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* Incident forced pilot to abort takeoff
* Propulsion line was detached in engine component
By Andrea Shalal-Esa
WASHINGTON, Jan 18 (Reuters) - The Pentagon and U.S. Navy on Friday grounded the Marine Corps version of Lockheed Martin Corp's F-35 fighter jet after an incident that occurred during a training flight at Eglin Air Force Base in Florida on Wednesday.
The Pentagon's F-35 program office said the grounding affected all 25 F-35B model jets, while flights of the Air Force's A-model and the Navy's C-model were unaffected. Ground operations of the B-model planes continued, it said.
The program office it ordered the temporary suspension of flight operations after a propulsion line associated with the B-model's exhaust system failed prior to takeoff. The pilot aborted the takeoff without incident and cleared the runway, the program office said in a statement. There were no injuries to the pilot or ground crew.
The incident came just days after the Pentagon's director of testing and evaluation released an 18-page report detailing an array of problems which he said underscored the "lack of maturity" of the $396 billion fighter program.
The report and Friday's grounding of the B-model highlighted the continued growing pains of the ambitious Lockheed fighter program, which began in 2001 and has been restructured three times in recent years.
The grounding affected F-35B models, which can take off from short runways and land vertically like a helicopter, at a Maryland naval air station, the Florida air base, Marine Corps Air Station Yuma in Arizona, and Lockheed's F-35 production facility in Fort Worth, Texas.
"Implementing a precautionary suspension of flight operations is a prudent response until F-35B engineering, technical and system safety teams fully understand the cause of the failure," said Joe DellaVedova, spokesman for the Pentagon's F-35 program office. "Safety of pilots and ground crew is the top priority of the program."
The fuel line in question enables actuator movement for the exhaust system associated with the B-model's engine. Instead of traditional hydraulic fluid, it instead uses fuel as the operating fluid to reduce weight.
Matthew Bates, spokesman for Pratt & Whitney, a unit of United Technologies Corp, which builds the engines for the singe-engine, single-seat fighter jet, said an initial inspection discovered a detached propulsion line in the rear part of the engine compartment.
The affected fuel line is not used in the A- or C-models, which are still permitted to fly.
"A team of Pratt & Whitney and Rolls-Royce engineers is investigating the cause of the incident and working closely with Lockheed Martin and the F-35 Joint Program Office to resolve the matter," Bates said in a statement provided to Reuters.
He said there had been no previous issues with the component that triggered the grounding. Rolls-Royce Holdings builds the lift fan used in the B-model engines.
DellaVedova said teams of government and contractor experts were reviewing data from the incident to determine what caused the line to fail and decide on any potential mitigating actions.
Once the issue was understood, the program would determine when to resume flights, he said, adding that the impact on the developmental flight tests and training was being assessed.
The Pentagon report sent to Congress last week said F-35 flight tests were ahead of schedule in 2012, but lagged in some areas due to unresolved problems and newly discovered issues. About 34 percent of the test program has been completed.
The Marine Corps version of the plane flew more than planned but lagged its target for test points by 49 percent. Durability testing of the B-model had to be halted in December after multiple cracks were found on the underside of the plane's fuselage, the report said.
Lockheed is building three different models of the F-35 fighter jet for the U.S. military and eight countries that helped pay for its development: Britain, Canada, Italy, Turkey, Denmark, the Netherlands, Australia and Norway.
The Pentagon plans to buy 2,443 of the warplanes in coming decades, although many analysts believe U.S. budget constraints and deficits will eventually reduce that overall number.