Photo/IllutrationAn Osprey transport aircraft of the U.S. military spews smoke from near its left engine at Oita Airport in Kunisaki, Oita Prefecture, on Sept. 7. (Hiroyuki Kikuchi)

The serious accident rate of the U.S. Marines’ Osprey surged to the highest level since its deployment to Japan in 2012, dealing a blow to the government’s reassurances about the safety of the tilt-rotor transport aircraft.

The accident rate for the MV-22 Osprey was 3.27 per 100,000 flight hours as of the end of September, the Defense Ministry said Nov. 8, quoting figures provided by the U.S. Defense Department the previous day.

The rate is calculated based on occurrence of Class A accidents, which are defined as incidents causing at least $2 million (227 million yen) in damage or death.

In comparison, the accident rate was 2.72 for all aircraft deployed by the U.S. Marine Corps around the world.

“The Osprey has recently been involved in a series of serious accidents,” Defense Minister Itsunori Onodera said in response to reporters’ questions on Nov. 8. “We will call on the U.S. side to operate the aircraft safely.”

The Japanese government has often defended the safety record of the Osprey by saying that its accident rate is either on par with or lower than the overall average of all aircraft deployed by the U.S. Marines.

The Pentagon said the Osprey’s accident rate surged because of three Class A incidents that occurred over the past year.

In December, an Osprey crash-landed in waters off Nago, Okinawa Prefecture. Another Osprey went down in waters off northeastern Australia in August, killing three Marines. The following month, an Osprey crashed in Syria, causing injuries to two service members.

The first two accidents involved Ospreys stationed at the U.S. Marine Corps Air Station Futenma in Ginowan, Okinawa Prefecture, the only U.S. base in Japan where the aircraft is deployed.

Protests were held in Okinawa Prefecture and elsewhere in Japan when the first fleet of Ospreys was deployed to the Futenma base in 2012.

In response to widespread concerns about the Osprey’s safety record, the government pointed to its accident rate of 1.93 at that time, compared with the average of 2.45 for all U.S. Marines’ aircraft.

The ministry insists that the accident rate is just “one indicator of the safety records.” It also said it uses data other than the accident rate to reassure locals about the Osprey’s safety.

The ministry tried to ease concerns of officials with the Saga prefectural government, saying the Osprey’s accident rate of 2.62 at the end of September last year was comparable with the average for the U.S. Marines’ entire fleet.

The Ground Self-Defense Force plans to introduce Ospreys at Saga Airport in the prefectural capital of Saga. The first batch is expected to arrive in Japan in autumn next year.