By Andrea Shalal
WASHINGTON, July 29 (Reuters) - U.S. authorities have loosened some flight restrictions on Lockheed Martin Corp's F-35 fighter jets, allowing pilots to fly faster and take tighter curves, as military officials investigate a massive engine failure that grounded the entire fleet of warplanes for over three weeks.
Pilots can now fly at speeds of up to 1.6 Mach, up from 0.9 Mach, and carry out turns with a gravitational load of 3.2 Gs, up from 3 Gs, a U.S. defense official and other sources familiar with the program told Reuters on Tuesday.
The changes will allow F-35 test pilots and trainers to carry out additional maneuvers and flight tests, but it remains unclear how much of an impact the grounding and restrictions will have on the program's flight test schedule for the year.
U.S. airworthiness officials are still requiring inspections of the Pratt & Whitney engines that power the jet every three hours, but that requirement and other restrictions may be revised as the investigation winds down, the official said.
The F-35, the world's most expensive weapons project with a price tag of about $400 billion, was grounded for weeks after an engine failure and fire that damaged a U.S. Air Force F-35 plane as it prepared to take off from a Florida air base on June 23.
The incident prevented the jets from making their international debut at two British air shows this month, and could compound software development challenges detailed in a report prepared before the grounding by the Pentagon's chief arms buyer. A summary of the report was obtained by Reuters.
The report criticized a March 2014 master schedule developed by Lockheed and forecast delays of up to 14 months in delivery of certain software still under development, but said it did not expect those delays to affect the schedule for the Marine Corps and other services to start using the jets in combat.
The Pentagon's F-35 Joint Program Office (JPO) said it agreed with eight recommendations made by the Pentagon report to improve software development, and was implementing the changes.
Joe DellaVedova, F-35 program spokesman, said the program had delivered 7.4 million lines of software code so far. The overall program will include 8 million lines of code for the aircraft, and 15.9 million lines for ground systems such a computer-based logistics system, he said.
Lockheed spokeswoman Laura Siebert said the company was working with the Pentagon to assess the impact of the grounding and flight limitations on the F-35 flight test program.
"The development schedule has margins built in to allow for discoveries and occurrences like these, and we remain confident that we will complete 2B software to support Marine Corp (initial operating capability) in July 2015," she said.
The flight restrictions will be lifted when the root cause of the incident have been clearly identified, DellaVedova said.
Sylvia Pierson, F-35 spokeswoman at a southern Maryland air base, said the Marine Corps' F-35 B-model jets and the Navy's C-model jets were now carrying out five or six flight tests a day.
Officials have determined that the engine failure occurred when a component in the third stage fan blade rubbed too hard against an adjacent seal during a certain flight maneuver, according to a briefing provided to U.S. lawmakers.
Details about the maneuver were not immediately available. A second defense official said other jets in the F-35 fleet that carried out the same maneuver did not exhibit signs of the "excessive rubbing" seen in the engine that failed.
(Reporting by Andrea Shalal; Editing by Mohammad Zargham)