Japan's Maritime Self-Defense Force says April's crash between two helicopters was caused by insufficient altitude management and inadequate monitoring.
Two MSDF helicopters collided and fell into waters off Torishima in the Izu Islands during a nighttime submarine detection drill, killing all eight personnel.
According to the MSDF accident investigation report released on Tuesday, the two helicopters were flying in different locations under the orders of separate commanders.
It says both aircraft then headed to the same location based on an assumption that a submarine had been detected.
The report says the front of one helicopter, which was flying straight, is thought to have collided with the left side of the other one, which was turning around. It concluded that the crews were not keeping a proper watch.
Additionally, the report states that when the two helicopters were heading to the target point, information-sharing between their commanders was insufficient. It says no orders were issued to fly at different altitudes to prevent a collision.
The report says that to prevent recurrences, officials will ensure that the crew keep careful watch and when multiple aircraft are in the same area, their commanders will strictly manage altitudes.
Exercises involving multiple patrol helicopters have been suspended following the accident, but the MSDF says they will resume such training from Tuesday.