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9 septembre 2011 5 09 /09 /septembre /2011 06:35



Members of the U.S. Air Force's 525th Aircraft Maintenance Unit work on an F-22 Raptor that just landed at Joint Base Elmendorf-Richardson, Alaska, in June 2009. The plane crashed in November 2010 in Alaska. (File photo / U.S. Air Force)


8 Sep 2011 By DAVE MAJUMDAR DefenseNews


The November 2010 crash of a U.S. Air Force F-22 Raptor was caused by a malfunction with the aircraft engine's bleed air system, an industry source said. The pilot, Capt. Jeff "Bong" Haney of the 525th Fighter Squadron, was killed in the accident.


Another source, a pilot, confirmed that information. The fighter squadron is based in Alaska.


An Air Force accident report said the F-22, tail number 06-4125, had a bleed air problem that caused both the stealth fighter jet's Environmental Control System (ECS) and On-Board Oxygen Generating System (OBOGS) to automatically shut down, the sources said.


The report has been released to Air Force officials at Pacific Air Forces, but has not been made public, the industry source said. The F-22 fleet was grounded May 3 after pilots suffered more than a dozen hypoxia-like incidents while flying.


The bleed air system siphons off air from a jet engine's compressor section to generate power, supply oxygen and inert gases, and handle heating and cooling.


If the ECS and OBOGS shut down, the pilot would not have air coming into the cockpit, and would have to switch to his emergency oxygen supply and dive to 10,000 feet, another source said.


"If the ECS is out … there is no conditioned air pressure pushing through the OBOGS, so he would be sucking rubber," the source said. However, as the aircraft descended, "the cabin pressure would be gradually rising as long as the canopy was still intact completely," he said.


But Haney's F-22 never recovered from its dive. The twin-engine jet hit the ground, and it is unclear whether the pilot had switched to his emergency oxygen supply, the industry source said.


"The rate at which he descended, though, he would have been at a hypoxia-safe altitude within time to have not fully succumbed to hypoxia and should have only had symptoms versus unconsciousness," the pilot source said. "The green ring [emergency oxygen bottle] in the Raptor is a tough pull, and it was altered to give the pilot some pressure."


Activating the emergency oxygen system is tricky in the Raptor, the source continued.


"It is a double pull that has to be practiced and experienced a few times before you end up in that bad situation, or you will panic," he said.


The industry source said the report declared that the accident was not related to the OBOGS.


But there are skeptics who say the OBOGS can't be ruled out as a culprit.


"Around May, the aircrew were briefed that the mishap OBOGS unit was operating fine on [Haney's] flight," the pilot source said.


The source said that if the report's findings are accurate, though he is not convinced it is, it could be that other physiological factors with pilot's g-tolerance and the oxygen levels in his body could have played a role in the crash. Haney was attempting a maneuver called a "rejoin" and made a fairly aggressive turn during the procedure, the pilot source said.


"I would have done the same thing with a Raptor in my hands," he said. "It's just that if OBOGS and the whole ECS was working nominally, physiological stuff is what might have crept up on him and impaired his normal ability."


The pilot source said the investigation would have had to determine Haney's oxygen supply and g-tolerance in that exact instance, but a precise assessment would not have been possible because of the condition of the pilot after the crash.


"I don't see how you can absolutely rule out OBOGS by checking a smoked and crushed system and using what aircraft data was available based on a lack of an [Integrated Caution and Warning] showing unacceptable [oxygen] concentration or pressure," the pilot source said. "You have to look at what testing was done to call those concentration and pressure limits as good, and that goes back before the flight of Ship 4001," the first F-22 test plane.


Why Did Bleed Air Fail?


Questions remain as to the nature and cause of the bleed air system malfunction.


Hans Weber, who owns Tecop International, a San Diego-based aerospace consulting firm, said that while bleed air systems are ubiquitous, they are complex and occasionally malfunction.


"It's a fairly complicated system," said Weber, a former member of the U.S. Federal Aviation Administration's Research, Engineering and Development Advisory Committee. "So there can be failures in it."


Bleed air is very hot when it is sucked from the compressor; it goes through a series of heat exchangers to cool it to about 450 degrees Fahrenheit, he said. From there, it is further processed and cooled before it is used. Failures are rare, but they do happen, Weber said.


What is particularly worrisome is that aircraft bleed air systems have built-in safety gear, and whatever this malfunction was, it managed to overcome them, he said.


Further, Weber said that even if the OBOGS is exonerated in this incident, there have been more than a dozen hypoxia incidents. It is possible the problem is related to the other oxygen system incidents, he said.


"Might that apply to the others? Is this an outlier or at the core of the problem?" Weber asked.


The Air Force did not respond to queries by the afternoon of Sept. 8

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