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​The F-35 Lightning II makes its first appearance at Luke AFB, Ariz., on March 10, 2014. The Air Force grounded all 55 F-35s at Luke, including USAF and international aircraft, on June 9, 2017, after multiple pilots experienced hypoxia-like symptoms in flight. Air Force photo by SSgt. Darlene Seltmann.

—John A. Tirpak

After engineers, physicians, pilots, and maintainers wracked their brains for a week, the cause of five physiological incidents among F-35A pilots at Luke AFB, Ariz., remains a mystery.

Even so, an “initial” return to flight status is contemplated as early as the afternoon of June 20, 56th Fighter Wing commander Brig. Gen. Brook Leonard told reporters Friday via teleconference from Luke.

Leonard said there was no “root cause” identified among the five incidents, which occurred over five weeks and caused F-35 flying to be halted at Luke, nor were there any “common threads” pointing clearly to a specific problem. Flying was stopped because “the risk snowball was building,” he said.  

The incidents affected five different pilots and five different aircraft from different production lots, so there were no common factors, Leonard said, nor were any of the procedures in maintenance or in the life support shop found to be improper or inadequate. The only consistency, he noted, were that the incidents occurred at about the same altitude or “cabin altitude”—the air pressure inside the cockpit. The intense scrutiny partnered “experts and engineers with operators,” and while flying is set to begin again, the investigation will continue.

In the meantime, Leonard believes flying can safely resume with several restrictions and “more robust” procedures. Those include avoiding the “flight regime”—the altitude and maneuvers—associated with the five incidents, but Leonard declined to identify what those are for fear that it would preclude an open-minded approach to finding the true root cause of the problem.

Leonard said scrutiny continues of the On-Board Oxygen Generating System (OBOGS), which was involved in a similar issue with the F-22 several years ago. He allowed that it’s “a possibility” the F-35 suffers from a problem similar to Navy F/A-18s and T-45s, which also have had OBOGS issues in recent months, “but we have not been able to test that, yet.”

Leonard said he will meet with pilots in the program and they will have to assure him “they are comfortable” with the additional safety measures in place. Those comprise extra pre-flight checks of the backup oxygen system, which “worked very well” in the recovery of the jets affected, as well as protecting pilots from breathing too much exhaust gas on the ground in their pre-flight preparations. Temperatures on the Luke flightline “are very hot,” he said, and jets in close proximity can put a lot of fumes into the air. Pilots also will get more frequent refreshers in physiological training so they can recognize and avoid both hypoxia—insufficient oxygen—and hyper- or hypocapnia, which involves the amount of carbon dioxide in the blood, and the effects of which are not as immediately discernible as those of hypoxia. In addition, pilots will begin wearing sensors that detect the levels of gases in the cockpit.

There have been 23 cases of physiological incidents with the F-35 since the first examples began flying, and 10 of those—including the five affecting Luke jets—remain unsolved, Leonard observed. Of the 23 incidents, 10 affected F-35As, three affected F-35Bs, and five involved F-35Cs. He declined to discuss the specific symptoms of the five affected Luke pilots, noting that physiological symptoms of hypoxia and other breathing-related physiological issues differ from person to person.

Leonard said he has conducted a “town hall” with spouses and families of F-35 pilots to keep them informed and assure them that safety will be the top priority as the solution is sought.

Five US pilots and one international pilot were affected, and five US jets and one international jet were involved, but Leonard declined to identify either the foreign pilot or the owner of the foreign aircraft.  One of the incidents affected a Luke jet while it was operating at Mountain Home AFB, Idaho.

There will be no surge in flying after the grounding is lifted, because that would violate the “slow and steady” principle of training, Leonard asserted, adding that he believes the lost training time will have little to no effect on the proficiency of the students in the F-35 program at Luke.  

The base has 55 F-35s: 40 of which belong to the US Air Force, while the remainder belong to Australia, Norway, Italy, and Japan. It is the center of advanced training in the “A” model of the aircraft. Basic F-35 training takes place at Eglin AFB, Fla.