The new chairman of the US Joint Chiefs of Staff has raised concerns about the cost of building three variants of the Lockheed Martin F-35 Lightning II.
"I am concerned about the three variants and whether we can go forward in this fiscal environment with all three, but I am eager to learn more about that," said Gen Martin Dempsey, speaking at a House Armed Services Committee hearing on 13 October.
"Three variants create some fiscal challenges for us," he added.
Dempsey's comments come nine months after then-Secretary of Defense Robert Gates placed the short take-off and vertical landing (STOVL) F-35B on a two-year probation, with the explicit threat of cancellation.
Leon Panetta, who succeeded Gates, also offered no promises about the future of any of the programme's three variants, which also include the F-35A for the US Air Force and the F-35C for US Navy aircraft carriers.
The F-35 is "a remarkable plane. It really does the job well," Panetta said. "But what we want to do is to make sure as it goes through this test period we're able to understand all of the issues involved with it and are able to be fully confident that this plane once it goes into production is going to be totally effective and totally capable of serving the mission it is required to do."
Ironically, affordability is what drove the Department of Defense to launch the Joint Strike Fighter programme in the late-1990s with three variants. It was believed that a common production line would allow the same basic fighter to meet the diverse needs of the air force, navy and US Marine Corps.
Since contract award 10 years ago, the programme has been delayed at least five years for the marines' F-35B variant, which has required a redesign to reduce weight and faced several structural and propulsion system glitches that delayed flight tests.
The STOVL variant has been making progress since Gates announced the probation status. In early October, the USMC landed two flight test aircraft on the amphibious carrier the USS Wasp to launch shipboard trials.
In his testimony, Panetta emphasised the importance of flight tests for the F-35 programme's future.
"Give us a chance to test it," Panetta said. "If it performs well then obviously it will be able to make the grade."