* Chief weapons tester says software development lags
* Problems with weapons integration, air refueling slow testing
* Cracks in fuselage halt durability testing of Marines' B-model
Lockheed Martin Corp's 's new F-35 fighter jet has completed over a third of its planned flight tests, but it is still facing problems with the helmet needed to fly the plane, software development and weapons integration, according to a report by the Pentagon's chief weapons tester.
The 18-page report, sent to Congress on Friday, included a detailed account of those issues and others, which it said underscored the "lack of maturity" of the $396 billion weapons program, the Pentagon's most expensive ever.
The program exceeded the number of flight tests and specific system tests planned for 2012 but lagged in some areas due to unresolved problems and newly discovered issues. The program has already completed over 20,000 specific tests of items and capabilities on the plane, but has 39,579 more such tests to go.
The report highlighted the continued growing pains of the ambitious Lockheed fighter program, which began in 2001 and has been restructured three times in recent years to slow down production and allow more progress on the development program.
Lockheed is building three different models of the F-35 fighter jet for the U.S. military and eight countries that helped pay for its development: Britain, Canada, Italy, Turkey, Denmark, the Netherlands, Australia and Norway.
The Pentagon plans to buy 2,443 of the warplanes in coming decades, although many analysts believe U.S. budget constraints and deficits will eventually reduce that overall number.
"The lag in accomplishing the intended 2012 flight testing content defers testing to following years, and in the meantime, will contribute to the program delivering less capability in the production aircraft in the near term," said the report prepared by Michael Gilmore, the Pentagon's director of operational test and evaluation.
Gilmore said the program remained saddled by a high level of concurrency or overlap between development, production and testing. The Pentagon planned that overlap from the start, but its top weapons buyer, Frank Kendall, last year described that approach as "acquisition malpractice."
CONDUCTING FLIGHT TESTS
The report said the program conducted 1,092 flight tests in 2012, 18 percent more than the 927 flight tests planned, running more tests than scheduled for the Marine Corps B-model and the Navy's C-model or carrier variant.
But it fell short of the flight tests planned for the Air Force's conventional takeoff A-model. That model completed 30 percent less test points than planned due to operating limits on the plane and problems with the weapon bay doors, it said.
It said flight tests were also limited by problems with the air refueling system, which led to restrictions on all A-model planes and required new instrumentation to isolate the cause.
The plane's stealthy coatings - which make it nearly invisible to enemy radars - were also peeling off on horizontal tail surfaces due to higher-than-expected temperatures during high-speed, high-altitude flights, the report said.
The Marine Corps version of the plane flew more than planned but lagged its target for test points by 49 percent due to issues with the weapon bay doors and an engine lift fan needed for that B-model's vertical landings, the report said.
The lift fan is built by Rolls Royce, a supplier to the engine maker, Pratt & Whitney, a unit of United Technologies Corp.
The weight of the new plane remained fairly steady over the past year, and the mean time between critical failures increased, but the plane's performance remained below the level expected for this point in the program, the report said.
The report also cited continuing delays with Lockheed's delivery of software for the new fighter, noting that software packages needed to support flight test were delayed or not complete when delivered.
It said the complex helmet that integrates data for the pilot from all the plane's sensors was still facing issues, as is a computerized logistics system.
Weapons integration testing was delayed by a number of factors, including problems with the performance of a radar system and in tracking targets.
Durability testing of the Marine's B-model had to be halted in December after multiple cracks were found on the underside of the plane's fuselage, the report said.
It also cited problems with the ability of the Navy's C-model to transfer video and imagery data to ships, and said one live-fire test revealed a potentially serious problem with the coolant system, which was now being addressed.
More work was also needed on a system aimed at protecting the plane from fuel tank explosions caused by lightning, the report concluded, noting that flight operations were currently banned within 25 miles of known lightning conditions.
No immediate comment was available from Lockheed or the Pentagon's F-35 program office.