A catastrophic fire that nearly engulfed a Marine Corps MV-22B Osprey late last year was sparked by a leaking hydraulic line in the left-side engine nacelle, investigators found.
The fire broke out about half way through a nearly five-hour training mission, when fluid from a key hydraulic system that powers landing gear, opens the rear door and helps filter the air inlets to the Osprey's engines poured out of the lines after spikes in pressure fractured the thin-walled tubes.
The fluid drained onto the infrared suppressor section of the nacelle -- where hot exhaust from the engine is cooled to cut down on the plane's heat signature -- sparking the mid-air fire which caused more than $16 million in damage to the aircraft, according to the Judge Advocate General Manual Investigation report obtained by Military.com.
Both pilots and three crew members who were aboard the MV-22 for the Nov. 6 night vision goggle training flight survived the incident after landing the aircraft in Landing Zone Phoenix at Camp Lejeune, N.C. The aircraft has not been repaired and returned to flight status, the Corps said.
The fire occurred about seven months after the service admitted another blaze in the same part of the aircraft had ignited just before takeoff. The Corps called the earlier incident a "minor nacelle fire" in a news release at the time, and told Military.com in an email response to questions regarding the November fire that the service "was in the process of implementing appropriate aircraft modifications when this incident occurred."
"All Ospreys in flight operation have the modifications, including those that are deployed," wrote Maj. Eric Dent, a spokesman at Marine Corps headquarters in Washington. "The modifications have also been fully incorporated into the V-22 production line so that new aircraft will not require further modification after leaving the factory.
The investigation report, which was released to Military.com after a Freedom of Information Act request, also cites the maintenance control division of the New River, N.C.-based Marine Medium Tiltrotor Training Squadron 204 for allowing the MV-22 to fly a nearly five-hour training mission before undergoing an inspection of the engine air particle separator -- the area where the hydraulic lines failed.
"The evidence supports that there was not enough time remaining on the [mishap aircraft] to complete the 4.5 hour event that was scheduled," the report states. "It was not inspected due to an error in tracking flight hours accumulated on the" mishap aircraft.
Potentially more worrying, the system designed to control a fire in the Osprey's two nacelles -- a compartment situated at the end of each wing that houses the engine and tiltrotor propulsion system -- failed to extinguish the blaze even though it was activated by the pilot before he escaped the burning plane.
The Corps says it is looking into a new solution to the fire suppression failure but explained that new modifications will allow more hydraulic fluid to drain should another rupture occur. Dent added that the VMMT-204 maintainers "misinterpreted" the inspection requirement and that a new automatic logging system will correct the problem.
No one was disciplined as a result of the incident.
Pictures of the MV-22 provided in the JAGMAN report show a twisted hulk at the end of the left wing, the Osprey's huge rotors bent downward, melted composite material solidified in mid-air as it dripped toward the ground.
Problems with the Osprey's ultra-lightweight hydraulic system are not new. In 2000, a hydraulic tube ruptured after a wire bundle chafed the thin-walled titanium, causing a crash near New River that killed four Marines.
Despite a thorough redesign after the fatal crash, the Bell-Boeing manufactured Osprey suffered another hydraulic failure in March 2007 that caused an engine fire -- a failure of the same hydraulic system investigators point to in the November mishap.
An Airframe Change Notice dated Aug. 3, 2007, included in the report indicated the Osprey's engine air particle separator needed to be retrofitted with thicker hydraulic tubing.<p."Engineering investigations have shown that EAPS blower failures can cause pressure spikes of 7,000 [to] 8,000 psi ... into the EAPS hydraulic system," the report said. "Combining these pressure oscillations with any existing preload in the hydraulic tube routing can cause a catastrophic failure of the pressure tube."
The modifications were made to newer, so-called "Block B" aircraft -- the ones sent to Iraq on the Corps' first deployment of the MV-22 in combat. But some "Block A" Ospreys that were awaiting the retrofit still flew despite the danger of a hydraulic rupture.<p.But all Ospreys have now been modified to prevent the problem from happening again, the Corps said.<p."Since this incident, 100 percent of the V-22 fleet has had the mods installed and there have been no additional occurrences of incidents of this type," Dent said. "We have confidence in all of the MV-22s Marines are flying today, both in training and in combat."