The Air Force is keeping a squadron of its newest F-35 fighter aircraft on the ground while it continues to probe why some pilots have reported experiencing symptoms of oxygen deprivation while in the cockpit.
The temporary grounding affects only the F-35s at Luke Air Force Base in Arizona, where five pilots in the past five weeks have reported symptoms consistent with "hypoxia," or lack of oxygen.
In all five cases, backup oxygen systems engaged and the pilots were able to land the high-tech jet safely.
"The 56th Fighter Wing will continue their pause in local F-35A flying to coordinate analysis and communication between pilots, maintainers, medical professionals and a team of military and industry experts," said Maj. Rebecca Heyse, a spokeswoman for Luke Air Force Base, in a statement Monday.
The problem was first reported by the Air Force Friday, along with the decision to stop flying the fifth generation planes at the Arizona base until further notice.
"The safety of our Airmen is paramount and we will take as much time as necessary to ensure their safety," Heyse.
The F-35s will not return to the air until a technical analysis of the incidents has been completed and "risk mitigation options" have been identified.
F-35s are built by Lockheed Martin as part of the Pentagon's most expensive weapons acquisition program ever, with a total cost of more than $1 trillion.
The latest production lot of the Air Force version of the plane has a price tag of $95 million a copy.
It is flown, not just by the U.S., but also by its allies including Great Britain and Israel.
This is not the first time the U.S. military has had a problem with oxygen deprivation in fighter aircraft. In 2011, the Air Force grounded its F-22 Raptors after reports of pilots blacking out and exhibiting other systems of hypoxia, and more recently the Navy has been investigating oxygen problems on board its F/A-18 Hornets and T-45 training aircraft.