The head of the US military's F-35 programme insisted on 26 March that Lockheed Martin and other suppliers must make improvements to the Joint Strike Fighter before the government awards contracts for higher production rates.

In prepared testimony to the US House Armed Services Committee, US Air Force Lt Gen Christopher Bogdan says Lockheed and its partners must address the slow pace of software development, which he says is several months behind schedule.

Bogdan says improvements must also be made to the F-35's reliability, "which is not growing at an acceptable rate".

"Additional progress must be demonstrated before awarding a contract for higher production rates," he says.

Although critical, Bogdan does note programme improvements in the last year. For instance, he says costs for production aircraft continue to decline, and software development changes are paying off.

However, Bogdan warns the Block 3F software – which will allow the aircraft to achieve "full warfighting" capability – is "tracking approximately four- to six-months late". This standard will be in the US Navy's F-35Cs, which the service hopes will reach initial operational capability by August 2018.

Lockheed, however, tells Flightglobal it “remains confident” it will deliver “initial warfighting capability” as planned.

Bogdan also singled out Pratt & Whitney, maker of the F-35's F135 engine.

"Far too often engine deliveries are interrupted by technical issues and manufacturing quality escapes, resulting in product holds and material deficiencies that increase overall risk to meeting future production goals," Bogdan says.

He adds, however, that engine delivery rates were stable in 2013, and that deliveries for low-rate initial production lot six aircraft are running "slightly ahead" of schedule.

In response, P&W says it has "had few true quality escapes that have stopped production". The company has an internal review process and discontinues delivery of F135 engines as it examines potential problems, it says.

"That process at times can be lengthy, and we are proposing ideas and approaches to shorten the process, [which] would enable even better delivery performance," the company adds.