REUTERS SUMMIT-Pratt sees clarity on 'root cause' of F-35 engine issue by end Sept.

(Adds military comments)

By Andrea Shalal

WASHINGTON, Sept 9 (Reuters) - United Technologies Corp's Pratt & Whitney unit said the "root cause" of the June 23 engine failure on a Lockheed Martin Corp F-35 fighter jet should be clear by the end of September, which would pave the way for over $1 billion in contracts for another 84 engines.

Engine maker Pratt & Whitney said it has delivered six of its F135 engines to the U.S. government since Aug. 29, when it resumed deliveries that had been halted after the June incident which grounded the F-35 fleet for weeks and prevented its international debut.

Bennett Croswell, president of Pratt & Whitney Military Engines, said he still hoped to keep the government's flight test plan on track through a series of measures aimed at addressing the issue and lifting restrictions on test aircraft.

"I believe that through the combination of things we will keep the flight test program on track," Croswell told the Reuters Aerospace and Defense Summit.

Separately at the Reuters summit, officials from the military and Lockheed echoed the timeline for identifying the cause of the engine incident.

"We're reasonably confident we're looking at the right thing, and then we're reasonably confident it's going to prove out in the next couple of weeks," Rear Admiral Randy Mahr, deputy director of the F-35 program, told the summit.

In June, an Air Force F-35A training jet caught fire after its Pratt engine broke apart and ripped a fuel line as the jet prepared to take off from Eglin Air Force Base in Florida. U.S. officials blame the incident on excessive rubbing of part of the engine's fan section against an adjacent seal.

The incident has raised fresh concerns about the $400 billion F-35 Joint Strike Fighter program, the Pentagon's largest-ever arms program, which is about 70 percent over initial cost estimates and years behind schedule.

U.S. officials say the program is back on track after earlier problems, although Air Force Lieutenant General Chris Bogdan last week said the engine issue had put the development program about 30 to 45 days behind schedule.

Croswell said Pratt was taking the issue very seriously and expected it to be resolved "in relatively short order." He also noted that such problems were meant to be found during the design and development phase of any new program.

The fix should be moderate in cost, Croswell said.

"It should not be a very expensive solution," he said, noting that retrofits would be possible without an expensive tear-apart of the F-35's powerful engine.

Croswell said Pratt had essentially wrapped up negotiations with the Pentagon on the 84 engines in the seventh and eighth low rate production contracts, but Bogdan wanted a clearer understanding of the root cause of the incident before signing off on an agreement valued at over $1 billion.

The Pentagon is negotiating a separate contract with Lockheed for an eighth batch of jets.

Croswell confirmed that Pratt would pay to fix the current issue and said the company had also agreed to share in the cost of future problems, but details were still being finalized.

"We have agreed in principle to provide a warranty to the U.S. government for future LRIP contracts. We have not yet finalized a warranty agreement, but P&W would likely pay a percentage of the total cost of certain performance specification shortfalls," spokesman Matthew Bates said.

Croswell said the engine failure came about eight hours after an unusual maneuver involving a roll, yaw and heavy gravity loads during sharp turns. "This is a maneuver that the engine should be able to withstand," he said.

Croswell said Pratt had already completed a baseline test of the current engine at its West Palm Beach, Florida facility, and had begun testing possible fixes. He said the company was investigating whether the density of the polyimide, a rubbery material made by Dupont, had played a role.

The part that broke apart ties the fan blades together. It is made by Cobham Plc (Other OTC: CBHMF - news) , Croswell said.

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(For more summit stories, see ) (Additional reporting by Lewis Krauskopf; Editing by Leslie Adler)