A US Air Force Scientific Advisory Board (SAB) panel investigating a series of hypoxia-like incidents afflicting pilots flying the Lockheed Martin F-22 Raptor has not discovered what is causing the problem, but service officials vow they will find the root cause.

"I am convinced there is a root cause," says Maj Gen Charles Lyon, Air Combat Command's (ACC) director of operations. "I want everyone to know--particularly those who operate it and their families--we will not rest until we find that root cause."

The USAF is continuing to test the F-22's life-support systems to try to determine what is still causing these "physiological incidents."

Those efforts started in September 2011 under the auspices of ACC, says retired Gen Greg "Speedy" Martin, who led the SAB study. "Those are ongoing today," he says.

The most recent of these physiological incidents happened on 26 March where a pilot at Joint Base Langley-Eustis in Virginia had to declare an emergency after his pulse-oximeter alerted him that there might be a problem. The small device is part of the new ensemble of safety gear F-22 pilots are now required to fly with to mitigate the potential risk from life-support system anomalies.

Lockheed Martin F-22 Raptor 

The USAF first noticed a pattern of unexplained hypoxia-like physiological incidents in April 2008. The service initially restricted the F-22 fleet to altitudes below 25,000 ft but eventually grounded the jet in May 2011 after a series of some 14 incidents. The USAF lifted the flight-ban last September after taking precautionary measures such as installing activated-charcoal filters, oxygen sensors, and having pilots wear a pulse-oximeter. But despite the "mitigation efforts," there continues to be incidents recorded every month-which have prompted a number of temporary local groundings by individual wing commanders.

"If there is any indication of abnormal oxygen levels, we terminate the flight," Lyon says. A team of medics will greet the pilot upon landing to quickly run tests and make sure that he or she is treated if need be.

Lyon says that there are only incidents on 0.1% of flights and pilots have flown over 10,000 sorties since September. Lyons says the F-22 is now safer to fly than it ever has been before.

Some of the "findings" of the SAB to date include the fact that the Raptor's onboard oxygen generation system (OBOGS) and emergency oxygen system were not classified as safety critical items, according to a document released by the USAF. Additionally, there is no backup system that automatically supplies oxygen to the pilot if there is a failure of the primary life-support system. Nor was there a requirement to inspect or maintain OBOGS components until recently.

"It was a fly to warn, fly to fail system," Martin says. "We now have routine inspections."

But perhaps more importantly, there is a general lack of institutional expertise in the field of aerospace physiology and human systems integration. And in the case of the Raptor, the modelling, simulation and integrated hardware testing of the Raptor's life-support system "were insufficient to provide an 'end to end' assessment of the range of conditions likely to be experienced by the F-22," the document reads.

"During that same period of time, we reduced the emphasis and numbers of people associated with aviation physiology research and science," Martin says "That needs to be re-established."

Part of the problem is that the Raptor routinely operates above the normal 50,000 ft operational ceiling of conventional fighters like the F-15 or F-16, Lyon says. How life-support systems behave at those altitudes is not understood very well, he says. This is particularly true of the Raptor's systems which were designed to operate in a chemical and biological warfare environment and thus require the OBOGS to supply a constant pressurized oxygen-mix to the pilot rather than blending it with cabin air.

While Martin says the Raptor's exact ceiling is classified, it is known that USAF regulations restrict the jet to 60,000 ft. Raptor pilots receive a waiver to fly the jet above 50,000 ft while wearing the Combat Edge g-suit-which counts as a partial pressure suit for the USAF's purposes. The restriction is due to the Armstrong Limit, which is found at an altitude of between 62000ft and 63000ft, where the outside air pressure is so low that water will start to boil at 37°C (98.6°F).

While the USAF continues to investigate, the service has a set of recommendations from the SAB which it is implementing. These include developing a new backup oxygen supply, developing new standards for breathable cockpit air supply, adding an automatic ground collision avoidance system to the jet, and installing a new handle for the troublesome emergency oxygen supply.

70% of the jets have the new handle-which cost $47 a piece-and the rest will have it in a couple of weeks, Lyon says.

There are a host of other recommendations that the USAF is implementing. The service is establishing quarterly review to monitor how well the USAF is doing in implementing the SAB recommendations.

(UPDATE: An earlier version of this story incorrectly attributed a quote in the 10th paragraph to Lyon. It should have been attributed to Martin)

Source: Flight International