Source: ABC News

Pilots for the U.S. Air Force’s F-22 Raptor fighter jets have been ordered to take off a portion of their flying suits, specifically the G-suit vest, during routine training missions as the service continues to investigate a rare but mysterious breathing problem some pilots have experienced in the $420 million-a-pop jets.

As a recent ABC News investigation found, in at least 25 cases since 2008 F-22 pilots have reported experiencing symptoms of oxygen deprivation in mid-flight. In one case, a pilot became so disoriented that his plane actually skimmed the tops of trees before he managed to save himself. Another pilot, Capt. Jeff Haney, was killed in a crash after an unexplained malfunction cut off his oxygen supply during a training mission in November 2010.

READ Exclusive: Family Demands Truth in Air Force F-22 Pilot’s Death

Despite grounding the whole $79 billion fleet of jets for five months last year, the Air Force has been unable to discover the source of the problem.

Air Force spokesperson Tadd Scholtis told ABC News today that the G-suit vest, designed to help pilots’ bodies cope with extreme G-forces during maneuvers, “appears to be contributing to breathing difficulties” for pilots, but is not believed to be the root cause of the prior incidents. It is being removed, he said, because of some “vulnerability and reliability issues.”

Last month Defense Secretary Leon Panetta ordered new flight restrictions for the F-22 while the breathing problem remains under investigation, but the Air Force has claimed those orders have not curbed the planes’ operations. Panetta also ordered the Air Force to expedite the installation of an automatic emergency backup system, a safety measure that Haney’s family told ABC News would have saved his life.

The F-22 Raptor, which is made by defense contracting giant Lockheed Martin, officially went combat operational in December 2005 but has yet to see an actual combat mission. From Iraq and Afghanistan to last year’s U.S.-led no-fly zone over Libya, the Air Force said it simply has not needed the advanced capabilities of the most expensive jet fighter in history.