Crew Was Able to Land Tilt-Rotor Safely; Problem Has Appeared Before

Investigators have determined an in-flight fire that broke out onboard a Bell-Boeing Marine Corps MV-22B Osprey during a training flight last year was caused by hydraulic fluid leaking onto an exhaust cooling component in the aircraft's left engine nacelle.

According to the Judge Advocate General Manual Investigation report -- obtained by Military.com through a Freedom of Information Act request -- a leaking hydraulic line spilled fluid onto the engine's infrared suppressor section, sparking a blaze that nearly consumed the aircraft in the November 6, 2007 incident.

The fire occurred about halfway through a scheduled five-hour training mission at Camp Lejeune, NC. Fortunately, the Osprey's crew was able to land the stricken aircraft, and escape before flames spread.

In a revelation that will do nothing to downplay intense criticism of the tilt-rotor aircraft, the JAG report also notes a similar fire broke out in the same section on another Osprey earlier that year. In that case, a fire broke out just before takeoff.

The Marines described that incident as a "minor nacelle fire," and noted modifications were underway to alleviate the problem, which stems from spikes in hydraulic pressure that may fracture the lines.

"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."

Those modifications -- essentially, thicker lines in the problem areas -- have since been retrofitted to all new "Block B" Ospreys, including aircraft sent to Iraq. The Corps is also looking at a more effective fluid dumping system, to drain more fluid quickly in the event of another rupture.

"All Ospreys in flight operation have the modifications, including those that are deployed," wrote Marine Corps spokesman Maj. Eric Dent. "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... 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."

In its report on the November 2007 fire, the JAG found fault with the Training Squadron's maintenance division, saying technicians should not have allowed the MV-22 to fly such a long training mission before undergoing a 4.5-hour inspection of the hydraulic lines in the engine air particle separator, where both failures occurred. The report cited errors in tracking flight hours on the accident aircraft.

The Osprey features a unique, lightweight hydraulic system... designed to power the aircraft's landing gear, rear cargo door and air inlet control. Military.com notes that system also contributed to a fatal 2000 training crash, in which a wire bundle chafed against the thin-walled titanium lines. Four Marines died in the New River, NC crash.

Lastly, the JAG report also notes the Osprey's fire suppression system failed to deploy in the November 2007 accident, despite having been triggered by the MV-22's pilot as he bailed out of the aircraft. Officials say they're looking into that problem, as well.

FMI: www.marines.mil