About a quarter of the F-35 fighter jets made by Lockheed Martin Corp, or 55 planes, have stopped flying until further notice because of irregularities in pilots’ oxygen supplies, U.S. Air Force spokesman Captain Mark Graff said on Monday.

Training flights at Arizona’s Luke Air Force Base, where the 55 jets are based, were canceled on Friday and scheduled to resume on Monday, but the grounding was extended indefinitely. More than 220 F-35s are flying worldwide.

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, U.S. Air Force officials said.

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

Luke Air Force Base, located northwest of Phoenix, is home to the 56th Fighter Wing. The base canceled local flying operations for its F-35A Lightning II aircraft due to five incidents in which pilots experienced symptoms resembling hypoxia, or oxygen deprivation, Graff said on Friday at the Pentagon.

SOURCE REUTERS, read more..