55 US F-35s Grounded Due To Oxygen Deprivation

Around a quarter of the F-35 aircraft currently in use have been grounded indefinitely, after five incidents in which pilots experienced symptoms resembling hypoxia, or oxygen deprivation.

55 of the stealth jets, made by Lockheed Martin, are based at Arizona's Luke Air Force Base, out of over 220 flying worldwide.

Its training flights have been grounded until further notice after the aircraft experienced irregularities in pilots' oxygen supplies, the US Air Force said, according to Reuters.

The Air Force will now investigate the issue with pilots, maintenance workers and medical professionals, according to a base spokeswoman.

F-35As typically fly 25 training missions every weekday at Luke, the largest F-35 base in the world, which trains pilots from the US and other allied countries.

The incidents occurred from 2 May to Thursday. In each case the aircraft's backup oxygen system worked as designed, with the aircraft able to land safely, the air force said.

A Lockheed representative said the company would help the US military rectify the issue. 

The F-35 accounts for about 37% of Lockheed's income, with revenue from its aeronautics business rising 8% to $4.11bn, led by increased sales of the F-35, during the first quarter.

The 220 F-35s currently in operation across the world have collectively done more than 95,000 flight hours, although the plane has not yet seen combat.