AN SA 227 METRO LINER ON INITIAL CLB AT 700 FT DECLARED AN EMER AND DIVERTED DUE TO A DOOR WARNING LIGHT FOLLOWED BY THE DOOR FULLY OPENED.

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FLT XXX WHICH I WAS THE FO ON; BEGAN AT ZZZ ARPT WITH PASSENGERS (2) ONBOARD. RAMP AGENTS CLOSED CARGO AND CABIN DOORS. ALL BEFORE ENG START CHKLISTS COMPLETE. AFTER START CHKLISTS COMPLETE WE WERE CLRED FOR TAXI FROM PARKING. SE CORNER OF ZZZ TO RWY. NW CORNER OF ZZZ APPROX 7 TO 10 MIN TAXI. TAXI CHKLISTS WERE COMPLETED; WE WERE CLRED FOR TKOF RWY TO JOIN THE 'ZZZ1' SID. WITH LINE UP CHKLIST COMPLETE. BLEED AIR SWITCHES ARE TURNED OFF HERE. DURING TAXI THEY ARE ON ONLY FOR ENVIRONMENTAL AIR; THEY CANNOT PRESSURIZE BECAUSE SQUAT SWITCH DUMP VALVE IS OPEN AT NOSE OF ACFT. THERE IS A CAB LIGHT ON THE ANNUNCIATOR PANEL THAT WOULD BE ON IF DOOR WAS NOT CLOSED. WITH ANNUNCIATOR PANEL CLR WE DEPARTED AND STARTED TURN TO R AT 400 FT AGL. I TURNED 1 BLEED AIR ON; THE CABIN APPEARED TO PRESSURIZE NORMAL. I TURNED ON 2ND BLEED AIR SWITCH APPROX 1-2 SECONDS LATER I HEARD A LOUD AIR NOISE THAT SOUNDED LIKE A FAULTY CAB DOOR SEAL. THE CAPT WITH CABIN PRESSURE CTLR ON HIS SIDE SAID 'CAB IS CLBING WITH THE ACFT. NOW WE HAVE A CAB LIGHT.' I WAS BUSY WITH ATC; WE HAD BEEN HANDED OFF TO DEP AND I HAD JUST MADE CONTACT WITH THEM. THE CABIN DOOR BLEW OPEN AND DEPLOYED ALL THE WAY DOWN. I IMMEDIATELY DECLARED AN EMER WITH ATC AND ASKED FOR A RETURN TO LNDG I DO NOT BELIEVE THEY COULD HEAR ME AND I CERTAINLY COULD NOT HEAR THEM. I WAS IN A WIND TUNNEL AND ALSO HAD ENG AND PROP NOISE AS CABIN DOOR IS JUST BESIDE AND BEHIND MY SEAT. I TOLD DEP I COULD NOT UNDERSTAND THEM AND WAS GOING BACK TO TWR. I TRIED WITH TWR AND DO NOT BELIEVE THEY COULD HEAR ME EITHER. THE CAPT WHO IS SITTING AROUND THE CORNER FROM OPEN DOOR YELLED AT ME; SAID HE COULD HEAR THEM AND INDICATED HE COULD COMMUNICATE. I HAD A VERY HARD TIME COMMUNICATING WITH THE CAPT ALSO. TWR GAVE US CLRNC TO LAND AND WE WERE ABLE TO DO THAT BASICALLY NON EVENTFUL; ONCE ON THE GND WE TAXIED CLR; I GOT UP PULLED DOOR UP AND HELD IT UP FROM MY SEAT WITH DOOR CABLE. WE TAXIED BACK TO RAMP. ACFT PROC FOR CLOSING CABIN DOOR IS BY RAMP AGENT. FLT CREWS DO NOT HAVE A PROC TO HELP WITH CLOSING CABIN DOOR AT AIR TAXI POSSIBLE CONTRIBUTING FACTORS. SA227 CABIN DOORS HAVE A DOOR CABLE ON EACH SIDE OF DOOR THEY CAN GET CAUGHT IN DOOR JAM WHEN DOOR COMES CLOSED AND SOMETIMES DO WITH THE LONG TAXI THIS COULD HAVE HAPPENED AND LET DOOR JIGGLE OPEN. DOOR HANDLE DOES NOT LOCK BY DESIGN UNTIL CABIN IS PRESSURIZED 1 TO 2 LBS PSI. CABLES HAVE ENOUGH SLACK IN THEM TO POSSIBLY EVEN GET A LOOP OVER THE DOOR HANDLE WHEN DOOR COMES CLOSED. IF THIS HAPPENED DOOR HANDLE COULD HAVE BEEN MOVED DURING LONG TAXI AND TKOF ROLL. BACK TO COMPANY. FLT CREW PROCS. THE ONLY CHK FLT CREWS DO IS CHECK CABIN DOOR LIGHT ON ANNUNCIATOR PANEL. SUPPLEMENTAL INFO FROM ACN 529833: THE FO AND I WERE THE FLT CREW MEMBERS ON AIR TAXI DEPARTING ZZZ WITH THE FINAL DESTINATION OF ZZZ2. AFTER OUR TWO PASSENGERS WERE BOARDED AND THEIR BAGS LOADED. THE RAMP AGENTS CLOSED THE CABIN AND CARGO DOORS. WE THEN COMPLETED THE 'BEFORE START' AND CLRED FOR START' CHKLISTS AND PROCEEDED TO START BOTH ENGS. AFTER THE ENGS WERE STARTED; WE COMPLETED THE 'AFTER START' CHKLIST RECEIVED OUR TAXI INSTRUCTIONS AND BEGAN TAXIING FROM OUR PARKING SPOT TO RWY 18L AT ZZZ. WE COMPLETED THE 'TAXI' CHKLIST AND HELD SHORT OF RWY 18L AFTER 7-10 MIN TAXI. AFTER BEING CLRED FOR TKOF. WE COMPLETED THE 'LINE-UP' CHKLIST AND DEPARTED RWY 18L. AT 400 FT AGL WE BEGAN OUR CLB FLOWS AND I TURNED THE AIRPLANE TO A 270 FT HEADING AS DEPICTED ON THE ZZZ1 DEP. THE FO TURNED THE FIRST BLEED AIR SWITCH ON AND THE CABIN APPEARED TO PRESSURIZE NORMALLY. HE THEN TURNED THE SECOND BLEED AIR SWITCH ON AND APPROX 1-2 SECS LATER WE HEARD A SUDDEN AND CONTINUOUS AIR NOISE THAT SOUNDED LIKE A FAULTY DOOR SEAL. I NOTICED THAT THE CABIN ALTITUDE WAS NOW CLBING ALONG WITH THE AIRPLANE AND THE CABIN DOOR LIGHT WAS NOW ILLUMINATED ON THE ANNUNCIATOR PANEL. THE CABIN DOOR THEN BLEW FULLY OPEN AND THE FO IMMEDIATELY DECLARED AN EMER WITH ZZZ DEP CTL AND REQUESTED LNDG BACK AT ZZZ. BECAUSE THE WIND NOISEWAS SO LOUD ON HIS SIDE OF THE COCKPIT; THE FO COULD NOT HEAR ATC'S RESPONSE. I COULD HEAR ATC FINE; HOWEVER; SO I SWITCHED BACK TO THE TWR FREQ AND RECEIVED CLRNC TO LAND ON RWY 18R. WE THEN PROCEEDED TO LAND ON RWY 18R AND TAXI CLR OF THE RWY WITHOUT FURTHER INCIDENT. THE FO PULLED THE DOOR CLOSED AND HELD IT CLOSED BY ONE THE DOOR CABLES FROM HIS SEAT AS WE TAXIED BACK TO OUR ORIGINAL PARKING SPOT. THE CABIN DOOR OF THE METRO 23 (SA-227) IS REQUIRED WITH A LOCKING DEVICE THAT WILL NOT ALLOW THE HANDLE TO BE MOVED FROM THE INSIDE WHEN THE CABIN IS PRESSURIZED TO APPROX 1-2 PSI DIFFERENTIAL PRESSURE. UNTIL THE CABIN IS PRESSURIZED; THE HANDLE CAN READILY BE MOVED AND THE DOOR OPENED. IN OUR SIT. THE BLEED AIR WAS ON FOR ONLY A FEW SECS BEFORE THE DOOR BLEW OPEN. THE DIFFERENTIAL PRESSURE WAS NOT YET HIGH ENOUGH FOR THE LOCKING MECHANISM TO BE EFFECTIVE. IT IS CURRENTLY THE POLICY OF OUR COMPANY FOR THE RAMP AGENTS TO CLOSE THE CABIN DOOR AND ASSURE THAT THE DOOR CABLES ARE CLR OF THE INSIDE HANDLE AND OTHER COMPONENTS OF THE DOOR. OUR PROCS DO NOT CALL FOR THE PLTS TO VISUALLY INSPECT THE DOOR HANDLE OR THE DOOR CABLES AFTER THE DOOR HAS BEEN CLOSED. I FEEL THAT IF A DOOR CABLE WAS CAUGHT ON THE INSIDE DOOR HANDLE; THE HANDLE COULD BE MOVED AND THE DOOR OPENED IF THE CABIN WAS UNPRESSURIZED. I WILL NO LONGER DEPART BEFORE VISUALLY CHECKING THE CONDITION OF THE DOOR HANDLE AND CABLES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE CABIN DOORS ARE NOT PLUG TYPE DOORS. THE RPTR SAID THE DOOR IN THIS INCIDENT IS LOCATED DIRECTLY BEHIND A BULKHEAD AFT OF THE CAPTS SEAT. THE RPTR STATED THE DOOR WHEN OPENED IS LOCATED DIRECTLY IN FRONT OF THE LEFT ENG PROPELLER. THE RPTR SAID THE DOOR IS SECURED BY SEVEN LOCK PINS AND TWO ALIGNMENT PINS. THE RPTR STATED THE DOOR IS LOCKED WHEN A BELLOWS MECHANISM SENSES CABIN PRESSURIZATION IS AT TWO POUNDS WHICH PREVENTS OPENING THE DOOR WITH THE CABIN PRESSURIZED. THE RPTR SAID WHEN THE DOOR IS OPENED ON THE GND IT IS SUPPORTED BY TWO CABLES ON THE LEFT AND RIGHT OF THE AIRSTAIRS. THE RPTR STATED WHEN THE DOOR OPENED THE LEFT CABLE CONTACTED THE LEFT PROPELLER CUTTING A 12 FT PIECE OF CABLE AND THROWING THE CUT CABLE INTO THE COCKPIT AT THE FO'S FT. THE RPTR STATED THE CARRIER HAS HAD NUMEROUS DOOR PROBS WITH THIS DOOR AND BELIEVES THE PROB BEING ALL RELATED TO AGING ACFT. THE RPTR SAID THERE WAS NO DOOR WARNING DURING TAXI AND MAINT COULD FIND NO MECHANICAL PROB. THE RPTR STATED THE CAUSE IS BELIEVED IMPROPER CLOSING OF THE DOOR BY THE RAMP AGENT BUT DOES NOT EXPLAIN LACK OF DOOR WARNING. CALLBACK FROM ACN 529833: THE RPTR STATED THE DOOR CAME FULLY OPEN. THE RPTR SAID THE AIRPLANE FLEW ADEQUATELY WITH SOME BUFFETING. THE RPTR STATED THE DOOR CLEARS THE PROPELLER WHEN OPEN BY A FOOT OR SO. THE RPTR STATED THE DOOR DID NOT DRAG ON LNDG WHICH WAS MADE WITH PARTIAL FLAPS AND EXTRA CARE. THE RPTR SAID MAINT DID NOT DETERMINE THE CAUSE. THE RPTR STATED A NEW COMPANY PROC IS TO VERIFY PHYSICAL LATCHING BY THE CREW BEFORE DEPARTURE INSTEAD OF DEPENDING ONLY ON THE DOOR WARNING SYSTEM.