* Pentagon spokesman says fuel line improperly crimped
* Similar problem found on six other F-35Bs
* Suspect lines made by unit of Parker Hannifin
* F-35B Marine Corps variant expected to resume flights soon
By Andrea Shalal-Esa
WASHINGTON, Jan 28 (Reuters) - Pentagon and industry officials said on Monday a manufacturing problem was the most likely cause of an engine failure that led to the grounding of all 25 Marine Corps versions of the Lockheed Martin Corp F-35 fighter jet 10 days ago.
The investigation found that a fuel line built by a unit of Parker Hannifin Corp had been improperly crimped, which resulted in it detaching and failing just before a training flight took off at a Florida Air Force base, said Joe DellaVedova, spokesman for the Pentagon's F-35 program office.
He said engine maker Pratt & Whitney, a unit of United Technologies Corp, and Britain's Rolls Royce Plc , which build the engine for the F-35B model were taking steps "to improve their quality control process and ensure part integrity."
The F-35B should be able to resume flights as soon as the parts supplied by a unit of Parker Hannifin are replaced, said Matthew Bates, a spokesman for Pratt & Whitney.
"The team continues to work diligently toward ... implementing corrective actions with the supplier. We anticipate a return to flight for the (short takeoff, vertical landing) variant soon," Bates said.
The Pentagon's F-35 program office said it was working with the Navy to resume flights of the F-35B model, which can take off from shorter runways and land like a helicopter, but gave no timetable for when training and test flights would resume.
The grounding did not affect the Air Force or Navy versions of the new fighter since they do not use the same part.
The speedy conclusion of the investigation is good news for the F-35 program, which is racing to complete an aggressive schedule of flight tests this year. The program has completed about 34 percent of its planned test flight program, but Lockheed is already building production models of the new plane.
DellaVedova said the investigation ruled out any design or maintenance problems, but revealed that the faulty fuel line had been improperly crimped during the manufacturing process.
Similar problems were found with six additional fuel lines used on the F-35B, and all the faulty parts had been removed from the planes and shipped to Pratt for replacement.
A spokeswoman for Parker Hannifin said the company, which makes many components for the aircraft, was working around the clock to support the investigation.
The Pentagon grounded all 25 F-35B jets on Jan. 18 after the "fueldraulic" line, associated with directing the B-model's exhaust, failed just before takeoff during a training flight at Eglin Air Force Base in Florida.
Instead of traditional hydraulic fluid, the line uses fuel as the operating fluid to reduce weight.
An initial inspection after the incident discovered a detached propulsion line in the rear part of the engine compartment, and subsequent tests showed the line was not built to specifications by Stratoflex, a unit of Parker Hannifin.
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.