The nation's F-22 fighter jets went back into service Wednesday, four months after they were grounded over pilot complaints about a lack of oxygen.
Air Force instructor pilots began flying the stealth jet fighters at six bases across the U.S. This followed a stand-down order, issued in May, and imposed over hypoxia issues reported by at least 12 pilots in the past three years. Hypoxia is when the body does not receive enough oxygen.
"It's Day One on a road to get our F-22s back in the air and back to their full operational capability," said Lt. Col. Derek France, 3rd Operational Group commander at Alaska's Joint Base Elmendorf-Richardson. Forty of the 170 F-22 Raptors are stationed at the Anchorage base.
In the past four months and even before, the aircraft's oxygen related systems have been the focus of an ongoing safety investigation, France said.
"While they never pinpointed, or have yet to pinpoint, an exact cause of these incidents, they got to a point where they felt that we could, based on risk mitigation, training of air crews and inspection of the aircraft itself, get to a point where we can safely fly again," he said. "And so that's the decision, we passed a safety line where senior Air Force officials said that we can go ahead and train again."
He couldn't provide additional information about what led to the decision.
"I can't go into the real details," France said after the first four Raptors raced down the runway and took off on a training run over Alaska.
"They did a thorough investigation of some of the life support systems in there and some minor modifications within the cockpit to ensure the safety of the pilot," he said.
Air Force Chief of Staff Gen. Norton Schwartz announced the end of the stand-down order in a statement issued earlier this week.
"We now have enough insight from recent studies and investigations that a return to flight is prudent and appropriate. We're managing the risks with our aircrews, and we're continuing to study the F-22's oxygen systems and collect data to improve its performance," he said in the statement.
Each $143 million plane was thoroughly inspected before being allowed to fly and will be subject to daily inspections. Pilots will also undergo physiological testing.
The first pilots to fly are instructors, who will then train other pilots once they shake off four months of inactivity.
France said no pilots have expressed concern to him about flying the F-22s, since the investigation has yet to determine the cause for the hypoxia-like symptoms.
"I think they are all fired up and ready to fly," he said.
France expects it to take a few months before crews are back to pre-stand-down functionality.
The F-22 Raptor was introduced in 2005, and the Air Force said it has flown more than 300 homeland security missions but none in combat.
The fleet is stationed at five other bases besides Alaska: Joint Base Pearl Harbor-Hickam, Hawaii; Joint Base Langley-Eustis, Va.; Nellis Air Force Base, Nev.; Holloman Air Force Base, N.M.; and Tyndall Air Force Base, Fla.
Associated Press writer Pauline Jelinek in Washington contributed to this report.