INFORMED OUR PASSENGERS OF THE NATURE OF THE PROBLEM; AND COMPLETED A NORMAL LNDG AT ZZZ ARPT APPROX 15 MINS LATER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS AIRPLANE HAD FOUR SERIOUS INCIDENTS OF RAPID DEPRESSURIZATION WITHIN A PERIOD OF ONE MONTH. THE RPTR SAID TWO OF THE INCIDENTS OCCURRED WHILE HE WAS FLYING THE AIRPLANE AND THIS RPT COVERS THE THIRD INCIDENT. THE RPTR STATED THE FIRST AND SECOND INCIDENT OF RAPID DEPRESSURIZATION WERE CORRECTED BY COMPANY MAINT WITH THE REPLACEMENT OF THE CABIN PRESSURE CTLRS. THE RPTR SAID IN THIS THIRD EVENT AN EMER WAS DECLARED AND DSCNT TO 10000 FT WITH NO INJURIES TO CREW OR PASSENGERS. THE RPTR STATED THE DIVERSION ARPT JUST HAPPENED TO BE A AUTHORIZED MANUFACTURERS SVC CENTER AND THE AIRPLANE WAS TURNED OVER TO THEM FOR A POSITIVE FIX. THE RPTR SAID AT THIS MAINT FACILITY CLAMPS WERE TIGHTENED; CONNECTIONS AND HOSES WERE REPLACED AND THE AIRPLANE WAS RETURNED TO SVC. THE RPTR STATED AFTER DEPARTING THE AUTHORIZED MANUFACTURERS REPAIR FACILITY AND CRUISING AT FL350 THE FOURTH INCIDENT OF CAB DEPRESSURIZATION OCCURRED. THE RPTR SAID THE AIRPLANE WAS FLOWN DEPRESSURIZED TO THE COMPANY BASE WERE MAINT FOUND A DEFECTIVE INTERMITTENT SQUAT SWITCH THAT WOULD FAIL IN THE GND MODE CAUSING THE OUTFLOW VALVE TO GO TO THE FULL OPEN POSITION IN FLT.