About a quarter of the F-35 fighter jets made by Lockheed Martin have stopped flying until further notice because of irregularities in pilots’ oxygen supplies, the US air force has announced.

Training flights at Arizona’s Luke air force base, where the 55 jets are based, have been grounded indefinitely. There are more than 220 F-35s flying worldwide.

Luke air force base, located north-west of Phoenix, is home to the 56th Fighter Wing. The base cancelled local flying operations for its F-35A Lightning II aircraft due to five incidents in which pilots experienced symptoms resembling hypoxia, or oxygen deprivation, spokesman Captain Mark Graff said on Friday at the Pentagon.

The air force said the incidents occurred from 2 May to Thursday and in each case the aircraft’s backup oxygen system worked as designed, with the jets were able to land safely. The base’s F-35A Lightning IIs typically flew 25 training missions each weekday, base officials said.

The grounding was extended so the air force could study the issue with pilots, maintenance workers and medical professionals, said Major Rebecca Heyse, a base spokeswoman.

The grounding comes at a critical time for Lockheed, which plans to demonstrate the advanced jet at the Paris Air Show next week. F-35 air operations at other bases continued, US air force officials said.

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

Luke is the largest F-35 base in the world and trains US pilots as well as those of allied countries.



A Lockheed representative said the company would help the air force address the issue. Lockheed and its main partners, Northrop Grumman, Pratt and Whitney and BAE Systems have been developing and building F-35s for the US military and 10 allies.

The 220 F-35s around the world have collectively done more than 95,000 flight hours, though the plane has not yet seen combat.