Officials have determined that an engine fire in a Marine Corps V-22 Osprey probably caused more than $1 million in damage, putting the incident in the most serious category of mishaps.

The fire, which remains under investigation, occurred Dec. 7 around 10 p.m. after the V-22 landed at Marine Corps Air Station New River, NC, according to Staff Sgt. Angela Mink, a spokeswoman for the station.

The fire occurred in the left nacelle of the V-22. The Osprey has two nacelles, one per wing, each containing an engine and a transmission that drives a large rotor.

Marine Corps and Air Force V-22s are continuing to fly, but they are operating under a flight clearance restriction issued last week by Naval Air Systems Command, said Navy V-22 spokesman James Darcy. Before flying, V-22 pilots must turn off the engine air particle separator, which is designed to filter out dirt and debris, he said. Though the root cause of the mishap remains undetermined, engineers believe this restriction cuts the risk of a similar incident, he said.

An aviation mishap safety board is investigating the Dec. 7 incident. The board labeled the incident a class A mishap because the damage is expected to cost more than $1 million, according to the Naval Safety Center’s Web site. By definition, class A mishaps are those which involve deaths or serious injuries, total destruction of the aircraft or damage to the aircraft and/or property exceeding $1 million.

Asked about the posting on the Naval Safety Center’s Web site, Mink first denied any decision had been made about the category of the mishap. Then she said a preliminary assessment labeling the incident class A was released by the mishap board to the Naval Safety Center. She said this is not a final conclusion, as the investigation is ongoing.

“The assessment can be changed as necessary, until the investigation is complete,” she said.

According to the Navy Department’s JAG manual, officials must determine within 60 days from the time of an accident whether it is a class A mishap. Any accident deemed a class A mishap is supposed to trigger a judge advocate general probe that would examine safety, command and criminal responsibility, and corrective action. This would be in addition to the standard mishap board investigation.

But Mink told Inside the Navy that officials have not started a JAG investigation into the Dec. 7 incident. Mink also said she is not aware of any plans for a JAG investigation. Mink said her understanding is there is no need to immediately start a JAG investigation based on the mishap board’s preliminary decision to label the incident a class A mishap.

Prior to the fire, the V-22 involved had just performed a training flight, she said. The aircraft belongs to Marine Medium Tiltrotor Training Squadron 204.

“The aircraft was shut down and the fire was extinguished on the New River flight line,” she said. “No personnel were injured and the incident is currently under investigation.”

The V-22 is a helicopter-plane hybrid developed by Bell Helicopter Textron and Boeing. The aircraft is scheduled to reach initial operational capability next summer. Officials may deploy the V-22 to Iraq or elsewhere next year.

Darcy said turning off the engine air particle separator will not cause a safety problem. However, if that practice remains in place for the long term, it could gradually increase wear and tear on the engines, he acknowledged. This sort of wear and tear was partly to blame for a V-22 emergency landing in Iceland earlier this year.

Some V-22s flew without the engine air particle separator (EAPS) between April 2005 and April 2006. The V-22s had to go without the system because it was being redesigned after a March 2005 incident, when a leak in the right-hand EAPS case drain line triggered a small fire aboard an Osprey on the ground. Darcy said the Dec. 7 mishap is unrelated to that incident.