The U.S. Air Force is putting the F-22 back to work as of Sept. 21 with a new plan for making sure pilots have enough oxygen, Capitol Hill was told early Sept. 19.

The Raptor fleet has been grounded since May 3 because pilots were showing symptoms of hypoxia. The Air Force Scientific Advisory Board investigating the problem has not pinpointed a “single, definitive root cause” of the symptoms, according to a release provided to Congress. Rather, investigators are looking at multiple factors to explain why the onboard oxygen-generation system (Obogs) does not deliver air to the pilot as it should.

To work around the problem, the Air Force is drawing on a 2007 plan to reconstitute the F-15C fleet. The service is beefing up aircraft inspections, adding training and protective gear, providing physiological testing to pilots and continuing to study the problem. “Some flight restrictions will be in place for a short period as pilots regain proficiency,” according to the notice to lawmakers.

The advisory board is continuing to “resolve remaining issues,” and a final report is expected later this fall.

An Obogs problem is considered the most likely cause for the Nov. 16, 2010, crash of a 525th Fighter Sqdn. F-22 operating from Joint Base Elmendorf-Richardson in Alaska. The pilot did not eject and was killed, and the aircraft left a deep crater suggestive of a steep-angle, high-speed impact, Aviation Week reported in July.

The Obogs on the F-22 is made by Honeywell in the U.K., and systems fitted to the F/A-18 are produced in the U.S. by Cobham. (Cobham acquired the unit in 2003 \that was previously Bendix, then Litton and subsequently Northrop Grumman.) However, they work on the same principle, passing engine bleed air through a molecular-sieve oxygen generator that absorbs nitrogen and other gases and delivers near-pure oxygen to the pilot.

The Air Force has been tight-lipped — or at least officially from the top — about the situation all year. The grounding, juxtaposed with Western operations in Libya — where the Raptor was once eyed for use in its first kinetic combat environment — is leading to further scrutiny in Washington about the high-profile program.