Finding Solutions for Physiological Episodes in Naval Aviation

Photo by Lance Cpl. Andy Martinez

By Lt. Todd R. Seech, MSC, USN; Lt. Adam T. Biggs, MSC, USN; and Dr. Matthew Funke


Healthy warfighters are lethal warfighters. Although equipment maintenance and training are important, the capabilities of our fleet begin with the good health of our personnel. Critical health issues range anywhere from our annual physical readiness programs to staving off performance detriments in extreme environments. This latter example serves as a central problem for the aviation community, in which aircrew members must operate in settings that often are at odds with human physiology. As a result, these warfighters can experience a variety of medical issues that directly impact their readiness and can compromise operational performance.

Medical issues that manifest in-flight are generally referred to as physiological episodes (PEs), and can range from vague physical sensations (e.g., dizziness, headache) to complete incapacitation. Needless to say, these impairments—whether minimal or pervasive—markedly increase the operational risk to the affected aircrews. Alarmingly, we have seen a dramatic rise in frequency of reports in recent years. Although PEs occur in all aviation communities, the Navy’s tactical jet community appears to be the most affected. From the Navy’s jet trainer, the T-45 Goshawk, to the classic workhorse F/A-18 Hornet, to the new F-35 Lightning II, no aircraft model appears immune to this emerging crisis. This phenomenon has resulted in numerous aircraft and aircrew groundings for maintenance/aeromedical examination as well as temporary suspension of flight for entire airframes, which has stymied the aviation training pipeline and threatens to downgrade our operational readiness. Most concerning of all, however, is that the definitive cause of these PEs has not yet been identified, and remains one of the most pressing aeromedical issues facing military aviation.

To understand how these events affect aircrew members, aeromedical experts are examining reported PE symptom sets across a wide variety of scenarios. Multiple health issues also emerge across different missions as well as different aircraft. Hypoxia, pressure fluctuations, and contaminants in breathing air are among a long list of proposed potential causes of PEs. Regrettably, this aeromedical challenge has been exacerbated by the vague, inconsistent, and idiosyncratic presentations of symptoms, which make prediction and mitigation especially difficult. Here we will discuss several issues in the pursuit of the causal agent of PEs, including: the scope of the problem, the prime medical challenges already known to affect aviation, and several projects the naval aeromedical community has undertaken in response to the health and operational needs of our aircrews.

Scope of the Problem

PEs are not a new phenomenon; they have been a recognized risk in aviation since Jacques Charles experienced one of the first PEs in his hot air balloon in 1783. Instead, the current problem involves a sharp increase in reported PEs in recent years. Regarding the T-45, reported PEs occurred at a yearly rate of 11.86 events per 100,000 flight hours during 2012. The rate increased to 16.22 per 100,000 flight hours in 2013, then 18.43 in 2014, and peaked recently with 44.99 and 46.97 in 2015 and 2016, respectively—thereby increasing nearly 300 percent in five years (as reported publicly to the House Armed Services Committee).

In light of this increasing rate, the chief of naval air training recently ordered a stand down of all T-45Cs, which temporarily shut down the aviation training pipeline and constituted one of the largest impacts an aeromedical concern has ever had on naval aviation training. Although a reduction of T-45 training flights provided a temporary solution, a definitive aeromedical cause remains elusive.

Leaders cannot help but compare the current situation to the Air Force’s F-22 Raptor PE crisis of 2011, during which the entire F-22 fleet was grounded due to a rapid increase in reported PEs. The current problem facing naval aviation, however, is occurring in more than one aircraft model. All variants of the F/A-18 are exhibiting increased frequencies of PEs, including the EA-18G Growlers. In the aforementioned House report, the PE rate per 100,000 flight hours for F/A-18A-F increased from 19.55 in 2010/2011 to 88.29 during the same time frame in 2015/2016. With concerns now mounting regarding the F-35, the pressure to identify the causal aeromedical and engineering issues is higher than ever, with stakes including potential loss of resources, clogged training pipelines, and even fatal mishaps.

In April 2017, the Navy issued an “operational pause” to all flights by the T-45C Goshawk, the Navy’s primary jet trainer aircraft. Although limited flights at restricted altitude soon resumed, the Navy’s pilot training program was severely affected for several months. Photo by MC3 Nathan T. Beard

The Usual Suspects

 Although a few primary points of failure were generally agreed upon during the F-22 PE crisis in 2011 (e.g., poorly fitted upper pressure garment), many experts argued that the actual cause involved a complex interplay between human physiology and aircraft design. As the PE issue now appears to be resurfacing in several new venues, experts warn that we may be facing a multifaceted problem. Many of the same suspected causes from the F-22 issue remain for the current crisis, which include hypoxia (insufficient oxygen to the brain), pressure oscillations, and carbon dioxide imbalances in the body (hypocapnia or hypercapnia). Also worthy of note, some remnants of the F-22 crisis may prove to further confuse investigators trying to identify causes of current PEs, including a fixation on oxygen system contaminants causing PEs despite a lack of corroborating data.

Further compounding an already complex problem is the myriad of less recognized issues that could also contribute to the transient and vague symptoms associated with a PE. Issues such as hyperthermia, dehydration, motion sickness, and fatigue are only a few of the physiological insults that could result in PEs in flight. The combination further complicates any attempt to find a solution because no single source may be responsible for the increase in PEs—and certainly no single source is responsible for all PEs in general. Still, many current initiatives in aircraft design are focused on finding the best sensors and fitting them into the cockpit, which represents even more obstacles in identifying the causal factors behind the increased PEs.

The More Sensors We Come Across, the More Problems We See

Today, experts are engineering methods to assess the intricate physiological patterns and fluctuations of human operators. Physiologic sensor suites that collect real- time data in pulse-oximetry, heart-rate variability, internal temperature, hydration, and other areas seem to rule the day. The resulting new philosophy appears to involve the assumption that “Big Physio Data” will identify the culprit as well as the mitigation strategy. In other words, we seem to believe that there is a sort of critical mass for physiological data that will unveil the secrets of human performance in flight—that we just need to strap enough sensors onto our aircrews.

This strategy has an inherent flaw: the idiosyncratic nature of human physiology and its transient connection to performance. Each person’s physiological response to hypoxia, rapid pressure change, hypocapnia, and other physical insults is somewhat different than that of other people, which makes predicting subtle performance decrements using physiological data difficult. A possible solution to this problem would be to move the focus of assessments from traditional physiological sensors to more direct measures of brain functioning, which may have a more direct connection to performance, as physiological inputs must filter through the brain before behavioral performance is produced.

At Naval Medical Research Unit-Dayton, multiple ongoing research projects are under way focusing on assessing real-time neurological data through noninvasive and nondistracting sensors. A prime example is a study currently funded by the Office of Naval Research’s basic biomedical portfolio, which examines how an electroencephalograph, or EEG, signal known as Mismatch Negativity can give real-time information about an operator’s ability to perform given his/her neurological state. While this research is encouraging, the immature status of the technology (e.g., sensitivity to electromagnetic noise) means that it is unlikely to be ready to solve today’s PE issue. Even so, assessing operator performance by focusing on a more universal predictor of performance deficits, such as brain functioning, may have the propensity to prevent the PE crises of tomorrow by providing a more universal and consistent measurement scheme.

To achieve this end, military medicine must continue to support basic biomedical research and solutions, even if they may not solve the most immediate crises. Taking this broader view of PEs will allow scientists to stay ahead of the next aeromedical challenge by investing in a future technology that can keep our aircrews informed about their neurological state—no matter what the physiological cause—and ultimately keep our aviators healthy, ready, and ahead of their aircraft.

A New Approach 

These aeromedical problems will persist as long as there are human operators placed at extreme altitudes and within dynamic envelopes. The research challenge is to reduce these physiological and cognitive impairments as much as possible, in light of the dramatic increase in recent PE rates. To that end, the recent episodes provide insights regarding how we should move forward.

It is unlikely there will be a “smoking gun” that explains the variety of aeromedical issues being observed. As long as there are humans in the cockpit, operators will be subject to any number of physiological and cognitive impairments that have interactive and multiplicative relations with one another. Perhaps there might be, however, a “silver bullet” solution to detecting PEs prior to performance decrements that involves shifting the primary focus from physiological evidence to neurological evidence, which might help to limit potential sources of error. Physiological symptoms are inherently complicated to address because they generally do not correlate with actual changes in performance. Based on the inherent variability in physiology and behavior from individual to individual, this solution would have to involve measurement at the “bottleneck” between physiological inputs (e.g., pulse oximetry, respiration, temperature) and performance outputs (e.g., aircraft control). This approach then takes a step toward identifying the actual problem rather than merely recognizing that a problem exists. Further, while data collection in flight using technologies such as EEGs requires notable miniaturization and hardening before being field-ready, this direction presents a viable way forward in detecting the PEs of the future, no matter the causes.

The persistence of PEs across aircraft types also demonstrates the importance of medical research to naval aviation. Human operators are the one commonality across aviation platforms, which makes aeromedical problems ubiquitous in aviation. In addition, when these issues become too severe, they can, and have, resulted in the grounding of entire aircraft types in the fleet—a prospect that deals a critical blow to our national security. Medical research in this sense thus matters most outside of the hospital rather than within one. Resolving these aeromedical issues, or staying ahead of them, is fundamental to the operational readiness of our fleet.

About the authors:

Lt. Seech is an aerospace experimental psychologist at the Naval Medical Research Unit-Dayton. Lt. Biggs is a research psychologist at the Naval Medical Research Unit-Dayton. Dr. Funke is a research psychologist at the Naval Medical Research Unit-Dayton.

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