A CE402B COMMERCIAL RATED PLT TRIED TO SECURE THE CREW DOOR INFLT AFTER OBSERVING THE DOOR ANNUNCIATOR ILLUMINATED. THIS DOOR SUBSEQUENTLY OPENED AFTER THE PLT MADE AN ATTEMPT TO SECURE IT CLOSED.

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I WAS FLYING A CE402B. AFTER A SERIES OF TURB; THE DOOR WARNING LIGHT CAME ON. I DISREGARDED IT BECAUSE THAT AIRPLANE HAS A HISTORY OF FALSE WARNINGS (DUE TO STICKY MICROSWITCHES ON CREW AND CARGO DOORS). THAT PROB WAS RPTED TO MAINT SEVERAL TIMES; BUT WAS NEVER REALLY CORRECTED. AFTER A MORE SEVERE JOLT OF TURB; THE BACK SIDE OF THE CREW DOOR CRACKED OPEN. WITH THE TURB AND THE DOOR VIBRATIONS; I DECIDED TO TRY TO CLOSE IT; AS I WAS FEARING TO LOOSEN THE DOOR COMPLETELY (THERE IS NO CHKLIST OR PROCS TO FOLLOW IN THAT CASE IN 402 FLT MANUAL; HOWEVER; IN THE CESSNA SUPPLEMENT IT IS STATED THAT A PLT MAY DECIDE TO RECLOSE A DOOR IT IF OPENS INFLT). I BARELY TOUCHED THE HANDLE TO TRY TO LATCH THE BACK SIDE OF THE DOOR BACK IN; THAT THE DOOR FLEW OFF. AFTER LNDG; I DISCOVERED THAT THE DOOR WAS STILL ATTACHED TO THE AIRPLANE; BUT SEVERELY DAMAGED. I WAS UNABLE TO SEE THE INVESTIGATION RPT FROM MY COMPANY'S MAINT PERSONNEL; BUT I WAS INFORMED THAT THE BACK LATCH WAS DEFECTIVE. I ALSO FOUND OUT THAT THE HANDLE THAT CTLS BOTH FRONT AND BACK LATCHES FOR THAT DOOR IS SPRING LOADED TO OPEN. THE CREW DOOR IS HINGED TO THE AIRPLANE AT THE TOP; WHICH MAKES IT IMPOSSIBLE TO RECLOSE INFLT ONCE OPENED. DURING TRAINING; THAT SIT WAS NEVER ADDRESSED; EVEN THOUGH IT HAPPENED TO OTHER PLTS IN THE PAST. ALSO THE CESSNA SAFETY SUPPLEMENT; FOUND WITH THE AIRPLANE MANUAL; THAT STATES THAT A PLT CAN TRY TO RECLOSE THE DOOR IF OPENED INFLT; IS GENERAL AND NOT SPECIFIC TO THAT MODEL. I LEARNED A VALUABLE LESSON FROM THAT: IF A DOOR OPEN INFLT THERE IS ONLY 2 THINGS YOU CAN DO: 1) TIGHTEN UP YOUR SEATBELT AND SHOULDER HARNESS. 2) LAND AS SOON AS PRACTICAL; THEN DEAL WITH THE DOOR.