More than six weeks after the US Air Force indefinitely grounded all Lockheed Martin F-22A Raptors, the scope of the safety investigation has widened beyond the Honeywell-supplied onboard oxygen generating system (OBOGS).

Although internally described as the "OBOGS safety investigation", the probe launched after the 3 May safety stand-down of the F-22A fleet is "not limited" to that particular system, Air Combat Command (ACC) said in emailed responses to questions.

"We are still working to pinpoint the exact nature of the problem," the ACC said. "It is premature to definitely link the current issues to the OBOGS system."

The stand-down was originally linked to five reports by F-22 pilots of potential oxygen system malfunctions, including one reported instance when an F-22 scraped treetops on final approach. The pilot could not remember the incident after landing, exhibiting a classic symptom of hypoxia.

The OBOGS is not the only device involved in the supply of oxygen to the pilot. Wedged into a space behind and slightly below the pilot's seat, it uses a molecular sieve to filter pressurised air diverted from the engine compressor section into pure oxygen.

In between the OBOGS and an F-22 pilot's lungs, the filtered oxygen passes through two major components - the MBU-22P oxygen mask and the breathing regulator/anti-g (BRAG) valve. The latter "ensures safe delivery of oxygen to the pilot" through the face mask, according to a fact sheet published by the USAF human systems integration office.

"The BRAG is fast-acting due to the manoeuverability of the aircraft and [is] compatible with the existing upper and lower g-garments, which keeps blood in the upper portions of the body during aircraft manoeuvres," the fact sheet added.

The USAF investigation is also comparing the F-22's life support system with other strike aircraft in its fleet, including the Lockheed F-35A Lightning II, Fairchild Republic A-10, Boeing F-15, Lockheed F-16 and Hawker Beechcraft T-6A, the ACC said.

The review is aimed at casting a "broad net for comparison", the command added.

Complaints about the F-22's OBOGS equipment had not surfaced until recently. In January, the ACC ordered the fleet to remain below a service ceiling of 25,000ft (7,620m). The order came two months after an F-22 crashed near Joint Base Elmendorf-Richardson, Alaska. That accident, in which the pilot was killed, is under investigation and has not been linked to the oxygen system concerns.

Source: Flight International