DASH 8 LNDG GEAR DID NOT RETRACT AFTER TKOF. FLT CREW DISCOVERED THAT THE ALTERNATE GEAR DOOR WAS OPEN; PREVENTING THE GEAR FROM RETRACTING.

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ON TKOF; GEAR WAS SELECTED UP AND THE GEAR DID NOT RETRACT. WE MADE THE DECISION TO RETURN TO THE FIELD. RECEIVED RADAR VECTORS FOR A DOWNWIND LEG. APPROX 5 MINS AFTER TKOF; CAPT LOOKS ABOVE MY HEAD AND NOTICES THE ALTERNATE GEAR EXTENSION DOOR IS OPEN AND CLOSED IT. THE GEAR RETRACTS AND WE CONTINUE ON TO ZZZ. WE RECEIVED THE ACFT APPROX 5 MINS BEFORE SCHEDULED DEP TIME AFTER BEING TOWED TO THE SPOT FROM MAINT AS OUR SCHEDULED ACFT WAS ON A MECHANICAL. I COMPLETED THE EXTERIOR INSPECTION AND MOVED IN TO THE COCKPIT AND ACCOMPLISHED MY NORMAL DUTIES. SOMEHOW I MISSED THE ALTERNATE GEAR EXTENSION DOOR ABOVE THE FO'S HEAD. THE DOOR AREA TO THE FLT DECK IS A BUSY PLACE WHICH COULD HAVE LEFT ME FEELING RUSHED. I'M NOT SURE IF IT WAS FULLY OPEN OR JUST NOT CLOSED ALL THE WAY AS I WAS NOT THE ONE WHO CLOSED IT. WHEN THE GEAR DIDN'T RETRACT; I AM SO USED TO HAVING MAINT PROBS WITH THE -400 THAT IT DIDN'T EVEN CROSS MY MIND THAT IT COULD HAVE BEEN CAUSED BY ME. THE LOCATION OF THE DOOR WOULD LEAVE IT TO NEVER BE CAUGHT BY A CREW EXCEPT FOR THE CHKLIST. FEELING RUSHED; I TRY TO REMEMBER TO STAY CALM AND DO MY CHKLISTS METHODICALLY AND NOT ALLOW THE NORMAL DUTIES OF OTHERS TO DISTRACT ME.