WASHINGTON -- A potentially faulty pressure vest is the latest clue in a yearlong mystery over why Air Force pilots flying Lockheed Martin's F-22 Raptor keep getting dizzy and disoriented.
Pilots have been instructed to stop using the vest during routine flight operations as the Air Force works on a fix, the service's Air Combat Command said. The vest, part of a "G suit" used to help pilots avoid blacking out during high- speed maneuvers, "increases the difficulty of pilot breathing under certain circumstances," according to an e-mailed statement.
Unable to explain episodes of dizziness, the Air Force is looking at everything from the prosaic -- hoses, masks and now G suits -- to the top-secret coatings and adhesives used in the plane's radar-absorbing stealth skin that makes it harder to track. So far, all the engineers and investigators have come up short of a solution to symptoms that include what's been called a "Raptor cough."
"The bottom line is we don't have a single causative factor," Brig. Gen. Daniel Wyman, the Air Combat Command's surgeon general, said in an interview this week.
About two dozen pilots and five ground-maintenance workers have reported symptoms associated with a lack of oxygen. There have been 11 reported incidents since the plane resumed flying operations last year after a four-month halt because of safety concerns.
Defense Secretary Leon Panetta imposed new safety measures last month that include limiting flight durations and speeding the installation of back-up oxygen systems.
No oxygen problem was detected before the Raptor was declared ready for combat in 2005.
"I don't have any ready answers to why we are experiencing a serious problem that apparently didn't surface during the supposedly extensive testing the Air Force did," Thomas Christie, who was the Pentagon's top weapons tester from 2001 to 2005, when the plane was in development, said in an interview.
The Pentagon has spent $67 billion buying 188 of the supersonic jets, which have never flown in combat. It plans to spend $11.7 billion to upgrade the planes at a time when the Pentagon is cutting spending after a decade of war in Iraq and Afghanistan.
The Raptor has been called "the most expensive, corroding hangar queen ever" by Arizona Sen. John McCain, the top Republican on the Senate Armed Services Committee.
The oxygen deficiencies promise to require even more money to fix. Bethesda, Md.-based Lockheed Martin, the world's largest defense contractor, won a $19 million contract last week for the back-up oxygen supply system.
"It really is a conundrum," said Jeffrey Sventek, executive director of the Aerospace Medical Association, whose annual conference last month included a briefing by the Air Force on the F-22 investigation.
The Air Force said Wednesday that it's looking at whether the equipment donned by F-22 pilots may be restricting their ability to breathe.
"Testing has determined that the upper pressure garment increases the difficulty of pilot breathing under certain circumstances," Lt. Col. Edward Sholtis, an Air Combat Command spokesman, said in an emailed statement. "We're also looking at the layering of other aircrew flight equipment as contributing to that difficulty."
The service is looking in particular at the flight suits, worn in combination with the pressure vests, by F-22 pilots at Joint Base Elmendorf-Richardson in Alaska and at Joint Base Langley-Eustis in Virginia, according to a government official briefed on the latest information.
The official, who spoke on condition of anonymity because the investigation is being handled in private, said investigators suspect the combination of clothing may be restricting a pilot's ability to expand his chest and take a full breath.
The Air Force isn't declaring the case solved.
"The upper pressure garment is not 'the' cause of physiological incidents, and we still have other variables to work through before we can determine what the major factors are and how they interact to produce the number of unexplained incidents we've seen," Sholtis said.
The probe focused initially on the plane's On-Board Oxygen Generating System built by Honeywell International. Honeywell, based in Morris Township, N.J., has said the system, which provides enriched oxygen to the pilot, is performing as designed.
Retired general Gregory Martin, who headed a study of the F-22 for the Air Force Scientific Advisory Board, told reporters in March that the system "might not produce as much oxygen as it would when it was not under G," or a high level of acceleration. Still, he said the oxygen level was "never in an area of concern."
The oxygen system is "a common design," Charles Oman, a senior research engineer and lecturer who specializes in aerospace physiology at the Massachusetts Institute of Technology, said in an interview. "That's why everyone was so mystified."
Wyman, the surgeon general, said the oxygen system in the F-22 is unique because it funnels highly concentrated oxygen directly to the pilot, instead of being mixed with air from the cockpit. While the F-22 also operates at higher altitudes and air speeds than other fighters, Wyman said evaluations show its oxygen system "works as advertised."
With answers still elusive, critics of the plane such as Pierre Sprey say the toxic coatings used for the stealth skin may be entering the plane's air intakes and fouling the oxygen flow.
"That's the No. 1 candidate," Sprey, who was an Air Force architect of the F-16 fighter and the A-10 ground-attack plane in the 1970's and 1980's, said in an interview. "There's no candidate that comes closer to filling the bill on the evidence."
Raptor pilots also have reported suffering from a persistent cough and vertigo, symptoms not associated with classic hypoxia, or deprivation of oxgyen, Sprey said.
The stealth coatings theory also may explain why five ground-crew technicians also have reported symptoms, even though they are never in the air using the oxygen system.
"This airplane is constantly being reglued, which is why the maintenance guys came down with these symptoms," Sprey said.
While the Air Force is investigating that theory, it has no evidence to support it, Sholtis said.
"If the stealth coating, adhesives or other materials were off-gassing or otherwise leaking contaminants into the pilot's air supply, you would expect to see significant amounts of harmful chemicals or other evidence of toxicity" when air samples are chemically analyzed, Sholtis said by e-mail.
"We don't see the evidence," he said. "So it's difficult for us to say with any confidence that that kind of contamination is occurring."
By DAVID LERMAN AND TONY CAPACCIO
BLOOMBERG NEWS