What was found
AFTER TAKEOFF WHEN THE LANDING GEAR WAS RETRACTED, THE DOOR UNSAFE LIGHT REMAINED ILLUMINATED. THE CREW RECYCLED THE LANDING GEAR AND THE INDICATION REMAINED THE SAME. THE AIRCRAFT RETURNED TO FRA, MAKING AN UNEVENFUL LANDING. MAINTENANCE INSPECTED THE AIRCRAFT AND FOUND THE MAIN GEAR DOOR UPLATCH CABLE SYSTEM MISRIGGED. MAINTENANCE CLEANED UPLATCH SWITCHES AND CORRECTED THE CABLE SYSTEM RIGGING IN ACCORDANCE WITH THE DOUGLAS MAINTENANCE MANUAL PROCEDURES.