WASHINGTON, Aug 13 (Reuters) - The U.S. Defense Department on Wednesday said it had further eased limits on flights of some Lockheed Martin Corp F-35 fighter jets that were imposed after a June 23 engine failure that grounded the F-35 fleet for over three weeks.
The Pentagon also said it was making progress in separate contract negotiations with both prime contractor Lockheed, and enginemaker Pratt & Whitney, a unit of United Technologies Corp , for the next batches of aircraft and engines.
The twenty-plus F-35 test planes in the U.S. fleet will now be able to fly six hours between engine inspections for aerial refueling missions and weapons testing, up from three hours, said F-35 program office spokesman Joe DellaVedova.
Other test flights and flights of dozens of F-35 training aircraft operated by the Air Force and Marine Corps are still subject to the three-hour mandatory engine inspections that were implemented after the engine mishap, he said.
The Pentagon had already relaxed some flight restrictions on the test aircraft last month, allowing them to resume flying at speeds of 1.6 Mach, up from 0.9 Mach.
Training jets operated by the U.S. Air Force and Marine Corps remain subject to speed limits that were in place even before the engine incident, which prevented the jet's debut at two UK air shows last month.
DellaVedova said military officials continued to investigate the root cause of the June 23 incident in which the engine on an Air Force training jet broke apart and caught fire just before takeoff from Eglin Air Force Base in Florida.
He said contract negotiations between the F-35 program office and the two companies were "progressing" but gave no indication when those deals could be signed.
Lockheed is in talks with the Pentagon about a contract for an eighth batch of 43 fighter planes. Pratt is negotiating with the Pentagon about the seventh and eighth batches of engines, which the government buys separately.
Air Force Lieutenant General Chris Bogdan, who runs the F-35 program for the Pentagon, earlier this year said he expected to reach agreement with both companies by late May or early June, although he told reporters at the time that he would "not rush into a bad deal."
Bogdan is due to speak to a conference of F-35 suppliers this week as the Pentagon continues work with Lockheed, Pratt and other contractors to drive down the cost of the F-35, the Pentagon's costliest new weapons program. (Reporting by Andrea Shalal; editing by Andrew Hay)