* Problem could cause engine stall
*"Minor" tweaks for fuel-pump software planned
*Impact on flight testing schedule uncertain (Recasts with Pentagon statement)
By Jim Wolf
WASHINGTON, Oct 1 (Reuters) - Flight testing of Lockheed Martin's (LMT.N) F-35 fighter jet, the costliest U.S. arms purchase, has been suspended after discovery of a fuel pump sequencing problem that could have caused engine stall, the Defense Department said on Friday.
All three F-35 models have been grounded as a precaution pending "minor" modification of software that controls signal timing, Cheryl Irwin, a Pentagon spokeswoman, said in a statement emailed 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.
A software fix has been developed and is due to be installed in test aircraft starting Oct. 5, Irwin said. The faulty sequencing was discovered during laboratory testing.
"It could possibly trigger a shutdown of all three boost pumps, potentially further causing engine stall," the spokeswoman said.
But she said a simultaneous shutdown of the United Technologies Corp's (UTX.N) Pratt & Whitney F135 engine's three fuel "boost pumps" was unlikely.
The grounding was the latest hitch in a high-profile multinational program revamped by Defense Secretary Robert Gates this year to deal with cost overruns and schedule slips.
"The aviation development process discovers technical challenges that force programs to pause, reassess, resolve, and continue," Vice Admiral David Venlet, executive officer for the Pentagon program, said in the statement.
The grounding's impact on the flight test schedule is unknown, John Kent, a Lockheed Martin spokesman, said in a separate emailed statement.
Lockheed, the Pentagon's No. 1 supplier by sales, said it had never experienced the fuel pump sequencing anomaly in any flight condition.
Lockheed and BAE Systems Plc (BAES.L), the fuel system software developer, have identified a fix and begun testing that software in their labs, Kent said.
BAE and Northrop Grumman Corp (NOC.N) are Lockheed's chief F-35 subcontractors. An alternate, interchangeable engine is being developed by a joint venture of General Electric Co (GE.N) and Britain's Rolls-Royce Group Plc (RR.L).
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)