The lead US Air Force investigator tasked with discovering the root cause of a series of "hypoxia-like" incidents that have plagued the service's Lockheed Martin F-22 Raptor fleet is adamant that the service has found the root cause of the problem.

"In the end, there is no smoking gun," says Maj Gen Charles Lyon, Air Combat Command's director of operations. "We have assembled the pieces of the mosaic, they reside in the cockpit."

F-22 Alaska - USAF 


Lyon reiterates that the root cause lies in the Combat Edge upper pressure garment, connector hoses and the valves. Specifically, the breathing regulator/anti-g (BRAG) valve that controls the air pressure within that vest is opening when it should not. That means the vest is pressurized when it should be empty. Consequently, that restricts the pilot's breathing, thus causing the problem.

"So this vest is always inflated on an F-22 pilot," Lyon says. "And it should not be inflated until they start to pull Gs."

The result is shallow breathing-essentially hyperventilation.

The reason the valve does not work correctly is because it was designed for the Boeing F-15 and Lockheed F-16-neither of which operates at the altitudes the Raptor does or has the same type of life support systems. Lyon says the USAF discovered the problem during its Scientific Advisory Board investigation.

However, Kevin Divers, a former USAF rated-physiologist and F-22 flight test engineer, says that the service has known about the Combat Edge pressure problem since 2000.

"I contacted Brooks AFB [Texas] physiologists about the vest inflating in 2001 and they assured me verbally, as well as through acceleration and altitude reports they sent me, that showed both the Boeing upper pressure garment (CSU-18/P) and the current vest (CSU-17/P) had been tested thoroughly without any issues and that they recorded the pressure of the vest at the inlet that is now being blamed for its design," Divers says.

Lyon acknowledges that there was some "rudimentary" testing done, but says that those trials were not comprehensive. "We did not execute end-to-end testing of the life-support systems," he says.

Moreover, the long-term physiological consequences were not explored during that time.

Additionally, Lyons adds, the standards the F-22 system was developed to in 1988 are obsolete.

The USAF is taking several steps to ensure the F-22 is safe to fly. It is adding an oxygen sensor and associated automatic backup oxygen supply. The service is also adding a helmet-mounted pulse-oximeter to replace a finger-mounted version. It has also developed a new prototype valve, which if successful in testing, will replace the current unit.

The current 44,000ft altitude restriction will be lifted only once the new valve is certified and the Combat Edge is back in place, Lyon says.

NASA is also conducting an independent review of the "hypoxia-like" events. The USAF expects in interim report from the agency by next month. Lyon says that the USAF has already incorporated one change recommend by NASA and will incorporate any further recommendations the agency makes.

The USAF hopes to finish implementing the fixes to the Raptor fleet by the fall. The entire fleet should have the new valve installed before the end of the year.

Meanwhile, the USAF leadership believes pilots are confident that the problem has finally been solved.

"I recently visited our F-22 bases and can tell you their confidence is high," Lyon asserts.

But a number of Raptor pilots say they are anything but convinced that the USAF has found the root cause of these "hypoxia-like" incidents.

"There's one thing I know for certain: The Combat Edge isn't the culprit," one F-22 pilot says. "But they're trying to show positive momentum."

Source: Flight International