The U.S. military has confirmed one thing: the physiological episodes (PEs) that pilots occasionally experience because of oxygen deprivation or decompression sickness present a “complex, perplexing issue,” to use the Navy’s words. A spate of recent incidents involving high-performance Navy as well as Air Force jets suggests that PEs are a recurring condition the services need to manage and not a problem they can simply fix.

There is also a wild card in the deck—the human in the loop known as the pilot. “There is no fixing the human; every human reacts differently to every situation,” Capt. Cliff Blumenberg, head of the Navy’s aerospace medicine branch, told reporters during a September teleconference. “Even the same person in the same aircraft on different days or different flights might experience a physiologic episode at one time and maybe not the next flight. It depends on what you’re doing, how hydrated you are, how well rested you are, what else is going on in your life. Do you have a mild cold that you didn’t recognize?”

Pressurization and Oxygen Ailments

Responding to concerns this spring involving its T-45 Goshawk training jets and an apparent rise in incidents reported by F/A-18 and EA-18G pilots, the Navy has focused on PEs relating to malfunctioning cabin pressurization systems leading to decompression sickness; and on-board oxygen generation systems (OBOGS) causing hypoxia, or oxygen deprivation.

In April, Vice Adm. Mike Shoemaker, commander of Naval Air Forces, ordered that T-45 training flights be paused after instructor pilots raised concerns about PEs caused by contamination of the jet’s OBOGS system, leading to cancelled flights at Naval Air Stations Kingsville, Texas; Meridian, Mississippi; and Pensacola, Florida. The service resumed flight training in September using T-45s fitted with upgraded CRU-123 solid-state oxygen monitors.

The T-45 incidents reinforced concerns about not only the single-engine training jet but also the Navy’s twin-engine F/A-18A-D Hornet, F/A-18E/F Super Hornet and EA-18G Growler jets, all of which have experienced increasing rates of PEs in recent years.

In written testimony submitted to the House Armed Services Committee in late March, naval officers reported that T-45 PEs per 100,000 flight hours increased annually from 2012 to 2016. Based on new reporting procedures adopted in 2010, the rate of PEs per 100,000 flight hours increased from 30.20 to 57.24 for legacy Hornets; from 28.02 to 31.05 for newer Super Hornets; and from 42.89 to 90.83 for EA-18G electronic warfare variants between Fiscal Years 2015 to 2016.

The Air Force in 2011-2012 conducted an investigation of PEs experienced by F-22 pilots that produced a number of safety recommendations but no determination of the root cause of the problem. Earlier this year, the service faced a similar problem with its newer fifth-generation fighter, the F-35A Lightning II. The 56th Fighter Wing at Luke Air Force Base near Phoenix, Arizona, paused F-35A flying operations there in June after several pilots reported experiencing PEs. The base distributed pulse oximeters to pilots to measure their blood oxygen levels and resumed flying F-35As on June 21 with an altitude restriction of 25,000 feet.

A multi-disciplinary team of engineers, maintainers and physiologists from the Air Force, the Pentagon’s F-35 Joint Program Office, the Naval Air Systems Command and manufacturer Lockheed Martin investigated possible causes, leading the service to revise its pilot training and preflight procedures. Brig. Gen. Brook Leonard, the 56th Wing commander, lifted the altitude restriction on August 30, though no root cause was identified.

When the incidents at Luke AFB came up during a briefing at the Paris Air Show in mid-June, Col. Todd Canterbury, director of the Air Force’s F-35 Integration Office, corrected a reporter who used the word “hypoxia” when referencing the F-35A standdown.

“We’re not labeling it a hypoxia incident, as it’s still too early to identify,” Canterbury advised. “There are theories that call it hypoxia, there’s hypocapnia, there’s hyperventilation—there’s a number of physiological terms that manifest the same symptoms. We’re very cautious to call them ‘physiological events,’ only because we don’t want to label it a symptom and chase something that’s incorrect. If you hear us talk around hypoxia it’s because it could be one of the others that has very similar symptoms; we just don’t know yet.”

In response to the T-45 incidents, Adm. William Moran, vice chief of naval operations, ordered the Navy to conduct a comprehensive review of the facts and circumstances attending PE episodes. The service released a 65-page report on June 15. It found that the majority of recent, serious PEs reported by F/A-18 pilots were attributed to the jet’s environmental control system (ECS) and cabin pressurization malfunctions such as fluctuating pressure, over-pressurization and rapid decompression, causing symptoms associated with decompression sickness. The majority of PEs reported by T-45 pilots “have an oxygen warning light with no symptoms,” leading the service to focus on breathing-air related PEs more than pressurization issues.

The ECS on F/A-18s is a system of inter-related components that uses engine bleed air, heat exchangers and a compressor/turbine to provide warm air for pressurization, heating and OBOGS operation, and cold air for avionics cooling. While most of the ECS design is common across F/A-18 models, legacy Hornets up to lot 12 use a closed-loop liquid oxygen (LOX) system to provide oxygen to the pilots, a system that operates independent from cabin pressurization.

On the T-45, the ECS and OBOGS are separately supplied bleed air from the compressor section of the jet’s Rolls-Royce Turbomeca F405-RR-401 Adour engine. The OBOGS uses a nitrogen scrubber mechanism called a sieve bed to remove nitrogen from the bleed air and provide pilots with concentrated oxygen.

According to the comprehensive review, the installation of OBOGS in both the T-45 and F/A-18 fleets was “inadequate to consistently provide high quality breathing air,” possibly allowing contaminants to enter pilots’ breathing air and potentially inducing hypoxia.” Separately, the report found that “aging parts, inadequate testing methodologies and numerous other factors” affect ECS systems that provide cockpit pressurization on the jets, “inducing several instances of decompression sickness.”

An apparent increase in PEs from 2010 likely was the result of improved pilot awareness and reporting, compromising any comparison with previous rates, the review determined. PE reports by F/A-18 pilots rose steadily from 31 in 2011 to 57 in 2012, 89 in 2015 and 114 in 2016, it found.

The review also determined that “numerous human factors such as fatigue, dehydration, diet, nutrition, anxiety, panic, hyperventilation and procedural error” can cause PEs. But missing was a smoking-gun explanation of exactly why PEs happen and how to stop them.

Human-machine Interface

“To date, finding a solution to the U.S. Navy and Marine Corps’ high-performance jet aircraft PE challenge has proved elusive,” the review conceded. “The complexity of aircraft human-machine interfaces and the unforgiving environment in which aircrew operate will continue to generate PEs whenever systems do not operate as intended or human physiology is a factor. The number and severity of PEs can and must be dramatically reduced with a unified, systematic approach.”

During the September teleconference the Navy held with reporters to discuss its efforts to analyze PEs, ranking officers emphasized that the human variable is key to understanding the problem.

“There’s something that comes out right away. This is a man-machine interface that we’re talking about,” said Capt. Sara Joyner, a rear admiral-designate the Navy appointed in August to lead a Physiological Episodes Action Team. “Human physiology and engineering come together, and in the middle, is the human who is operating the machine. That interface is where we’re really trying to work with the complexity of the program to make sure that we fully address it.”

The Naval Air Systems Command (Navair) has initiated a cockpit pressure and oxygen monitoring system (CPOMS) process to target which systems and conditions to monitor in the different aircraft platforms. Already, the Navy is replacing analog cockpit-pressure altimeters with digital altimeters in F/A-18s.

The service also was considering replacing the CRU-99 oxygen monitoring system installed on versions of the F/A-18 with the improved CRU-123 on nearly 1,500 jets. CRU-99 monitors oxygen content and provides an “OBOGS degrade” warning indication if it senses anomalous readings. The CRU-123 system used on T-45s monitors breathing gas pressure in addition to oxygen content and provides for post-flight download of OBOGS parameters.

“We’re trying to let the process play out to say: ‘What do I need to monitor on the airplane?’ and we’re using the CPOMS process to define what that monitor should be,” said Capt. David Kindley, program manager with Navair’s F/A-18 and EA-18G Program Office. “If CRU-123 is the right answer, the work is already done—we can install it right now. CRU-123 works, but right now the F/A-18 series is trying to define exactly what we need to monitor before we spend money installing the boxes. Does CRU-123 monitor the things I want it to monitor?”

As interim steps to a more permanent, installed solution, the service has provided F/A-18 aircrew with pressure-sensing Garmin Fenix watches and “Slamstick” memory devices for recording and post-flight download of pressure readings, both on a one-per-jet basis.

“We say ‘fix the machine’ would be a great approach, but [there’s] also a human involved,” said Joyner. “The number-one thing that I want to do is to alert the aircrew that there’s been some sort of a compromise in their health or safety. If we can alert them, they know how to react to emergencies, they can do their procedures and they can land this plane safely. But our longer-term goal is to fix the machine so it best supports the human in the loop. Where we’ve really come to is how do we optimize human performance in the aeronautical regime in fast jets?”

UK-based Cobham, which supplies OBOGS and breathing regulators on both the F/A-18 and T-45, was “actively supporting” U.S. Navy as well as the Air Force investigations into PE root causes. The company’s OBOGS manufacturing facility in Davenport, Iowa, in particular was supporting the Navy’s research, said Rob Schaeffer, product director for environmental systems.

In September 2016, before the latest PE incidents, the Air Force awarded Cobham’s Orchard Park, New York, facility a contract to supply eight versions of its Aircrew Mounted Physiologic Sensing System (AMPSS), which consists of inhalation and exhalation sensor modules that integrate with a pilot’s mask and breathing hose. Cobham calls this the first pilot-mounted sensor system to capture oxygen system, cockpit environment and pilot physiological data in real time in flight; the company completed deliveries to the Air Force School of Aerospace Medicine at Wright-Patterson Air Force Base, Ohio, this September.

The fatal crash of an Air Force F-22 Raptor in Alaska in November 2010, later attributed to loss of control by the pilot, led to development of the breathing sensor suite, Schaeffer said.

“I don’t think anybody would disagree that these PEs have been around for quite some time,” he observed. “I think the Air Force had an inkling that a research tool was necessary, and it came in the form of AMPSS. AMPSS started around the 2011-2012 time frame when the tragedy with the F-22 occurred. That was the genesis of the breathing sensors as they have evolved today.”

Honeywell Aerospace, which supplies OBOGS on the F-35 and F-22 fighters as well as on other U.S. and international aircraft, referred questions on the F-35 to manufacturer Lockheed Martin.