Lockheed Martin's F-35 fighter jet grounded

The F-35 Lightning II, also known as the Joint Strike Fighter (JSF), planes arrive at Edwards Air Force Base in California in this May 2010 file photo.

WASHINGTON (Reuters) - Flight testing of all three models of the F-35 fighter jet, the Pentagon's costliest arms purchase, has been suspended pending tweaks to the fuel system software, Lockheed Martin Corp, the plane's manufacturer, said on Friday.

The grounding's impact on the flight test schedule is unknown, John Kent, a company spokesman, said in an email after the close of markets.

The radar-evading F-35, also known as the Joint Strike Fighter, is the Pentagon's costliest acquisition at a projected total of up to $382 billion for 2,457 planes over the next two decades.

Lockheed said it had discovered, during normal ground testing, a software anomaly that could cause a fuel boost pump to shut down in flight.

The F-35 contains three fuel pumps that feed United Technologies Corp's Pratt & Whitney F135 engine.

"Although we've never experienced this anomaly in any flight condition, we have suspended all flight operations as a precaution," Kent said.

Lockheed and BAE Systems Plc, the fuel system software developer, "have identified a fix and begun testing that software in our labs," he said.

BAE and Northrop Grumman Corp are Lockheed's chief F-35 subcontractors. A competitive, interchangeable engine is being developed by a joint venture of General Electric Co and Rolls-Royce Group Plc.

The United States is co-developing the F-35 with eight foreign partners -- Britain, Italy, the Netherlands, Turkey, Canada, Australia, Denmark and Norway. Together, the partners are projected to buy about 730 planes.

(Reporting by Jim Wolf; editing by Carol Bishopric)

(additional reporting by Andrea Shalal-Esa)

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