Two weeks after hypoxia scares led the 71st Flying Training Wing at Vance Air Force Base in Oklahoma to ground its T-6A Texan II aircraft, it remains unknown when flying operations will resume.

Vance stopped flying its T-6As on Nov. 15 after “physiological incidents” happened during four flights since Nov. 1. The incidents affected four instructor pilots and one student pilot there.

Flight operations for the T-1A Jayhawk and T-38 Talon at Vance are still ongoing.

Vance public affairs chief Terri Schaefer said Wednesday that the pilots “experienced mental disorientations in conjunction with classic hypoxia systems.”

Hypoxia is too little oxygen in the body.

Schaefer said the cause of the hypoxia has not yet been found. Aviators, medical personnel and industry experts are still reviewing the incidents and looking for a solution, she said, but Vance is not sure when the pause in T-6A flight operations will end.

In a Nov. 20 release announcing the groundings, Vance said that “in each case, the aircraft’s backup oxygen system operated as designed, and the pilot followed the correct procedures, landing the aircraft safely,”

In all, over the last year there have been eight such physiological events involving T-6A sorties at two bases, Vance and Naval Air Station Pensacola in Florida.

“One event is too many,” Schaefer said in an email. “The Air Force takes this issue seriously and will continue to investigate these events with all major stakeholders in the T-6 enterprise to provide Vance AFB and all other T-6 bases solutions, including changes to checklist procedures, that protect our nation’s finest and ensure continued, safe and effective flying training.”

Vance has also briefed its instructor pilots and students on the hypoxia situation, and how the pilots in the incidents successfully landed their planes afterwards.