The Air Force lifted flying restrictions on many F-22 fighter jets after upgrading their oxygen system and life-support equipment, the service said in a statement.
The move comes days after the U.S. military flew a pair of the aircraft to South Korea amid escalating tensions with the North, whose young leader Kim Jong Un has declared a state of war on the peninsula and readied missiles for a possible attack.
The timing of the Air Force's April 4 statement "is strictly coincidental" and unrelated to the joint military exercises the U.S. is conducting with South Korea, said Kelly Sanders, a spokeswoman for Air Combat Command. The command is based at Joint Base Langley-Eustis, Virginia, and oversees the F-22.
The military's most advanced fighter jet is made by Lockheed Martin Corp., based in Bethesda, Maryland, and its oxygen system is made by Honeywell International Inc., based in Morristown, New Jersey. It was the Defense Department's most expensive acquisition program until Congress in 2009 voted to stop further production. The Pentagon spent about $67 billion buying almost 200 of the aircraft.
Then-Defense Secretary Leon Panetta last year restricted flights of the F-22 after two Air National Guard pilots appeared on the CBS news program, "60 Minutes," to say didn't feel safe flying the plane. They complained of dizziness, disorientation, even something called "The Raptor Cough," after the aircraft's name.
The complaints weren't new to the military. The Air Force temporarily grounded the planes in 2011 after more than a dozen pilots reported similar symptoms of oxygen deprivation, a condition known as hypoxia. It did the same in 2010 after pilot Jeff Haney died in a crash in the Alaskan wilderness. (The Pentagon's own inspector general criticized the Air Force's accident investigation.)
The service, which initially struggled to identify the cause of the problem, last year concluded that a lack of oxygen -- not the quality of it -- was causing the symptoms, due primarily to a faulty valve on the pilots' life-support vest.
The Air Force fielded new vest pieces in January and expects to finish installing automatic back-up oxygen systems on the rest of aircraft in the fleet by July 2014, according to the statement. Planes that have already received the upgrades are no longer restricted to flying at certain altitudes and within 30 minutes of an airfield, it said. Crews in Alaska have resumed missions, it said.
"Feedback from the field shows the F-22 community, including pilots, maintainers and family members, are confident in the safety of the F-22," Sanders said in an e-mail.