How USAF is Tackling Hypoxia-Like Incidents in its T-6, Fighter Fleets

A pilot runs a pre-flight checklist with the T-6 Texan II on March 30, 2018, at Laughlin AFB, Texas. The T-6A Texan II is a single-engine, two-seat aircraft, designed for initial flight training during Specialized Undergraduate Pilot Training. Air Force photo by A1C Anne McCready.

The Air Force Physiological Episodes Action Team has recommended the service stand up two new program offices to help it monitor and reduce the number of hypoxia-like incidents in its trainer and fighter fleets. The move comes as both the Air Force and Navy work to overhaul the On-Board Oxygen Generation systems in their T-6 trainer fleets, following a spike in such incidents and an extended grounding last year.

The Breathing Systems Program Office would be charged with managing and sustaining “operational safety, suitability, and effectiveness,” while carrying out data analysis to prevent “disjointed sustainment practices and communication,” according to an AF PEAT infographic that was shared with Air Force Magazine.

The second program office would be responsible for testing and purchasing cutting-edge technology to monitor pilot physiology. It would be tasked with ensuring the service has aircrew data needed for mishap investigations and follow-on actions, according to the infographic. Senior service leaders are slated to consider these proposals “in the next few months,” AF PEAT lead Brig. Gen. Edward Vaughan told Air Force Magazine.

The frequency of confirmed physiological events in the Air Force’s T-6A Texan II fleet tended to be higher after the February 2018 stand down than before, newly released data from the Air Force Safety Center shows.

From June 2017 to December 2017, the fleet averaged just 1.4 physiological episodes (PE) a month, but that number skyrocketed to 17 reported episodes in January 2018 prompting the service to ground the fleet from Feb. 1 to Feb. 27.

“We believe the January [20]18 uptick is multi-factorial, but primarily caused by ROHC [rapidly oscillating hyperoxic conditions], combined with increased vigilance in reporting,” Vaughan told Air Force Magazine.

ROHC refers to a situation in which pilots are getting sufficient oxygen, but not in the correct quantity, Vaughan explained. For the T-6A, the current design of the trainer’s OBOGS system, its age, and other maintenance factors collectively create “an oscillation in the oxygen,” he said. “So, you might get some percentage of oxygen for a few seconds, and then a much higher percent, and then back down to that” lower percentage.

While both oxygen percentages “are perfectly acceptable,” he said, the human body’s resiliency and eagerness to quickly adapt to these changes winds up being its downfall.

“If you were to climb a mountain, your body would start to adapt to the lower oxygen content in the air, and if you were actually physically climbing, you’d probably have the time to adapt, assuming you don’t go too high,” Vaughan said. “The problem with the aircraft is that it oscillates so quickly that the body gets out of sync.”

When the body undergoes an uptick in oxygen percentage during this fluctuation, it takes it as a signal to “start constricting veins because” it thinks it’s receiving too much, he said. “What happens is it quickly goes to a lower oxygen content, and the body’s behind, and that’s when people start to feel [what] we call hypoxia-like symptoms.”

Once the grounding was lifted, the monthly average reported cases of PEs increased to 5.8 per month from March 2018 until December 2018, according to the AF PEAT data. That jump in monthly averages was due to actual PE incidence as well as the way the statistics were crunched, Vaughan said.

However, he emphasized that T-6A pilots aren’t actually experiencing “classic hypoxia,” which he said comes in various forms, but is generally defined as a lack of “oxygen, whether delivered or in the lungs.”

AF PEAT defines a PE as “any anomaly in the interaction among the aircrew, equipment, and environment that causes adverse physical or cognitive symptoms” that might make it harder for a pilot to fly an aircraft.

The top five symptoms that pilots report when experiencing PEs are “cognitive impairment,” being unable to focus or taking longer to react than normal, feeling dizzy or lightheaded, having a hard time concentrating, and having their extremities tingle or go numb, according to the Air Force.

Since the physiological event phenomenon isn’t exclusive to the T-6 trainer fleet—incidents with the F-15, F-22, and A-10 fleets also have been reported—the Air Force calculates the rate of PEs per 100,000 flying hours so it can “compare apples to apples across airframes, assuming there are enough data points,” he explained in an email response to questions.

“During unusual circumstances like stand downs or introduction of new airplanes, there are situations where the number of flying hours is low and so the rate value can be skewed,” he said. “Immediately following the stand down, the AF took a gradual approach to resuming full flying as they inspected aircraft and responded to many clues. This graduality is reflected in those numbers, on top of actual reports of PE.”


To combat physiological episodes across the T-6 fleet—and the service, in general—the Air Force is planning multiple training, procedural, maintenance, mechanical, and administrative updates.

The service has changed T-6 emergency procedures. Now, if a pilot experiences “some kind of oxygen anomaly,” they’re told to prioritize getting below an altitude of 10,000 feet where oxygen is more plentiful. They should then drop their mask so they can breathe this ambient air, and cut the mission short. “It’s a training mission, right? We’ll get it later,” Vaughan said. “What we need you to do is get on the ground safely.”

He said T-6 pilots are encouraged “to be the most vigilant” about reporting potential aircraft issues. “Every little thing you think may have gone wrong, we want you come down and report it,” he said.

When it comes to overhauling the T-6’s On-Board Oxygen Generation System, or OBOGS, Vaughan said the Air Force is incorporating lessons learned from both the Air Force and the Navy.

For example, the service pulled inspiration from the way the F-15E’s oxygen-generation system—the Molecular Sieve Oxygen Generating System, or MSOGS— is designed and maintained. While Vaughan said the system “is not high tech,” it is “robust and proven.”

Like the Strike Eagle system, USAF now plans to add a moisture separator and an automatic backup oxygen system—or ABOS—to the T-6 (the latter of which is pending, since the system is still in the design phase). “An ABOS is like a shock absorber for the system helping to level out the peaks and troughs to ensure more consistent flow to pilots,” Vaughan explained.

USAF is also considering whether to upgrade the T-6’s oxygen regulator to the type currently used with MSOGS, since it “is more comfortable for many pilots,” he said.

The Navy, on the other hand, inspired the incorporation of new oxygen concentrators to the trainer.

In addition to these tweaks, USAF will start using stainless steel “and other improved materials” to build OBOGS components “to prevent corrosion and improve reliability,” and it will upgrade “software-based algorithms for the concentrators that can be periodically upgraded with a software push as needed,” he said. It’s also improving preventative maintenance schedules for the trainers, as well as building a cleaning valve into the moisture separator.

The total price tag for the T-6 overhaul is still to be determined, because “each of those line items” are priced separately so the service can “grab efficiencies and get the best bang for the buck,” Vaughan said.

“We gotta balance because we don’t want to overspend and not get what we want, but we also don’t want to let cost concern get in the way of safety,” he said. “It’s a balancing act.”

The Navy, which has suffered similar issues in its fleet, has taught the Air Force a lot about maintenance, such as the value of periodically removing moisture from the OBOGS system, which Vaughan said is the subject of controversy since “some technicians don’t believe it improves system performance, while others believe it does.”

“Empirically, the USAF has recorded fewer PEs in aircraft where such purging occurs,” he noted.

The Navy also changes out the oxygen concentrator valve every 1,500 hours, but the Air Force previously did not. Vaughan suggested the service replace the valve like the Navy, saying even if it doesn’t add value, it can’t hurt.

The Navy also takes the OBOGS out of the aircraft and thoroughly inspects it after a physiological event occurs. Vaughan said it’s important USAF understands its OBOGS system as well as it does its engines. “They’re equally important,” he said.

While the Navy’s focus right now is to “minimize breathing effort variations across all aircraft and system designs,” the Air Force is focusing more on optimizing “oxygen concentration consistency during all phases of flight,” according to a USAF infographic provided to Air Force Magazine.

“Since we work jointly, both services benefit from this complementary approach,” Vaughan said.


To train pilots to deal with conditions like hypoxia and hypocapnia (a dip in carbon dioxide levels in the blood), USAF currently uses a device to simulate these kinds of in-flight issues so pilots can learn how they uniquely respond to them. If a pilot is especially prone to such medical issues, he said, Air Force physiologists will give them more or personalized training so they can have action plans for how to minimize and manage their symptoms. The goal is not to discourage these pilots from flying, but rather to ensure they’re protected while doing so, said Vaughan, who noted this device-assisted training is also paired with in-classroom training.

“Right now, our only sensor is the aviator” in flight, he said. But Vaughan said the Navy also taught the Air Force the necessity of placing sensors on pilots if the service is ever to decode “exactly what happens” during a physiological event.

“Currently we infer what happened based on pilot reporting post-flight medical exam,” he explained. “Some of the blood oxygen content issues that would help us better understand what’s happening are transient in nature and can’t be readily detected in post flight blood draws. This has informed much of the current research and simulation and modeling conducted by AFRL [the Air Force Research Laboratory] and NAMRU-D [Naval Medical Research Unit-Dayton].”

USAF has begun ramping up its investment in research and development regarding “the human-machine-environment interface” and is working to build sensors that can gauge “critical metrics,” Vaughan said. According to the AF PEAT infographic, this would help the service bridge “knowledge and data gaps” with respect to “safety specifications for aircrew life support and related systems.”

Vaughan said AFWERX and the Defense Innovation Unit are helping the team choose sensors to monitor pilot physiology. AF PEAT is also partnering with the Civil Air Patrol, which offered to assess any commercial products that don’t impede flight safety. CAP students are slated to assist AF PEAT with “analysis and algorithms,” he said. Human-machine sensors are currently being developed to monitor the amount of oxygen in pilots’ tissue and blood, in addition to their carbon dioxide levels, gas flow, pulse, blood pressure, and skin temperature, according to the AF PEAT infographic.

“My iPhone can tell me how many steps I walked, and then it can analyze that. … Why don’t we have that for the airplane?” Vaughan posed. “Why don’t the pilots have something?”