Utilizing a rigorous Root Cause Corrective Action (RCCA) analysis process to eliminate contaminated breathing gas as a cause of the physiological episodes being experienced by F/A-18 and T-45 pilots, the two teams tasked with investigating the issue continue to narrow down the list of possible factors.

Each RCCA Core team—one for F/A-18 Hornet and Super Hornet and EA-18G Growler jets, another for the T-45 Goshawk training jet—determined last fall that the quality of pilots’ onboard oxygen was unaffected by asphyxiates, carbon monoxide and external or internal contaminants, such as fuel vapor or pyrolysis byproducts, respectively.

"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 (PEAT). "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."

The T-45 team reached its conclusion in September, with the F/A-18 team following in October, after a joint 16-month effort that saw 21,000 samples taken across 11 sites from pilots’ breathing gas, ground sampling and blood analysis. In total, roughly 1,800 compounds were evaluated by an independent panel of toxicologists and multi-disciplinary panel of aeromedical professionals, who determined that none of the compounds played a role in physiological episodes, or PEs.

“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.

The RCCA teams include Naval Air Systems Command (NAVAIR) engineers along with instructor pilots, independent doctors and scientists, along with support from dozens of other subject matter experts.

The F/A-18 team is now focused on two potential factors, one being the maintaining of cabin stability by preventing unexpected pressure fluctuations that have been correlated with PE events but not yet shown to be a causal factor, said Don Salamon, deputy assistant program manager for system engineering for the F/A-18 and EA-18G Program Office.

The second focus is on breathing dynamics and factors that can impact gas exchange during respiration, such as hyper/hypocapnia, hypoxic hypoxia, work of breathing, and adsorption/acceleration atelectasis.

“There is likely no single ‘smoking gun’ that will be found as a result of the investigation,” Salamon said. “However, we have identified multiple contributors that are being aggressively worked through the [F/A-18 program] with near-term corrective actions.”

The T-45 team has closed more than 90 percent of the nearly 350 branches on its RCCA “fault tree,” 50 of which were related to contamination, team lead Ann Dickens said. The team is now focusing on optimal breathing pressure and oxygen concentration as potential factors.

The notion that PEs could be caused by contaminants infiltrating the aircraft’s Onboard Oxygen Generation System (OBOGS) was an early assumption made in the absence of alternative explanations.

“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.”

But following seven years of data collection where compounds other than oxygen in OBOGS-generated breathing air were consistently measured in the parts per billion—levels so low as to be functionally nonexistent—the RCCA teams determined contamination could safely be ruled out as a root cause of PEs.

“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.”

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

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