US Air Force tests have been unable to replicate the hypoxia-like conditions experienced by some pilots of the Lockheed Martin F-22 Raptor.
Speaking to analysts following Lockheed's second-quarter earnings, chief executive Bob Stevens said the company was working closely with the USAF in order to get to the bottom of the issue which has led to the stand-down of the F-22 fleet.
The USAF has fitted one F-22 with "very sophisticated instrumentation" as part of an "incredibly detailed and thorough review".
He said: "It has been flying that airplane to see if those conditions exist and can be observed and then we can work backwards together." So far they have not been replicated and the cause remains a mystery, he said.
Meanwhile, Lockheed has completed 448 test flights of its F-35 Joint Strike Fighter so far this year - 18% above target and 51% of the total planned for 2011.
Stevens said that of the 63 aircraft ordered in the early production blocks, 43 are in build: 21 in component assembly, 16 in final assembly and a further nine on the flight line for flight-qualification testing.
Recent reports produced by the US government suggested that the first 28 aircraft for the US armed forces would cost as much as $900 million more than target. Stevens said this assumed a "worst-case scenario" on the fighter. He added: "We are absolutely committed to working with our industry team to reduce these costs as much as possible."
Lockheed has identified $200 million of cost savings on the programme, which are being evaluated by the customer, he said. "The costs of the F-35 will continue to come down and we will deliver a high-performance, effective and affordable solution," he added.
Stevens also pointed out that the government's projected sustainment costs of $1 trillion over 50 years needed contextualising as the F-35 was the first aircraft to be subject to this evaluation. The sustainment of the "legacy fighters" to be replaced would exceed the F-35's costs, he said, without bringing any of the fifth-generation benefits.
He said the first two F-35A conventional take-off and landing aircraft had been delivered to the USAF, with a further four in acceptance testing.
Negotiations with the US government over a price for lot-5 production was still ongoing, he said, with this likely to continue next month.