The 19th Air Force has halted all T-6 Texan II operations after a “cluster” of unexplained physiological events — a term used to describe conditions such as hypoxia — at three bases over the last week.

In a release Wednesday evening, Air Education and Training Command said the problems happened at Columbus Air Force Base in Mississippi, Vance Air Force Base in Oklahoma, and Sheppard Air Force Base in Texas.

Maj. Gen. Patrick Doherty, commander of the 19th Air Force, ordered the “operational pause,” which will begin Thursday.

The release did not specify how many events have taken place, or what problems pilots experienced. But in addition to hypoxia, the Air Force has also used the term “unexplained physiological event,” or UPE, to describe conditions such as hypocapnia, hypercapnia, or disorientation.

Such events can be dangerous, especially for pilots. Hypoxia, for example, happens when there is too little oxygen in the body, and can put someone at risk of becoming confused, faint, or even losing consciousness.

T-6 photos from Laughlin Air Force Base, Texas, in 2004. (Master Sgt. Jeffrey Allen/Air Force)
T-6 photos from Laughlin Air Force Base, Texas, in 2004. (Master Sgt. Jeffrey Allen/Air Force)

Doherty ordered the grounding to give the Air Force a chance to study the root causes of these problems, listen to the T-6 aircrew, educate them about the problems, come up with solutions, and put them into place.

“The safety of our instructors and student pilots is paramount and has been our priority and focus,” Doherty said. “We’re acting swiftly, making temporary, but necessary, changes to everyone’s training, general awareness, checklist procedures, and possibly modify aircrew flying equipment to mitigate risk to the aircrew while we tackle this issue head-on to safeguard everyone flying T-6s.”

Earlier on Wednesday, AETC confirmed that the 19th had suspended all solo flights in T-6s due to concerns about hypoxia and other related problems. Doherty also authorized all T-6 pilots to fly with their masks partially unlatched to allow them to breathe cockpit air.

Vance temporarily suspended its T-6 flights last November after a series of hypoxia events, but was unable to find the cause of the problem and resumed flights in December.

The Air Force has also created a team, headed by Brig. Gen. Bobbi Jo Doorenbos, to look into hypoxia and other related problems.