PEs (Physiological Events) in Navy Jets

On 5 April 2017 Navy authorities grounded its fleet of T-45 training jets, after instructor pilots vigorously expressed concerns about an apparent rise in the number of PEs (“Physiological Episodes”) experienced in that fleet of aircraft.

It turns out that the Navy has experienced a “sharp increase in hypoxia-like physiological events in the last decade” in its entire combat fleet of airplanes and T-45s, according to an article in Aviation Week’s Aerospace Weekly online. These episodes appear to take two forms: those mimicking true hypoxia – “oxygen starvation” or decreased oxygen in the blood – euphoria and loss of judgement, headache, nausea, light-headedness or dizziness, paresthesias (pins-and-needles sensations), breathlessness; and decompression sickness – from exposure to low barometric pressure, in aviation typically the result of loss of cabin pressure, which results in formation of tiny nitrogen bubbles in tissues (think what happens when you pop the top off a bottle of ginger ale) – joint pain in shoulders, elbows, knees or ankles (“the bends”), headache, confusion, dizziness, nausea, breathlessness and chest pain, paresthesias. Except for the bends, the similarity of symptoms of hypoxia and decompression sickness is striking.

Most recent data for the Navy’s operational aircraft fleet report 101.42 PEs per 100,000 flight hours in F/A-18AD aircraft, 66.52 (down from 90.83 a year earlier) in EA-19Gs and 30.37 in F/A-18EF series planes. Pilots of F-35s (from all services), the new “joint fighter”, have reported 27 PEs since the aircraft began entering service in 2011. I couldn’t find statistics for the T-45s. Since 2010, four aviator deaths have been attributed to PEs.

The Navy (and the Air Force and NASA, and Boeing, manufacturer of the F/A-18 series) are vigorously studying the matter using what’s described in a 5 April Navy release as an “unconstrained resources” approach. Investigators include aviation medicine types, flight physiologists and, of course, a plethora of engineers. It appears that, in the F/A-18 series, 75% of PEs are decompression sickness episodes are caused by the aircrafts’ Environmental Control System (“ECS”) while the remainder are hypoxia episodes attributed to the On Board Oxygen Generating System (“OBOGS”).

Finding the causes and solving the problems have not been easy, in part because of the very complicated interrelatedness of onboard aircraft systems – themselves being intricate – and the human pilot’s physiology and psychology. Slowly, however, the problems are being teased out, and they are multiple and often interrelated.

Regarding the ECS, NASA researchers think the problem – especially in the older F/A-18 AB series aircraft – is due to the fact that the system is designed to “service” the avionics, radars and other electronic systems preferentially. The amount of electronics has increased markedly over the years while key components including even the ductwork and the software haven’t really changed since the 1980s. Because the ECS is programmed to feed the electronics first, the OBOGS gets fed with air last. And that is the system that generates oxygen for the pilots.

NAVAIR is installing modifications to the equipment and software with urgency as causes are identified. So far, this has produced the previously noted short drop in PEs in the especially electronics-heavy EA-18G series. Results in the others remains to be seen. Says RADM Sarah Joyner, who heads up the PE team, “PEs are not going to go away, but we are going to try to do our best to mitigate them and make them milder in nature as best we can”.

This doesn’t sound very optimistic. Stand by for future reports.

Hat tip to my high school classmate Price Bingham, Lt Col, USAF, Retired, who has been keeping abreast of news on OBOGS and EXS.

Articles I consulted, thanks to Lt Col Bingham, include:

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  • bahms  On 17 Feb 2018 at 12:51


    Very interesting. In a way it’s surprising that there are not more PEs considering the speed and maneuverability of the aircraft.



    • thomaslsnyder  On 17 Feb 2018 at 13:32

      Gordon, considering the effort and cost, you can understand why some are arguing for the demise of human combat pilots altogether. Some naval aviators say they see no future in their trade.

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