Two Naval Air Systems Command teams are focusing on the cause of physiological episodes experienced by F/A-18 and T-45 pilots after ruling out contamination of oxygen.

Last fall, the Root Cause Corrective Action teams found the quality of pilots' onboard oxygen was unaffected by asphyxiates, carbon monoxide and external or internal contaminants, including fuel vapor or pyrolysis byproducts.

One team is investigating problems aboard the F/A-18 Hornet and Super Hornet and EA-18G Growler jets, and another for the T-45 Goshawk training jet.

"There is likely no single 'smoking gun' that will be found as a result of the investigation," Don Salamon, deputy assistant program manager for system engineering for the F/A-18 and EA-18G Program Office said in a news release Monday. "However, we have identified multiple contributors that are being aggressively worked through the [F/A-18 program] with near-term corrective actions."

The F/A-18 team is focused on two potential causes for the events. One involves maintaining cabin stability by preventing unexpected pressure fluctuations already linked to PE events, though not yet proven to be causal.

The team is also looking at breathing dynamics and factors that may impact gas exchange during respiration, including hyper/hypocapnia, hypoxic hypoxia, work of breathing, and adsorption/acceleration atelectasis.

Last fall, the teams determined the problem wasn't caused by contamination.

"We've done challenge testing in the labs with aircraft equipment that shows it is nearly impossible to force anything other than oxygen through the OBOGS," Salamon said. "Most importantly, the symptomatology of PEs does not match exposure to any type of contaminant.

"We've gotten smarter, and now we understand there are other things that could be happening that manifest as those symptoms, but it's not exposure to contaminants," Salamon said.

The T-45 team reached its conclusion in September and the F/A-18 team in October, after a joint 16-month effort that included 21,000 samples from 11 sites of pilots' breathing gas, ground sampling and blood analysis. Roughly 1,800 compounds were evaluated by an independent panel of toxicologists and multi-disciplinary panel of aeromedical professionals. None of the compounds played a role in physiological episodes, or PEs.

"We are happy to see that contamination has been ruled out and that all Navy aircraft are delivering clean air to our aviators," said Rear Adm. Fredrick Luchtman, Navy lead for the Physiological Episodes Action Team. "We still have work to do, especially with the Hornets and Growlers -- we need to ensure oxygen is being delivered at the right concentration and pressure, and that cockpit pressure stability is continually improving. And just as important, we are working on improving the process of treating aviators who have experienced physiological events so we can make sure they are healthy and can get back in the aircraft."

Other potential factors have also been ruled out, including electromagnetic exposure, but others have been determined to play a role in F/A-18 PEs, including maintenance-related issues and atelectasis, commonly referred to as a collapsed lung.

"Contamination was an explanation for people getting sick in the aircraft when we couldn't explain it very well," Salamon said. "We had people experiencing hypoxia-like symptoms at altitudes below 10,000 feet, and it's nearly impossible for you to get hypoxic at those altitudes ... other than a condition that affects your ability to exchange gases."

The RCCA teams include Naval Air Systems Command engineers, as well as instructor pilots, independent doctors and scientists, along with support from dozens of other subject matter experts, the Navy said.

"The Naval Aviation Enterprise took this very seriously and went through a rigorous process featuring an independent review by doctors, physiologists and toxicologists that determined definitively that contamination is not the cause of PE," said Capt. Todd St. Laurent, program manager of the Naval Undergraduate Flight Training Systems Program Office.

In 2017, the Navy briefly grounded the T-45C Goshawk after incidents.

A group of more than 100 Navy pilot instructors refused to fly the Goshawk trainer aircraft due to safety concerns.

In October that year, a military training jet crashed in a remote area of Tennessee after the grounding was lifted. The T-45C that crashed had the CRU-123 oxygen monitoring system installed as part of a fix for the previous issues with the oxygen system.

Another Navy team is conducting research on detecting symptoms associated with rapid pressure fluctuations in military jets.

Naval Surface Warfare Center Panama City Division's Fluctuating Altitude Simulation Technology team recently delivered an aircraft cabin simulator system to the Navy Experimental Diving Unit for their use conducting human subject research in Panama City, Fla.

"The purpose of the FAST system is to characterize the symptoms associated with rapid pressure fluctuation, and determine what symptoms may be most closely associated with PE," Navy research psychologist Lt. Jenna Jewell said in a news release. "This information allows us to conduct future research that can be more targeted, including focusing on specific symptoms and adding in factors present in the cockpit."

Before simulated flight, participants are examined to determine inner ear function, retinal tracking, and a neurocognitive exam. Participants then enter the FAST chamber and fly one of three predetermined flight patterns.

The vital statistics are tracked constantly during the flight, and a Doppler ultrasound test is conducted at four times to determine if venous gas bubbles are present in each participant's heart. Retinal function is tested mid-flight.

After the flight, participants undergo the same pre-flight testing to determine if there are any changes in physiological or neurocognitive performance from the rapid pressure fluctuations experienced during simulated flight.

"This study is the first-of-its-kind human subject research investigating PEs plaguing naval aviators by replicating the cabin pressure fluctuations observed in the fleet," NEDU research physiologist Lt. Travis Doggett said.

"The FAST system, coupled with the manned testing, will provide Navy leadership vital information needed to help solve the Chief of Naval Operations number one aviation safety concern -- impacting the Navy's ability to operate safely in the airspace of its choosing, without physiological hindrance."

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