The full force of America's most sophisticated and expensive fighter jets are flying again today even though the Air Force remains stumped as to what could be causing some pilots to suffer apparent oxygen deprivation mid-flight.
Two U.S. bases called for a "pause" in training and homeland defense operations after a pilot at Virginia's Joint Base Langley-Eustis reported experiencing hypoxia-like symptoms during a flight last week, but after studying the incident, both bases green-lit resumption of missions Tuesday, the Air Force said.
The full fleet of the $143 million-a-pop F-22 Raptors -- which have yet to see combat -- was grounded for more than four months earlier this year while the Air Force investigated the cause of 12 other separate, similar incidents since 2008. Nearly six months since the original grounding, the Air Force admits it still does not know why some of its pilots, on relatively rare occasions, showed different "hypoxia-like symptoms".
"It's not just that 'the problem' wasn't identified -- there was no conclusive cause or group of causes," Air Force spokesperson Lt. Col. John Haynes told ABC News Monday. "In different situations, there were different types of symptoms at different times. There was no common thread they [investigators] found to link all these together."
Hypoxia occurs when the brain does not receive enough oxygen and can cause dizziness, confusion and "poor judgment".
Haynes said that the Air Force had been "cautiously moving forward" and had successfully launched 1,300 missions since the original grounding was lifted before the incident at Langley. Though the Air Force has 181 F-22s stationed at a handful of bases around the U.S. and abroad, the "pause" over the weekend only affected Langley and Alaska's Joint Base Elmendorf-Richardson. Elmendorf-Richardson has not experienced an incident recently, but it is the home base of the late Capt. Jeffrey Haney, who was killed in an F-22 crash during a nighttime training mission in November 2010. An investigation into that crash is ongoing, the Air Force said, but Air Force Chief of Staff Gen. Norton Schwartz told reporters last month the oxygen system was definitely not the cause of the crash, despite news reports to the contrary.
Along with the F-35 fighter, which is slightly less expensive per plane, the F-22 marks America's foray fifth-generation stealth fighter jets that the Air Force said can dominate the air space anywhere in the world -- even if they've never had to prove it.
Not a single one of the Raptors -- which cost U.S. government $77.4 billion for a total of 187 planes from developer Lockheed Martin -- has been used in combat operations and isn't expected to "any time soon," an Air Force official told ABC News last month.
The Pentagon initially ordered more than 600 of the fifth-generation fighters, but Congress stopped at funding 187 in 2009 under a hail of criticism over the fact that the planes are designed to take on other rival high-tech fighter jets instead of the third-world militaries and insurgents the U.S. currently faces.
Only recently have rival major powers -- including Russia and China -- unveiled their prototypes for what are believed to be their own stealth fighters, designed to take on the F-22.
Haynes said that since the original grounding, base commanders everywhere are keeping a vigilant lookout for problems with Raptor pilots in the air.
"Everybody knows, everybody's watching," Haynes said.