An Air Force T-6 training squadron in Texas became the latest to pause operations after multiple pilots reported physiological episodes with the trainer aircraft’s OBOGS system, the Air Force confirmed Friday.

The 559th Flying Training Squadron at Joint Base San Antonio-Randolph paused flying operations Monday after pilots reported issues with the On-Board Oxygen Generating System. It resumed flights Tuesday, Air Education Training Command said in a statement late Friday. The pause was first reported by the San Antonio Express-News.

“A commander-directed Safety Investigation Board is ongoing with testing physiology and dynamic OBOGS components in [Air Force] labs, in which we are working with experts, resources and focused energy from both inside and outside the Air Force to look at the causes of the physiologic events,” the command said in a statement.

The Randolph pause follows a larger stand-down in February of the T-6 Texan II aircraft. The 19th Air Force grounded its T-6 fleet Feb. 1 after 13 pilots at three bases experienced “unexplained physiological events” during the last week of January.

Oxygen issues have plagued trainer and fighter aircraft across the military. Loss of proper oxygen levels, pressure or mixture have led to disorientation, consciousness and deaths for pilots that depend upon the systems to breathe as they maneuver at high altitudes.

Navy, Marine Corps and Air Force officials have previously said the rise in physiological events, or PEs, may in part be due to greater awareness by crews of potential symptoms, which has led to an uptick in reported events. Both the Navy and Air Force to date have been unsuccessful in finding a root cause behind the sharp increase in oxygen system failures.

Tara Copp is a Pentagon correspondent for the Associated Press. She was previously Pentagon bureau chief for Sightline Media Group.

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