The US Air Force has extended by several months a self-imposed deadline for releasing a scientific analysis of the oxygen problems blamed for two groundings of the Lockheed Martin F-22A Raptor.

The five-month old oxygen study by a scientific advisory board led by retired General Gregory "Speedy" Martin was due to be released in November, a schedule upheld by the USAF earlier this month.

Three days before the deadline, however, the USAF backed off a firm date for releasing the study. The board "now expects to finalise its complete study this winter," the USAF said without elaborating.

"A releasable report will be made available at that time," the USAF added.

The board is assigned to investigate the F-22's mysterious oxygen problems. More than 20 F-22 pilots have reported hypoxia-like symptoms in the last three years.

So far, the USAF has not been able to pinpoint the source of the problem. The entire fleet was grounded for four months starting on 3 May. Lockheed Martin and USAF officials tried to replicate the hypoxia symptoms during a series of flight tests, but were unsuccessful.

The USAF returned the F-22 fleet to flying status in September, but grounded the aircraft again in October for several days over the same concerns.

The F-22's oxygen system works by diverting bleed-air from the engine compressors. The air is filtered through a molecular sieve contained in a Honeywell onboard oxygen generating system (OBOGS).

A hose leads the air into the cockpit and through a breathing regulator/anti-G valve, which both inflates the pilot's pressure suit and delivers air into a face mask.