HOLLOMAN AIR FORCE BASE, New Mexico — U.S. Air Force officials remain stumped about the causes of hypoxia-like symptoms that affected F-35 pilots at Luke Air Force Base, Arizona, earlier this summer, but the service is beginning to carve out a plan for removing certain flight restrictions, a general said Wednesday.

Pilots at Luke have been required to fly at altitudes lower than 25,000 feet since June, when Brig. Gen. Brook Leonard, the 56th Fighter Wing commander, approved F-35 flight operations to resume after a weeklong grounding at Luke AFB. Leonard ordered that flight operations halt following five incidents at the base in which pilots reported they had experienced symptoms similar to hypoxia, or oxygen deprivation.

Leonard, who spoke with Defense News on Aug. 9 at Holloman AFB in New Mexico, said there is no new information from any of these efforts that point to a single root cause. However, once experts weigh in with more information, he might have enough data to make the decision to open the flight envelope again, he said.

“Once we kind of sit back down with the instructor pilots, once we sit back down with the experts and kind of analyze the data that we have, if we really haven’t found anything that pops out as causal — or if we have, and we can fix it with the mitigation [actions] that we’ve put in place and what we’ve learned so far — we’ll probably go and eliminate the 25,000-foot restriction,” he said. “But that one needs to be determined based on the data that we still have to analyze.”

Since flight operations were suspended in June, the U.S. Air Force and F-35 joint program office, or JPO, have conducted an investigation into the physiological events. Stakeholders, including the U.S. Air Force’s 711 Human Performance Wing, F-35 physiological event team and the Navy Medical Research Unit Dayton, are also analyzing data from sensors worn by the pilots and evaluating potential causes of the problems — such as whether the incidents occurred due to problems with the onboard oxygen generating system, or OBOGS.

“We have looked at the carbon monoxide issue on the ramp because it’s fairly congested,” he said. “We’ve actually found that the fuel that we use in our aircraft actually burns a lot cleaner than even car exhaust. So while I wish I could start to find substantive things that were wrong, that one ended up not being a problem.”

Many other assessments are still in the works, and Leonard noted that certain modifications to the plane could take months, or even a year, to be put in place.

Those longer-term fixes to the F-35 include updating the jet’s OBOGS system with new algorithms, which will optimize the flow of oxygen to the cockpit. There are also plans to install a new carbon monoxide filter to strip the toxin from air in the cockpit and in the pilot’s mask, he said.

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Additionally, Luke AFB made other immediate changes to its F-35 flight operations, including improving pilot training on how to breathe in the physiological taxing conditions regularly encountered in fighter jets, how to recognize symptoms of oxygen deprivation and to react accordingly.

“We keep our mask up, making sure we park the jets in certain places to reduce the carbon monoxide,” he said. “We’ve looked at the valves in our mask to make sure that they’re all working.”