How military researchers are working to understand—and prevent—hypoxia-like events in Air Force aircraft fleets.
After a U.S. Air Force F-35A pilot crashed their Joint Strike Fighter at Eglin Air Force Base, Fla., last May, an Accident Investigation Board determined that the “work of breathing,” or the physical effort required on the part of a pilot’s muscles for them to breathe mid-flight, contributed to the accident.
While investigators determined excessive landing speed was the main cause of the crash, issues related to breathing are a persistent problem that remains hard to pinpoint and explain. The Air Force cataloged 54 unexplained physiological episodes (UPEs) in fiscal 2020, not including that event, including five other F-35A incidents. The others were spread among A-10C, F-15C/D, F-16C/D, and F-22A aircraft.
The Air Force Physiological Episodes Action Team (AF PEAT) defines a physiological event as “any anomaly in the interaction among the aircrew, equipment, and environment that causes adverse physical or cognitive symptoms.” USAF said these can include “cognitive impairment,” inability to focus, slow reaction time, feeling dizzy or lightheaded, difficulty concentrating, and tingling or numbness in the extremities.
The objective is to drive physiological episodes to zero.Col. Mark Schmidt, director, Air Force Physiological Episodes Action Team
U.S. Navy researchers recently concluded that UPEs in its fleets are caused by “a complex relationship between aircrew, their flight gear, and their aircraft,” USNI reported last June. But, when it comes to USAF fleets, the case is far from closed.
AF PEAT Director Col. Mark Schmidt told Air Force Magazine in December his objective is “to drive physiological episodes to zero.”
“Increased awareness of, and focus on, maintenance practices for both aircrew breathing systems and aircrew personal flight equipment” are helping reduce UPEs, he said. “Through a larger network that is both joint and within the Air Force, we will continue to develop and refine more comprehensive solutions.”
Vetting Episodes
The search for answers begins by narrowing down the field of study.
“Things like G-induced loss of consciousness (G-LOC), trapped gas, and spatial disorientation are excluded,” the Air Force Safety Center explained in a statement. So are types of hypoxia that aren’t linked to altitude, such as hypemic hypoxia—when the blood can’t carry enough oxygen—stagnant hypoxia—when circulation is impeded—and hisotoxic hypoxia, when blood cells can’t absorb oxygen even if it’s available.
That leaves hypoxic hypoxia, which happens when the body cannot effectively transfer enough oxygen from the air through the lungs. The Safety Center said it searches for evidence of other possible explanations, such as “hyperventilation, air-sickness, … dehydration, contamination,” and high or low blood sugar. If none of those can be blamed, the incident is categorized as unexplained—a UPE.
Understanding UPEs
AF PEAT is working with the Air Force Research Laboratory’s 711th Human Performance Wing, the Naval Air Medical Research Unit-Dayton (NAMRU-D), and the Air Force Life Cycle Management Center to try to identify possible causes of UPEs.
On-Board Oxygen Generation Systems (OBOGS) were the subject of initial research dating back to at least 2014. The systems produce enriched breathing oxygen during flight by concentrating oxygen from engine bleed air or environmental control system air. This eliminates the need to carry liquid oxygen in the aircraft. While the OBOGS are just part of the aircrew breathing system, the systems are seen as a probable cause of hypoxia-like events.
While the 711th HPW’s OBOGS lab has focused on the T-6A Texan II trainer since 2018, AF PEAT Medical Lead Col. William E. Nelson said that its findings are broadly applicable to other aircraft, as well. Researchers believe they can rule out contaminated air and are now focusing on fluctuating in-flight oxygen levels, a phenomenon known as ROHC, or rapidly oscillating hyperoxic conditions.
Although OBOGS generate more than the “minimum amount of oxygen that a pilot needs to perform effectively,” AFRL 711th Human Performance Wing product line lead James Christensen explained, its production “can vary quite a bit, depending on flight conditions.”
That variation may at times be too difficult for the body to manage.
In a 2018 interview with Air Force Magazine, AF PEAT’s then-lead, Brig. Gen. Edward L. “Hertz” Vaughan, described the issue this way: “The problem with the aircraft is that it oscillates so quickly that the body gets out of sync.”
f they can establish a safe limit for the extent of oxygen-output variation, he said, they may be able to curb risky fluctuations.
Nelson said USAF and Navy researchers created an “accurate mockup of the F-35 Aircrew Breathing System—including the seat component and the angle that it’s set at … along with the types of replica air equipment that’s on a pilot’s chest when they’re breathing—so they can capture that data.”
Nelson said the Air Force is attempting to learn more about the F-35 because it’s new and many more aircraft are still to be built. “We’re trying … to get ahead of the curve so that if there are going to be more events, we’ve got the basic science in order to answer any questions that might come up in the future,” he said.
The Air Force Life Cycle Management Center has established a second lab, the Life Support Systems Scientific Test, Analysis, and Qualification Laboratory—not just for fighters and trainers, but for all aircraft—to help figure out whether issues with aircraft life support systems are causing hypoxia-like events.
“Testing will be across compressed oxygen systems, liquid oxygen systems, OBOGS” and Molecular Sieve Oxygen Generating Systems, wrote Andrew Klein, chief engineer with AFLCMC’s Human Systems Division. “Testing will be performed on both the system components, as well as the systems as a whole, to include the pilot-worn equipment, system tubing and piping, and all oxygen delivery, oxygen generation, and backup/emergency equipment.”
The new lab will backup both AFLCMC’s efforts to acquire new life support equipment and to keep “currently fielded capabilities” up and running, he said.
The 711th HPW OBOGS lab previously lent its equipment and expertise to the AFLCMC as needed to help determine if life support equipment from various aircraft might’ve contributed to unexplained hypoxia-like events.
“Ideally every piece of equipment would be removed after an incident (if deemed appropriate by the maintenance squadron), inspected, and tested thoroughly as part of the root-cause investigation,” Klein said.
The new lab will focus on testing equipment and failed hardware, while the 711th will research causes of UPEs and developing new technology, Klein said.
So far, the new lab has tested and studied an improved quick don oxygen mask for C-17 and C-130 aircrew, and will soon work on the qualification process for the T-7A Life Support system.
To help reduce physiological episodes in the F-22 fleet, the Air Force installed an automatic backup oxygen system in the Raptor, modified the schedule that dictated how much oxygen is delivered at various altitudes, and redesigned a valve in upper pressure garments Raptor pilots don to reduce the labor to breathe in certain flight conditions. The Air Force’s work with physiological episodes in the Raptor fleet helped spark “a realization that things besides hypoxia” could cause similar symptoms.
Pressure changes in F-15 C/D aircraft drove the Air Force to add a cockpit pressure warning system to alert pilots if an “insidious loss of pressure” occurs, Nelson said. To help prepare F-15 aircrew, they are put in a pressure chamber during training to learn to detect “rapid decompression,” but maintainers have also worked to inspect and repair seals between the canopy and the aircraft to further reduce the risk of incidents, according to Nelson.
Lastly, Nelson noted, “our aerospace physiologists have been aggressively enhancing the education program of the aircrew” to understand what causes physiologic symptoms and how to react if they experience them, he said, including breathing techniques and emergency procedures.
And while UPE mitigation efforts in the U.S. military’s F-35 fleets are owned by the F-35 Joint Program Office (JPO), Nelson said some of these initiatives have included modifying the Joint Strike Fighter’s “OBOGS system to provide a more consistent oxygen concentration,” ongoing work to improve the aircraft’s oxygen regulator (known as a “spa”), and adding a carbon monoxide filter to the jets to prevent the exhaust from one jet from contaminating the air in another when they’re parked “one behind another” on aircraft carriers.
In-Flight Insights
After physiological episodes in the T-6 fleet became a hot topic, researchers wanted to equip pilots with commercial medical sensors to monitor physiology mid-flight, but Christensen said they soon found the devices did not perform well “in a pressurized, maneuvering aircraft.”
Eventually, flightworthy sensors were found, but pairing them with other sensors proved difficult. Components made by different manufacturers had their own unique data formats, some were wired while others were not, and synthesizing data was difficult. If data from different in-flight sensors could be integrated, Christensen said, researchers could theoretically cross-check results to understand what was happening and alert aircrew accordingly.
In 2019, AFRL launched the Integrated Cockpit Sensing Program initiative, and the program released its first request for proposals at an industry day hosted that December.
“We’re less than six months into execution at this point, but … certainly very excited,” he said in November 2020.
Ball Aerospace is the prime contractor for the program, and “subcontract partners” include (but aren’t limited to) Rockwell Collins, Lockheed Martin, Human Systems Integration, Inc.Within the Defense Department, the program also collaborates with Air Combat Command and Air Education and Training Command, as well as NAVAIR, NAMRU-D, AFLCMC, the T-6 Program Office, and the F-35 JPO.
hich is supporting us to produce an early system prototype by the end of this fiscal year,” Christensen said. The prototype will be subjected to testing in a centrifuge and altitude chamber by the end of September.
About a dozen components are being tested for integration, including two developed by the wing specifically for combating hypoxia-like events in the T-6.
“The vision is not that … every pilot’s gonna be wearing tons of sensors, you know, forevermore,” Christensen said. “The goal is to have the data … to improve the flying environment.”
The team is also working with Nellis Air Force Base’s 422nd Test and Evaluation Squadron to obtain approval for prototype testing in the A-10, F-15, F-16, F-22, and F-35 fleets, he added.