The 56th Fighter Wing at Luke Air Force Base in Arizona suspended all F-35A flights Friday after the five pilots experienced hypoxia-like symptoms, Air Force spokesman Capt. Mark Graff said in a statement. The pilots all used their backup oxygen to land the planes safely.
"In order to synchronize operations and maintenance efforts toward safe flying operations we have canceled local F-35A flying," said Brig. Gen. Brook Leonard, commander of the 56th Fighter Wing. "The Air Force takes these physiological incidents seriously, and our focus is on the safety and well-being of our pilots. We are taking the necessary steps to find the root cause of these incidents."
Graff said that the pause was taken "not out of fear or out of danger, but out of an abundance of caution," and said the Air Force plans to resume operations on Saturday.
There are 55 F-35As at Luke Air Force Base. Graff said that it's still not clear what caused the oxygen incidents, but said that the pause was confined to Luke because "no other incidents have been reported" at any other Air Force bases since May 2.
But the Luke F-35 grounding is nevertheless the latest setback for the F-35, a long delayed and over-budget weapons program that's become the Pentagon's most expensive in history. The Air Force grounded 10 of its F-35 fighters last year due to insulation problems, and last month the Air Force announced it had resolved an ejection seat issue that had led to a weight restriction being imposed on pilots.
The Lockheed Martin-made F-35A fighter jets were declared combat ready by the Air Force last year, and F-35s have now deployed to Japan and Europe.
The F-35A is the Air Force variant of the Joint Strike Fighter: The F-35B Marine Corps variant was declared combat ready in 2015, and the F-35C Nary variant is supposed to be combat operational next year.
President Donald Trump has taken a personal interest in the F-35 program, slamming the costs as "out of control" and then getting involved in the Pentagon's contract negotiations with Lockheed Martin. He took credit for generating $700 million in savings in the $8.5 billion contract for the latest batch of F-35 fighters.
For military jets, oxygen deprivation has been a nagging problem.
The Navy's F/A-18 fighter jet pilots experienced a rising rate of "physiological episodes," Navy officials told Congress in March.
Navy investigators had identified 382 cases, including 130 that involved some form of oxygen contamination, and 114 with a failure of the jet's system that maintains cabin pressure.
The Air Force's F-22 fighter pilots also struggled with hypoxia-like symptoms back in 2012, which led to limitations on F-22 flights until the issue was resolved. The Air Force said Friday that the F-35 program office has created a team of "engineers, maintainers and aeromedical specialists to examine the incidents to better understand the issue."