The US Air Force has officially settled on the root cause of a series of "hypoxia-like" incidents that have been plaguing the service's fleet of Lockheed Martin F-22 Raptors.

The USAF had earlier narrowed down the potential root cause to either contamination or an air quantity problem. "We have eliminated one of the hypotheses that the air force scientific advisory board postulated as a potential root cause for the hypoxia-related incidents and that was contamination," says USAF chief of staff Gen Norton Schwartz. "We have the data that has confirmed that."

The USAF has also gathered data that indicates that the problem has to do with the amount of oxygen that is reaching the pilot, Schwartz says. Based on tests conducted inside an altitude chamber and a centrifuge, the USAF has concluded that a combination of hardware defects with the pilot's life support gear contributed to the problem.

"Part of that is the upper pressure garment of the g-suit assembly," Schwartz says. "Part of that has to do with hose and valve and connection hardware in the cockpit."

The USAF has a deliberate plan to modify and test that equipment under the "most demanding conditions," the general says. "That will begin to hit the field in September," he adds.

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But despite the USAF having found the root cause of the hypoxia problems, the service will maintain most of the restrictions imposed upon the F-22 fleet until the all of the modifications reach operational flight crews. The aircraft is flying with an altitude restriction of 44,000ft and must remain within 30 minutes of an airfield-but there are also a number of maneuvering limitations that are in place.

"We are confident that we have managed the risk associated with continuing operations in the F-22," Schwartz says. "Minimized, perhaps not eliminated, until the modifications are in place."

Asked why the problems with the Raptor's life support systems were not caught earlier during the jet's extensive developmental and operational test phases, Schwartz says that human physiology is not well understood at the combination of altitude and g-loadings that F-22 pilots routinely operate at.

"This is a unique airplane," Schwartz says. "You can pull 6Gs at 50,000ft. Tell me what other airplane, ever, can do that?"

There are aspects of the Raptor's performance at high altitude, which from the standpoint of human physiology, are not well understood. "In some respects, the testing did not reveal the shortcomings we have recently discovered."

In the early days of jet aviation, Schwartz says, the aerospace physiology community played a very large role. But in recent years, that base of expertise has atrophied. "The engineering know how that's associated with that has diminished, I think, even on a national basis."

Schwartz says the lesson the USAF must draw from the F-22 experience is that the service must pay much more attention to man-machine interfaces when developing an aircraft that offers such extreme performance levels. "We missed some things, bottom-line," he says.

US secretary of defense Leon Panetta has mandated that the USAF will use a phased approach to lifting the restrictions on the aircraft. The USAF must demonstrate the results to Panetta after every step it takes, Schwartz says.

Panetta was briefed 20 July on the USAF's conclusions by USAF secretary Michael Donley and Schwartz, says acting assistant secretary of defense George Little.

Panetta is easing one condition on the USAF's Raptor operations-the jets will be allowed to deploy to Kadena, Japan, via a route over the northern Pacific island chain, Schwartz says. That would necessitate increasing the time and distance the aircraft is allowed to maintain away from an airfield from 30 minutes to 1.5 hours.

The deployment will take place "in days," Schwartz says. But the USAF tankers that will accompany the squadron-sized deployment must have an F-22 pilot onboard to provide guidance in case of an emergency, Schwartz says. Moreover, the tankers will carry enough JP-8 to refuel the F-22s at low altitude in the event the fighters are forced to descend.

The USAF is deploying the jets now rather than in September "because there is an operational requirement," Schwartz says. "And the birds are ready to go."

Source: Flight International