The Lockheed Martin F-22 Raptor is to return to flight status after a four-month grounding - but only after an "extensive" inspection of the stealth fighter's life-support system and continued daily checks, the US Air Force has announced.

The USAF also acknowledged that the grounding order was prompted by 12 separate reports of hypoxia-like symptoms, suffered by F-22 pilots over a three-year period dating to April 2008.

F-22s have been voluntarily grounded since 3 May. At the time, the USAF reported five incidents of hypoxia-like symptoms reported by F-22 pilots in the previous three weeks.

 F-22 tanks - USAF
© US Air Force

The grounding of the air force's premier fighter fleet triggered a wide-ranging review of onboard oxygen generation systems (OBOGS) throughout the US military.

OBOGS units supplied by Honeywell and Cobham to a variety of military aircraft have not been directly implicated in the USAF investigation.

Both manufacturers have denied knowledge of any safety concerns about the equipment that they deliver.

The OBOGS technology was first introduced in combat aircraft in the late 1970s and early 1980s, replacing a requirement to store liquid oxygen tanks in aircraft.

OBOGS uses pressurised bleed air from the engine, which is filtered through a molecular sleeve. In the F-22, the cleansed air is processed through a breathing regulator/anti-g valve, before reaching the pilot's oxygen mask.

An official OBOGS safety investigation led by retired Gen Gregory "Speedy" Martin remains ongoing, but USAF officials said a releasable report will be issued later this year.

Martin's investigation has encompassed not only the F-22, but also the Lockheed F-35, Hawker Beechcraft T-6 and other aircraft.

Source: Flight International