Davis-Monthan Air Force Base grounded 28 of its A-10 aircraft after two pilots reported physiological incidents while flying.

During the week of Nov. 27, two A-10 Thunderbolt II pilots at the Arizona base said they experienced hypoxia-like symptoms, where an oxygen deficiency can cause impairment and loss of consciousness.

In both cases, the A-10’s backup oxygen supply system worked, and the pilots followed the correct procedures to safely land, Davis-Monthan spokesman Capt. Josh Benedetti told Air Force Times. The incidents were first reported by Aviation Week.

“The Air Force takes these physiological incidents seriously, and our focus is on the safety and well-being of our pilots,” Benedetti said via email Tuesday.

This was at least the third case of Air Force aircraft being grounded last year because of reported physiological incidents.

In June, the 56th Fighter Wing at Luke Air Force Base, Arizona, grounded its F-35A Lightning II aircraft for 11 days after five incidents. And in December, the 71st Flying Training Wing at Vance Air Force Base, Oklahoma, grounded its T-6A Texas training flights. In that instance, four instructor pilots and one student pilot in four different flights experienced hypoxia-like symptoms.

At Davis-Monthan, one of the A-10s was using an Onboard Oxygen Generation System, or OBOGS, and the other A-10 was equipped with a Liquid Oxygen System, or LOX.

“We quickly determined the issue with the LOX-equipped aircraft was related to a malfunction with the cabin pressure and oxygen regulator,” Benedetti said. “Those issues were fixed immediately.”

During the same week that pilots experienced issues while flying, a third pilot reported an issue with his A-10 while it was still on the ground, Benedetti said.

The pilot ran his pre-flight checklist and saw the OBOGS system had an error, so he didn’t take off.

The base, near Tucson, decided to ground the 28 A-10s equipped with OBOGS for about a week so experts could thoroughly inspect the system, he said.

“At this point, we have not determined a root cause,” Benedetti said Tuesday.

However, he said they have identified how to better maintain the system by cleaning the water separator drain and associated piping with pressurized air, which might help prevent corrosion that was found in some of the piping.

“This information has also been shared, and will continue to be shared, with other A-10 units and units equipped with OBOGS aircraft so that they can keep pilots and maintainers informed and take appropriate precautions,” he said.

There are 85 A-10s assigned to Davis-Monthan, so 57 of the aircraft were still able to fly while the other 28 were grounded, Benedetti said.

“We were able to execute our entire mission with our LOX-equipped aircraft while we inspected the OBOGS aircraft,” he said. “We resumed flying with all LOX and OBOGS aircraft less than a week later, and there have been no incidents since.”

The 355th Fighter Wing, based out of Davis-Monthan, trains and deploys A-10 pilots.

Charlsy is a Reporter and Engagement Manager for Military Times. Email her at cpanzino@militarytimes.com.

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