The aircraft took off with 17 parachutists. At 3500 feet the rear ramp was opened and one parachutist dropped. The intention thereafter was to climb to 5000 feet and drop the remaining parachutists. During the climb two of them tried to close the ramp. They had problems to lock it and pushed on the lower part of it. Suddenly the ramp broke in the middle, and the two parachutists fell out. They opened their parachutes and landed without problems. The ramp remained hanging below the aircraft, creating an aerodynamic pitch-down force. The aircraft lost 500 feet before recovery. The pilot was unsure about the low-speed flight characteristics and decided to drop all parachutists before landing. The landing was performed without problems. investigation revealed that the attempt to lock the ramp had been done with the lower hooks in a down-folded position, which was against the manufacturer’s procedures.