A C402'S LEFT SEATBACK FAILED ON DEP; REMOVING THE PLT FROM THE ACFT'S CTLS FOR 3 TO 5 MINS. THE PLT HAD DIFFICULTY RELEASING HIS SHOULDER HARNESS.

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AT THE ABOVE MENTIONED TIME; DATE; AND LOCATION; AND WHILE THE PIC OF A CESSNA 402C IN THE EMPLOY OF ABC ACR AND USING THE CALL SIGN ACR XXX THE FOLLOWING OCCURRED DURING AN ASSIGNED CLBOUT FROM ZZZ DIRECT TO ZZZ1. AT APPROX 7500 FPM; AND AT APPROX 120 KTS; THE BACK OF THE PLT'S SEAT IN WHICH I WAS SITTING GAVE WAY AND COLLAPSED; CAUSING ME TO FALL BACKWARDS IN A SEMI-RECLINING POSITION WITH THE SEATBELT AND SHOULDER HARNESS FASTENED AROUND ME; AND AS A RESULT CAUSED ME TO BE UNABLE TO REACH THE PLT'S CTLS. THIS RESULTED IN THE ACFT BEING UNCONTROLLED BY ME FOR APPROX 3-5 MINUTES WHILE I STRUGGLED TO UNFASTEN MY SEATBELT AND MOVE MYSELF TO THE RIGHT SEAT OF THE ACFT; WHICH AT ONE POINT I SAW WAS IN 40 DEG CLBING RIGHT TURN AT APPROX 100 KTS OR LESS. I ALSO NOTICED THAT I WAS FLYING IN AND OUT OF IMC CONDITIONS AS A TSTM WAS APCHING THE AREA FROM THE SW. AFTER SEVERAL FRANTIC MINUTES OF ATTEMPTING TO REPOSITION MYSELF TO THE RIGHT SEAT; I WAS ABLE TO SECURE MYSELF THERE AND NOTICED THAT THE ACFT HAD CLBED THROUGH 12300 FT MSL AND APPROX 24 TO 40 DEGS OFF MY ASSIGNED HDG. I ALSO NOTICED AS I STRUGGLED TO REGAIN CTL OF THE ACFT THAT THE HDG INDICATOR ON THE RIGHT/COPLT SIDE OF THE ACFT WAS PRECESSING AND SO I HAD TO FLY THE ACFT FROM THE RIGHT SEAT USING THE INSTRUMENTATION ON THE LEFT SEAT/PLT'S SIDE. AS I STRUGGLED TO REFAMILIARIZE MYSELF WITH FLYING A CESSNA 402 FROM THE RIGHT SEAT USING LEFT SEAT INSTRUMENTATION; I NOTICED THAT THE ACFT CONTINUED TO CLB AND DSND THROUGH MY ASSIGNED ALT AND HDG FOR ABOUT 15 OR 20 MINS; THIS WHILE I WAS ALSO IN THE MIDST OF REGAINING MY COMPOSURE FROM A PANICKED STATE OF MIND HAVING BEEN VERY FEARFUL FOR MY LIFE THAT I WAS GOING TO BE UNABLE TO CTL THE ACFT PROFICIENTLY ENOUGH TO AVOID A STALL AND CRASH. I WAS SUBSEQUENTLY ABLE TO CTL THE ACFT AND FLY TO ZZZ1 ON HDG AND ASSIGNED ALT. HOWEVER; ON APCH TO ZZZ1; I NOTICED THAT THE BRAKES ON THE RIGHT/COPLT'S SIDE OF THE ACFT WERE NOT FUNCTIONING AND I CALLED FOR EMER CREWS TO BE STANDING BY AT ZZZ1 FOR MY LNDG ON RWY 3 WHICH I WAS ABLE TO ACCOMPLISH WITHOUT FURTHER INCIDENT. IT SHOULD BE NOTED THAT THE SEATBACK WHICH COLLAPSED HAD BEEN PREVIOUSLY REPORTED TO ACR MAINT AS BROKEN -- WITH A MAINT SIGN OFF AS REPAIRED PRIOR TO MY TAKING POSSESSION OF THE ACFT FOR MY FLT. THE SEAT APPEARED TO BE IN GOOD CONDITION FOR THE BEGINNING OF MY FLT TO ZZZ1 AT XA48; 8/THURS/06.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS PLT HAS BEEN FIRED BY HIS ACR ON THE GROUNDS THAT HE SHOULD NOT HAVE FLOWN THE ACFT. HE STATES THAT HE HAD NO MEANS OF KNOWING THAT THE SEAT WAS DEFECTIVE UNTIL HE FLEW THAT ACFT. THE RPTR STATED THAT THIS WAS A HORRIFYING EXPERIENCE.