What was found
AFTER TAKEOFF, CAPTAIN OBSERVED LANDING GEAR WOULD NOT RETRACT. CAPTAIN ELECTED TO RETURN TO AIRPORT OF DEPARTURE. AIRCRAFT RETURNED TO BLOCKS WITHOUT FURTHER INCIDENT. MAINTENANCE PERSONNEL FOUND TRIM INTERLOCK CYLINDER WOULD NOT EXTEND AND ALLOW GEAR HANDLE TO BE RAISED. REMOVED AND REPLACED CYLINDER. INSTALLED ALL FOUR GEAR DOWNLOCK PINS, PRESSURIZED HYDRAULIC SYSTEM NR 3, AND RAISED AND LOWERED GEAR HANDLE. ALL OPS CHECK OK. AIRCRAFT RETURNED TO SERVICE. (X)