WASHINGTON — After a week of analyzing recent physiological incidents involving F-35A pilots at Luke Air Force Base, Arizona, a team of engineers and medical personnel have not been able to identify a single root cause linking all five events, a U.S. Air Force official said Friday.

But while F-35As at Luke AFB will remain grounded going into next week, Brig. Gen. Brook Leonard, 56th Fighter Wing commander, believes flight operations could be resumed as early as Tuesday afternoon, he told reporters during a conference call.

Base officials announced June 9 that it would halt F-35 flights after five pilots experienced hypoxia-like symptoms over the course of about a month. Five different pilots and five separate aircraft were affected, including one international operator and jet, Leonard said. Since then, a team of engineers, maintainers and aeromedical specialists from the F-35 Joint Program Office have been called in to analyze the occurrences.

"We did not find a specific cause that we could put our finger on and just fix," Leonard said. "In many ways, it was unsatisfying, but in other ways, it was very satisfying. … We scrubbed our maintenance practices, and we had the engineers and the folks from Lockheed [Martin] looking at those, and we found nothing we were doing incorrectly on the maintenance side as well as on the aircrew flight equipment side that would cause a physiological incident"

At this point, the U.S. Air Force and JPO have not been able to rule out a larger problem with the F-35’s On-Board Oxygen Generating System, or OBOGS.

"We do think the OBOGS system is not as robust as it can be, however, according to all of our testing, it meets the minimum standard, and then if you look at the number of sorties that we’ve had in the program overall, over 86,000 of them, it’s performed well in many cases," he said.

The F-35 will make some changes to the OBOGS system, which is produced by Honeywell, although Leonard declined to describe potential modifications.

Restoring flight operations

Although a root cause still remains a mystery, the Air Force is finalizing initial criteria that the base must meet before returning to normal flight operations. Over the weekend, base officials and the JPO team will review the criteria and will begin putting them in place on Monday.

"If everything is deemed to lead us to safe flight, and the pilots have confidence in it, the soonest the pilots would be able to return to flying would be Tuesday afternoon," Leonard said.

The tentative criteria are :

  • Temporarily restricting the F-35A’s flight envelope, as all five aircraft were operating at a similar altitude when symptoms occurred. Leonard declined to detail how the jet’s flight profile could be altered.
  • Having pilots wear sensors that measure his or her oxygen levels, which would allow analysts to make correlations between what the jet is doing with the operator’s physiological experience. Leonard said no decision has been made on this point yet, but the U.S. Air Force used similar measures when it dealt with hypoxia incidents involving the F-22.
  • Ensuring backup oxygen systems are kept "as full and capable as they can be" before every flight.
  • Mitigating physiological risk to pilots as they perform operations on the ground, so they aren’t exposed to excessively high heat or harmful exhaust gases on the ramp.
  • Improving pilot training on how to respond to physiological events and increasing communication across the medical and operations community.

On the final point, Leonard noted that hypoxia symptoms are similar to other conditions like oxygen contamination as well as hypercapnia and hypocapnia, terms referring to the state of having too much or too little carbon dioxide in the blood.

"While we don't know exactly what the pilots experienced," he said. "It's important to talk through each one. Because, for example, hypoxia symptoms go away pretty quickly when you get on 100 percent oxygen where hyper or hypocapnia does not necessarily do that. And if there is contamination on your oxygen source, while you think you might be applying 100 percent pure and good oxygen to help you recover, if there is contamination in that source, that will obviously not help you."

A wider problem?

Leonard said the overall F-35 program has documented 23 physiological events, including three involving B-model and five with the C-model. Of those 23, the program office was able to determine the causes of 13 incidents, but 10 — including the five recent episodes at Luke — have not been explained. Only the incidents at Luke were assessed by the JPO this week, but future analysis will involve expanding the data pool to include the other unsolved occurrences.

Aside from the five recent occurrences at Luke, there have been 10 physiological events in F-35As since 2011, but because they were spread out over a period of six years, they have been viewed as isolated incidents, said Capt. Mark Graff, a U.S. Air Force spokesman. Graff’s statement did not make it clear whether pilots in all 10 incidents suffered from oxygen deprivation or other conditions.

"Overall, physiological events occur at low rates in all Air Force aircraft. The Air Force reviews all physiological events to learn from them and to ensure the safety and well-being of our pilots," he said.

All quiet on Capitol Hill

Over the past week, response from lawmakers has been relatively subdued. Usually, technical or operational issues concerning the F-35 prompt immediate concern from the top members of defense committees, especially if they pose a safety issue to servicemembers. However, lawmakers have kept mum about the reports of hypoxia emanating from Luke AFB, even as Defense Secretary Jim Mattis and Joint Chiefs Chairman Gen. Joe Dunford made multiple appearances to Capitol Hill.

Sen. John McCain, the Arizona Republican who chairs the powerful Armed Services Committee and one of the F-35’s biggest critics, didn’t issue a statement about the incidents until Wednesday. More surprisingly, it expressed support of Luke AFB officials handling the problem and did not allude to any larger concerns with the health of the program.

"I am in close communication with Air Force officials as they continue to conduct a technical analysis of the F-35A program at Luke Air Force Base, which has been suspended following reports of physiological incidents in recent weeks," he said. "I share the concerns of our military commanders, and believe it is critical they identify the root cause of these incidents and take the necessary steps to ensure our pilots return to safe flying operations."

Valerie Insinna is Defense News' air warfare reporter. She previously worked the Navy/congressional beats for Defense Daily, which followed almost three years as a staff writer for National Defense Magazine. Prior to that, she worked as an editorial assistant for the Tokyo Shimbun’s Washington bureau.

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