WASHINGTON, Aug 1 (Reuters) - A U.S. military Osprey

aircraft crash off the coast of Japan in November that killed

all eight crew members was caused by a failure in the gear box

and the pilot's decision making contributed to it, an Air Force

investigation released on Thursday said.

The U.S. military grounded its fleet of V-22 Osprey aircraft

for months after the fatal crash, which happened during a

routine training mission on Nov. 29 off Yakushima Island, about

1,040 km (650 miles) southwest of the capital, Tokyo.

"The mishap was caused by a catastrophic failure of the

left-hand prop rotor gear box... decision-making was causal,

prolonging the mishap sequence," the investigation said.

It said that the crew did not have a sense of urgency when

they received warnings in the aircraft.

The pilot of the Osprey had received an advisory to "Land as

Soon as Practical" when the aircraft was still close to mainland

Japan and could have diverted to several closer airfields.

The investigation added that the pilot continued to fly at

8,000 feet above sea level, even though it would have been

prudent to fly below the clouds present.

At least 400 multipurpose Ospreys have been delivered and

are mainly used by the U.S. Air Force, Marines and Navy in Japan

and elsewhere, according to Boeing ( BA ), which manufactures

the Osprey along with Textron's ( TXT ) Bell Helicopter ( TXT ) unit.

The U.S. Navy aircraft carrier deployed to Japan, the USS Carl

Vinson, relies on them to deliver some supplies and personnel.

According to the Flight Safety Foundation, at least 50

personnel have died in crashes operating or testing the

aircraft. More than 20 of those deaths came after the V-22

entered service in 2007.