Pilots at the Luke Air Force base in the US state of Arizona noticed that they were becoming deprived of oxygen during flight tests, experiencing what the branch described as "physiological incidents while flying," according to a new report from Defense One.
Flights involving all 55 of the base's F-35s were grounded on Friday as engineers and technicians worked to determine the cause of the problem.
After noticing the issue, pilots were able to switch to the F-35's backup life-support system and land their aircraft safely, according to reports.
This is the second grounding of the beleaguered F-35 program since they were declared ready for service by the US Air Force last year, after embedded electrical lines inside of the aircraft's fuel tanks were found to have cracking insulation.
A US Air Force spokesperson claimed that the Virginia-based F-35 program head office has now implemented a "formal action team of engineers, maintainers and aeromedical specialists to examine the incidents to better understand the issue."
Pilots from nations including Australia, Norway, Italy, Japan, the Netherlands and Israel are trained to fly the F-35 at Luke air base.
The Arizona F-35s are anticipated to be cleared to fly on Monday, according to Defense One.