F-22s Grounded Pending Oxygen System Probe

Jul 18, 2011 By Bill Sweetman aviation week and space technology

Washington - The U.S. Air Force’s F-22 fighter remains subject to the longest full-force grounding of any combat aircraft in recent history, with no cause firmly identified. Meanwhile, documents show that the focus of the investigation—the onboard oxygen-generation system (Obogs)—has been a flight safety issue for many years on the F/A-18C/D Hornet, increasing the number of cases where aircrew were affected by hypoxia, or lack of oxygen, in flight.

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 aircraft crashed about 100 mi. north of Anchorage. The pilot did not eject and was killed, and the aircraft left a deep crater suggestive of a steep-angle, high-speed impact.

As the investigation continued, the F-22 fleet was restricted in January 2011 from flight above 25,000 ft., following reports of multiple incidents that pointed to occurrences of hypoxia. On May 3, a full safety stand-down order was issued. The only exceptions were a limited series of test flights, conducted with real-time monitoring, to verify software fixes.

The USAF is being tight-lipped about the investigation. The response to the question, “Has a cause been positively identified?” is, “The investigation is still ongoing.”

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.

According to a source familiar with some aspects of the investigation, Honeywell participants in the investigation believe the problem does not lie with the Obogs alone. An alternative hypothesis points to a problem with the sealing of the oxygen mask or with the “counter-pressure” garment worn by the pilot. The latter allows for positive-pressure breathing under high g-loading or in the event of loss of cockpit pressure. To stop the pilot’s chest from expanding under pressure and impeding blood circulation, the garment is inflated by the oxygen system and compresses the pilot’s upper body.

A complicating factor, one observer suggests, could be the fact that the F-22 (and F-35) Obogs are “relatively simple” systems with two flow rates, with high-flow triggered by specific events such as high g-loadings. A problem such as a mask leak or a faulty valve in the counter-pressure garment could be slower to trigger a rate increase, a potential problem in critical situations such as maneuvering at high altitude, with low ambient pressure in the cockpit.

The U.S. Navy, meanwhile, has been dealing with a post-2000 spike in hypoxia-related Class A mishaps (causing the loss of the aircraft or pilot or more than $1 million in damage) in the Hornet fleet. A 2005 article in the Navy aviation-safety publication Approach noted that the rate of such incidents in 2001-04 was almost 10 times that in 1980-2000, and that Obogs-equipped aircraft were suffering from hypoxia events at four times the rate of liquid-oxygen-equipped types.

A further analysis of 2002-09 data showed 64 hypoxia events in F/A-18s, with Obogs failure being the largest single cause (29%). Two events resulted in fatal accidents.

One factor appears to be that Obogs can result in “mask-on hypoxia” due to contaminants or other partial Obogs failures. In August 2005, the Naval Safety Center released a message that warned of “a remote possibility of contamination by gases such as acetylene, carbon monoxide and carbon dioxide.”

All Obogs include an oxygen-monitoring system to warn the user if the system is not delivering the required concentration of oxygen. According to a patent filed in 1990 on behalf of Normalair-Garrett (now Honeywell), early monitoring systems were based on electro-chemical technology used in medical laboratories and suffered from drift over time and limited life. Alternative systems added complexity because they required a second “reference” source of air bled from the engine. (The F-22 has a solid-state controller.)

Improved systems have been developed since. The original GGU-12 oxygen concentrator used on the F/A-18 family is being upgraded with the addition of a catalyst that converts carbon monoxide to benign carbon dioxide, and the Navy has been working to install an updated solid-state oxygen-monitoring system on all in-service F/A-18s that tracks oxygen pressure and concentration. A cockpit pressure warning system has also been introduced.

A fiscal 2009 Small Business Innovation Research solicitation, meanwhile, called for ideas for a “miniaturized, non-invasive sensor” that would monitor the pilot for signs of hypoxia.