Human error was behind an accident involving a Japanese Ground Self-Defense Force V-22 Osprey transport aircraft on Yonaguni, a remote island in Okinawa Prefecture, in October, the GSDF said Thursday.

During Japan-U.S. joint exercises on Oct. 27, the Osprey keeled over when taking off from a GSDF camp in Yonaguni, and the left wing touched the ground and was damaged.

The co-pilot forgot to press the start button to increase the engine speed and the captain neglected to check it, the GSDF said.

As a result, immediately after takeoff, the aircraft's altitude suddenly dropped due to insufficient power. The left wing touched the ground when the aircraft shook from side to side during an emergency landing attempt, the GSDF said.

The GSDF's investigation committee concluded that the co-pilot and captain forgot to take basic procedures, as a schedule delay and the addition of one passenger at the last minute burdened them psychologically.

The GSDF grounded its Osprey fleet in response to the accident. It plans to restart flying Ospreys after training crew members.