The Lockheed Martin F-35 Lightning II missed key software testing milestones necessary for the US Marine Corps to field combat-ready jets while those already delivered remain unreliable and difficult to maintain, says a new report by the US Defense Department’s director operation test and evaluation (DOTE).

The programme was heavily focused on completing the testing necessary for the fleet release of software iteration 2B, which will enable the Marine Corps to declare initial operational capability in 2015 with what the report calls “limited combat capability.”

“Slower than planned progress in mission systems, weapons integration, and F-35B flight sciences testing delayed the completion of the testing required for Block 2B fleet release,” the report says. “The program now projects this to occur by the end of January 2015, instead of the end of October 2014 as was previously planned.”

The Pentagon’s F-35 Joint Program Office (JPO) says in a statement that about 60% of the overall test programme is complete and that challenging work remains.

“This is the optimal time to discover issues through testing so we can provide solutions early, and deliver an extremely capable and lethal aircraft to the warfighter,” the JPO says in a statement.

Test flights in 2014 overall outpaced the planned schedule set by Lockheed and the Joint Programme Office (JPO), but test-point accomplishment for software blocks 3B and 3i lagged, the report finds.

The programme also failed to complete the weapons delivery accuracy tests planned for 2014 because progress was delayed when deficiencies were found in the aircraft’s navigation system. Such deficiencies likely will carry over into the development of later software iterations.

In reply, Lockheed says the report points out problems that are surmountable for a programme as large as the F-35.

“The report points to known challenges previously identified and are the normal discoveries found in a test program of this size and complexity,” Lockheed says.

Flight testing for all variants suffered after a June engine malfunction caused an F-35A to catch fire and grounded the fleet for weeks. Testing of the air force’s conventional takeoff and landing F-35A was further delayed because of the focus on the Marine Corps’ July 1 IOC deadline, the report finds.

Resultant flight restrictions “reduced test point availability and slowed progress in mission systems and flight sciences testing from July through November”, the report says. Mission system testing suffered a 17% loss in productivity as a result of the flight restrictions. Prior to the fire, the programme was checking off about 210 test points per month, the report says. The rate fell to around 175 points per month because of the flight restrictions.

The report also found “overall suitability” of the three variants to be lacking and that reliability and maintainability for fielded jets “relies heavily on contractor support and unacceptable workarounds.” In response, a JPO statement says availability for all variants is holding at 50% or above since October.