More than two dozen A-10 Thunderbolt II attack aircraft at Davis-Monthan Air Force Base, Arizona, didn't fly in late November after two pilots reported they experienced hypoxia-like symptoms while flying.
In both incidents, which occurred the week of Nov. 27, "the aircraft's backup oxygen supply system operated as designed and the pilots followed the correct procedures to safely land the aircraft," said Capt. Joshua Benedetti, spokesman for the 355th Fighter Wing.
Benedetti said one of the aircraft was equipped with the Onboard Oxygen Generation System, commonly known as OBOGS, while the other was equipped with the Liquid Oxygen System, or LOX.
"An additional pilot reported a ground incident associated with the A-10's oxygen system that same week," he said.
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The Air Force "quickly determined the issue with the LOX-equipped aircraft was related to a malfunction with the cabin pressure and oxygen regulator. Those issues were fixed immediately," Benedetti said.
To properly inspect the OBOGS-equipped aircraft -- 28 of the total 85 A-10s on base -- the service grounded those jets for about a week, he said.
Despite those efforts, the service is still seeking answers to figure out how the incidents occurred.
"At this point, we have not determined a root cause," Benedetti said.
"During the course of the investigation, we have identified how we could better maintain the system by cleaning the water separator drain and associated piping with pressurized air, which may help prevent corrosion found in some of the piping. Additionally, we made the pilot preflight OBOGS procedure more prescriptive," he said.
While the 28 aircraft stood down operations, missions on base continued with the remaining 57 LOX-equipped aircraft.
"We resumed flying with all LOX and OBOGS aircraft less than a week later, and there have been no incidents since," Benedetti said.
He added, "The Air Force takes these physiological incidents seriously, and our focus is on the safety and well-being of our pilots."
Officials at Davis-Monthan said they will continue to share information on the OBOGS aircraft with other A-10 units so proper precautions may be taken.
In June, the 56th Fighter Wing at Luke Air Force Base, Arizona, halted operations for all F-35As there after pilots complained of hypoxia-related issues.
In succeeding days, the Air Force established initiatives to keep pilots safe and to avoid experiencing symptoms -- shortness of breath, confusion, wheezing -- in flight.
Those initiatives include a backup oxygen system, wearable technology to monitor pilots' oxygen levels, and a restriction on how high pilots could take the craft.
The F-35s at Luke resumed flight that same month. Currently a total of 61 F-35As are assigned to Luke.
A root cause for those incidents has not been found.