For instructor pilot Maj. Kinsley “Trigger” Jordan, the first clue that something had gone seriously wrong was when he suddenly tasted something metallic.

Jordan was in a T-6 Texan II, on the back half of a routine training sortie with a student pilot near Vance Air Force Base in Oklahoma in early November 2017. The student was practicing basic touch-and-go landings at a nearby airport when Jordan became cognizant of the metallic taste. He first wondered if he hadn’t eaten enough for lunch, or if he had unknowingly bit his lip and drawn blood.

But then, Jordan looked at the T-6’s instrumentation. He could see the numbers clearly, but the experienced pilot suddenly could not comprehend what the basic altitude and airspeed readings meant — and whether they were good or bad.

“That’s when the severity of what it was made me realize, ‘I’m not exactly sure what’s happening right now,’ ” Jordan, who teaches flying at Vance’s 33rd Flying Training Squadron, said in a Jan. 23 interview.

Jordan had suffered either hypoxia, a lack of oxygen in his blood, or a similar “unexplained physiological event,” as the Air Force calls it. It was one of the first in a series of incidents that led Vance’s 71st Flying Training Wing to temporarily ground its T-6s.

A month-long, fleet-wide grounding of T-6s followed early in 2018, and an alarmed Air Force scrambled to solve the mystery of why its most basic training aircraft was putting dozens of student and instructor pilots at risk of passing out in mid-air — and possibly dying.

But now, the Air Force is rolling out a series of fixes that it thinks will solve the problem once and for all — and may have already started cutting down on hypoxia and hypoxia-like incidents.

In a Jan. 10 interview, Brig. Gen. Edward Vaughan — the officer in charge of solving this problem and head of the Air Force Physiological Episodes Action Team — said all the changes will be in place by fiscal 2023 at the latest, though the Air Force is making faster progress than expected and he’s optimistic it could get done sooner.

Key to the Air Force’s effort is a completely redesigned On-Board Oxygen Generating System, or OBOGS, that takes design ideas from the F-15E Strike Eagle, in which hypoxia is virtually unheard of, and from the A-10 Warthog.

The Air Force is also taking maintenance cues from the Navy, which service officials thinks are helping, though they’re continuing to study those changes. In all, Vaughan said, the Air Force has a slate of about 20 changes it’s making to the T-6. And some of those changes could also be adapted to other aircraft as well, to further reduce the likelihood of their pilots suffering hypoxia or other episodes.

An insidious threat

Besides hypoxia, unexplained physiological events, or UPEs, can include hypocapnia, a lack of carbon dioxide in the blood. They can cause problems such as shortness of breath, dizziness or disorientation, or even loss of consciousness — and when pilots suffer them, the results can be dangerous or even deadly.

It can be one of the most insidious dangers a pilot can face, said retired Gen. Hawk Carlisle in a Jan. 22 interview. The former head of Air Combat Command flew F-15s. Hypoxia, he said, sneaks up so slowly that the pilot often doesn’t realize what’s happening until the condition becomes serious.

Carlisle, now president of the National Defense Industrial Association, said he once became hypoxic during a nighttime training flight in an F-15C Eagle that had a leaky oxygen hose. The flight happened in 2002 at Eglin Air Force Base in Florida, while he was the commander of the 33rd Fighter Wing.

Carlisle suddenly couldn’t see clearly, and his first reaction was to wonder what had gone wrong with his night vision goggles. But because he had been trained to identify symptoms of hypoxia, he quickly realized what was happening wasn’t a night vision problem. That training, which included purposely having hypoxia induced in a controlled situation on the ground, taught him that his vision started to suffer when he experienced hypoxia.

Other pilots can experience hot and cold flashes or tingling in their fingers when hypoxia strikes. Wesley Hallman, a retired colonel and F-15 and F-22 pilot who is now senior vice president for policy at NDIA, said in the same Jan. 22 interview that’s what he typically felt.

Carlisle said that once he figured out he was hypoxic, he took the usual steps to resolve the problem: He declared an emergency, “gang loaded” his regulator to turn the oxygen all the way up and descended so the ambient air coming in would be more oxygen-rich. Pilots experiencing hypoxia must get below 18,000 feet, he said, though it’s best to get below 10,000 feet.

“Almost instantaneously, I went, ‘Oh! There’s my vision coming back,’ ” Carlisle said. “The recovery is pretty dramatic. Once you get oxygen, it comes back pretty rapidly.”

The Air Force was unable to provide statistics on hypoxia rates by press time.

Carlisle estimates he had five or six experiences with hypoxia or a similar condition over his career and said it’s extremely common.

“Anybody that flies a fighter that says they haven’t is lying,” Carlisle said.

Once the immediate danger of hypoxia or a similar incident has passed and the pilot reflects on what just happened, Carlisle and Hallman said, the experience is frightening.

“You recover very quickly, but once you do recover, you’re like, ‘That wasn’t good,’ ” Hallman said. “It’s so insidious, and there’s nothing you can do, once you get to unconsciousness.”

Carlisle said he lost a wingman in Germany during a high-altitude training flight in F-15s in 1982, after the other pilot lost consciousness due to hypoxia. Carlisle said he radioed his experienced wingman, Jeff “Wedge” Roether, to climb. Roether hit the afterburners and pulled his nose up.

That was the last thing Roether ever did before losing consciousness, Carlisle said. His F-15 peaked at 37,000 feet and then plunged into the ground, with full afterburners. The accident investigation board concluded hypoxia was to blame, he said.

“You know what can happen,” Carlisle said. “Every time I had a hypoxic incident, I went back, I wrote it down, I thought about what I felt like going into it, just to make sure I was conscious of what my indications were, so if it happened again, I would be ready to recover. Because you know it can sneak up on you.”

When Jordan had his scare in 2017, he was about to tell the student pilot they needed to bail out. But just then, he saw the runway, and realized they were in good shape.

“Even though I couldn’t understand what [the instruments] were telling me, something about the runway and where we were looked OK,” Jordan said.

The student pilot said he felt fine, so they lined up for a landing, touched down — and then the student pilot took off again.

“My mind was screaming, ‘No! Why didn’t we stop?’” Jordan said. “And I realized I never told the student, ‘Make this a full stop.’ My thinking was slow, my cognition of what was going on was slow. I wasn’t incapacitated, but it was hard to reason the simple, everyday tasks that an instructor pilot does in the aircraft. And I didn’t realize that until after the fact.”

Jordan dropped his mask, and after breathing ambient air for about 30 seconds, his faculties started coming back and things began to make sense again. He radioed in an emergency, they turned back to Vance and Jordan landed the T-6 safely.


The incidents at Vance were among the first in a series of hypoxic and hypoxic-like episodes that plagued the T-6 fleet. Five pilots at Vance, including Jordan, reported hypoxia-like symptoms during four flights, leading to the November 2017 grounding for three weeks.

Concerns began to mount, and frustrations grew, as the episodes continued without a solution in sight. In January 2018, the 19th Air Force suspended all T-6 solo flights and authorized pilots to fly with their masks down, so they could breathe cockpit air and reduce the risks of hypoxia. After the wave of physiological episodes the last week of that month, the Air Force grounded all T-6 flights for all of February and launched a serious effort to figure out what was wrong.

In early February 2018, Rep. Michael Turner, R-Ohio, lambasted Lt. Gen. Mark Nowland, then-deputy chief of staff for operations, for suggesting that training would be an important part of fixing the problem, implying that the onus was on pilots rather than the Air Force.

“Should we start doing hearing training where we ask you to come before us and then let’s have you hold your breath for a minute in the first hearing, and then in the second hearing we’ll have you hold your breath for two minutes?” Turner said. “It makes no sense.”

Nowland swiftly clarified he was not blaming pilots.

After months of study, the Air Force concluded that the physiological episodes were caused by rapidly fluctuating oxygen concentrations in the T-6. It wasn’t happening all the time, Vaughan said, or even most of the time, which made the problem tricky to pin down.

One thing the Air Force learned from consulting with the Navy is that periodically purging moisture from the OBOGS — as the Navy does every two weeks — might cut down on UPEs. Some maintainers and experts from the Air Force Research Laboratory believe the buildup of moisture over time degrades the OBOGS’ performance.

The Air Force has been purging moisture regularly for the past few months, and Vaughan said there are signs that it’s helping. The Air Force is also experimenting with moisture purges at different intervals — for example, some T-6s have weekly purges, others are purged every three weeks or so — to see which is most effective.

The Air Force is also in the midst of some long-overdue hardware upgrades to the T-6’s OBOGS. A new oxygen concentrator has been designed, Vaughan said, and deliveries began in December. The redesigned concentrator has new parts and materials, is built to more recent specifications, and is more reliable than the previous two-decade-old concentrator, he said. It will likely take a year to 18 months to install the new concentrators in all of the roughly 400 T-6s in the fleet, he said.

One of the new concentrator’s biggest improvements is that its software can be easily upgraded or adjusted if, for example, the Air Force decides to tweak the concentration of oxygen in the cockpit. This is similar to how engine software is upgraded today, Vaughan said.

The old concentrator wasn’t designed to accept software adjustments, Vaughan said, so maintainers would have to physically take the concentrator out of the plane and basically “break” the device to hack in the new software.

A closer look

Vaughan said that the AFPEAT group also took a closer look at the F-15E Strike Eagle, to figure out what it was doing right — and it learned a lot.

“If you look back over the years, the F-15E Strike Eagle … has virtually none of these UPEs,” Vaughan said. “When they have a physiological episode in a Strike Eagle, it’s very well known that this part failed, or there was some type of a mishap — it’s very attributable. We looked at the F-15E and said, what is it about that OBOGS system that makes it so reliable and effective that we can apply to the T-6?”

The Strike Eagle’s OBOGS has about a half-dozen features that could be applied to the T-6, and possibly other planes as well, Vaughan said.

For example, the Strike Eagle has a moisture separator upstream from the OBOGS, as does the A-10 Warthog. Adding one to the T-6 could help cut down on UPEs, Vaughan said. The Air Force could end up transplanting a Warthog moisture separator into the T-6, with only a few tweaks, because the specifications of the Warthog are close enough to the T-6. That would mean the Air Force wouldn’t have to go to the effort and expense of designing a new one.

Another main difference is that the F-15E has an automatic backup oxygen system included in its OBOGS — basically a second pair of oxygen tanks in reserve, for when its pilots push the limit and need more help breathing.

Vaughan said the Air Force is designing a similar backup system for the T-6, which now has a two-tank OBOGS, and he expects a prototype to be ready in a year.

But, he cautioned, it’s not certain the Air Force will move forward with the four-tank OBOGS for the T-6. If the other changes on their own eliminate the unexplained physiological episodes, the Air Force could decide it’s not worth the investment to field the backup tanks.

The T-6 will also get more sensors to better track how oxygen is flowing to the pilot, Vaughan said. The T-6 OBOGS currently has sensors that measure whether it’s producing oxygen, how much, and if the system is meeting its requirements, he said. But it doesn’t have sensors that show if the oxygen is flowing all the way to the pilot’s mask and regulator, like the Strike Eagle does, which could alert the pilot if there’s a leak in a hose or other problem.

When all is said and done, Vaughan said, the T-6 OBOGS will be almost entirely replaced, save for a few standard pipes and hoses. The oxygen concentrator upgrade alone accounts for 80 percent of the OBOGS, he said.

After the hypoxia wave began, pilots at Vance began talking to each other regularly about what they were experiencing, Jordan said, as well as their uncertainty about what was causing the problems. Jordan said it was great that talking about it became a cultural norm.

But recently, those conversations have all but dried up, which Jordan sees as a sign of progress.

“We don’t see what we were seeing a year ago,” he said. “It’s not a taboo topic at all. [But today] we don’t talk about it, because it’s just not happening that much anymore.”

Carlisle said the Air Force is likely on the right track with the maintenance, hardware and software upgrades to solve the unexplained hypoxia problem. Training is also vital, he said, so that pilots are able to recognize the symptoms of a physiological episode before it becomes too dangerous. Carlisle said he has no doubt Air Education and Training Command will continue to push hypoxia training.

But the fact that pilots are flying fewer hours, on average, than in previous years is worrisome, Carlisle said. The lack of more time in the cockpit could place them in greater jeopardy if a physiological episode develops. Pilots are spending more time in simulators, he said, but even the best simulators can’t prepare a pilot for how he will react if he’s suddenly faced with a life-and-death hypoxic situation.

Pilots can get anxious and upset and make mistakes in the heat of the moment, he said. But the more a pilot flies, the more comfortable he is, and he’s more likely to recognize even small changes in his environment alerting him that something’s wrong.

“A simulator’s great training, you need it,” Carlisle said. “But a simulator never killed anybody.”

Stephen Losey is the air warfare reporter for Defense News. He previously covered leadership and personnel issues at Air Force Times, and the Pentagon, special operations and air warfare at He has traveled to the Middle East to cover U.S. Air Force operations.

In Other News
Load More