FLC OF AN MDT ACR ACFT LEFT THE ALTERNATE GEAR EXTENSION DOOR OPEN DURING PREFLT CAUSING A RETURN LAND AFTER THE GEAR WOULD NOT RETRACT AFTER TKOF.

Narrative

AFTER DEPARTING RWY 13 AT CMX THE CAPT CALLED FOR GEAR UP. AFTER MOVING THE GEAR SELECTOR; THE GEAR DID NOT COME UP. WE CYCLED THE GEAR AGAIN BUT IT STILL DID NOT RETRACT. WE PUT THE LEVER IN THE DOWN POS AND RECEIVED A DOWN AND 3 GREEN FOR THE GEAR. WE LANDED AND TAXIED TO THE GATE. ON A POSTFLT INSPECTION; WE FOUND THAT THE ALTERNATE GEAR EXTENSION DOOR WAS NOT IN A FULLY CLOSED POS. IN THE DASH-8 THIS RESTRICTS THE GEAR FROM OP. DOOR WAS CLOSED; VERIFIED BY MECH AND THE FLT WAS REDISPATCHED WITHOUT FURTHER INCIDENT. THIS DOOR IS OPENED ON THE PREFLT SO THAT WE CAN OPEN THE GEAR DOORS. UPON COMPLETION OF THE PREFLT; EITHER THE DOOR WAS NOT CLOSED OR WAS NOT CLOSED ALL THE WAY. ACCEPTANCE FLOWS WERE COMPLETED BUT THIS ITEM APPARENTLY WAS NOT CHKED CLOSELY ENOUGH. THIS HAPPENED EARLY IN THE MORNING APPROX XA00 WHILE THE CREW WAS ON A CONTINUOUS DUTY OVERNIGHT TRIP. I WAS VERY TIRED AND THIS COULD HAVE CONTRIBUTED TO THE PROB. I ALSO FEEL THAT SINCE WE OPEN THIS DOOR; AND IT IS CRITICAL TO GEAR OP; THAT IT SHOULD BE INCORPORATED INTO OUR CHKLIST. AT THE CURRENT TIME IT IS NOT.