What was found
AFTER T/O FROM PIE, THE LANDING GEAR LEVER WAS PLACED IN THE UP POSITION, CREW REPORTED NO RESPONSE TO GEAR HANDLE SELECTION, GEAR REMAINED DOWN AND LOCKED AND MLG DOORS OPEN. RECYCLED THE GEAR HANDLE, NO HELP. RETURNED TO PIE. DURING THE POST FLIGHT INSPECTION THE MAINTENANCE BYPASS DOOR WAS FOUND IN THE DOWN POSITION. WHEN THE MAINTENANCE BYPASS DOOR WAS CLOSED AND WITH HYDRAULIC PRESSURE APPLIED THE MAIN GEAR DOORS CLOSED. THE LEFT GEAR DOOR WAS REPLACED IAW MM 32-12-1 AND A GEAR SWING WAS PERFORMED IAW MM 32-30-0 WITH ALL GEAR SYSTEMS OPERATING NORMAL. THE BYPASS DOOR LATCH WAS REPLACED DUE TO AN UNSERVICEABLE SPRING AND WAS DETERMINED TO BE THE PRIMARY CAUSE OF THE AIR TURN BACK. (M)