Lockheed Martin F-22A Raptors have been cleared to fly for the first time in four months, but the oxygen problem that grounded them remains a mystery to the US Air Force.
It will be two months before F-22A pilots regain full operational capability of the fighters after the four-month hiatus, Gen Norton Schwartz, USAF chief of staff, said on 20 September.
The USAF's wide-ranging safety investigation, which was prompted by the incidents that caused the grounding, could take even longer.
Sixteen flight tests were filed to identify the source of possible contamination of the F-22A's oxygen supply, which had caused 12 pilots to report hypoxia-like symptoms since April 2008.
However, the USAF has since determined that a fatal F-22 crash in November 2010, in Alaska, was not caused by a fault in the aircraft's oxygen system, Schwartz said. The actual cause of the crash has not been released.
"We do not have a smoking gun here," Schwartz said.
Pilots will wear "certain protective equipment" on F-22 missions, and receive new training on emergency procedures, he added.
The USAF also will continue to collect data on oxygen quality during daily flight operations.
A broader safety review of the onboard oxygen generation systems (OBOGS) on USAF combat aircraft also is ongoing.
The technology replaced liquid oxygen canisters in modern military aircraft in the 1970s.
The USAF became concerned about OBOGS on the F-22A after last year's fatal crash. On 3 May, US Air Combat Command put the Raptor fleet on a voluntary safety stand-down, as the investigation progressed.