F-35 pilots at Luke Air Force Base continue to experience unexplained symptoms similar to oxygen deprivation while flying, even after the base temporarily grounded all flights to investigate the problem.

Since flights resumed June 21, pilots at Luke have reported three more cases of hypoxia-like symptoms, 56th Fighter Wing spokeswoman Kiley Dougherty said Thursday.

Pilots flying the F-35 at other bases reported two additional cases in that span, she said. One was at Nellis Air Force Base in Nevada; the other was at Naval Air Station Joint Reserve Base Fort Worth in Texas.

Officials at Luke grounded all F-35 flights June 9 after five in-flight incidents of hypoxia-like episodes at the base in a roughly five-week span. The five incidents occurred between May 2 and June 8.

Base officials resumed the flights after retraining pilots to detect the symptoms of oxygen deprivation and implementing a suite of safety protocols, although a team of engineers and specialists was unable to pinpoint the cause of the problems on the most expensive weapons system in history.

Symptoms of hypoxia, or oxygen deprivation, include dizziness, disorientation and tingling in extremities. Extreme oxygen deprivation can make a pilot pass out.

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However, it’s still unclear if the pilots’ symptoms have been caused by hypoxia or another condition with similar symptoms like hypercapnia, Dougherty said. Hypercapnia is caused by elevated levels of carbon dioxide in the blood.

Dougherty noted that in the three most recent cases at Luke, pilots were able to switch on their backup oxygen systems and safely return to base, but their physiological symptoms did not immediately improve.

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That suggests the problem may not be caused by hypoxia, she said, because sufferers tend to experience rapid improvement when oxygen is restored. Because the cause is unclear, the military calls the incidents “physiological episodes.”

The 10 physiological episodes so far in 2017 represents a significant jump. According to Aviation Week, there were 10 such incidents on the F-35A in the previous 10 years, from 2006 until 2016.

“Five years ago, we didn't have nearly as many F-35s as we do now in the Air Force. Here at Luke, the F-35 didn't even exist five years ago and now we have 60 out on our ramp,” Dougherty said. “It's important to remember that although the numbers are increasing, so are the number of aircraft that we have available.”

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