ENID, Okla. — Flights involving the main training aircraft at an Oklahoma air base have been grounded for the second time in three months as military officials investigate several incidents involving a lack of oxygen onboard.

The “operational pause” of all the T-6 Texan aircrafts at the Vance Air Force Base began Thursday.

Maj. Gen. Patrick Doherty, the 19th Air Force commander, said the base is working fast to make changes that will minimize the chances of risk for “the aircrew while we tackle this issue head-on.”

The T-6 was also grounded in November after four instructor pilots and one student pilot reported “physiological incidents” due to incidents, according to a release from the Air Force Base.

“Physiological incidents” refer to problems with hypoxia, a condition where the body does not get enough oxygen.

The recent pause comes after the U.S. Air Force announced an investigation last month into problems with oxygen that have resulted in the grounding of some training aircrafts, including A-10 Thunderbolts, F-35A Lightnings and T-6A, over the last several months.

“As part of the integrated effort to address physiological events, the Air Force is providing more resources to understand (unexplained physiological events), standardize response actions to such events and assess options for more robust aircrew training to recognize and respond to these events,” said Brig. Gen. Bobbi Jo Doorenbos, who was picked to lead the investigative team. “Our ultimate goal is to prevent (unexplained physiological events).”

The T-6 is a single-engine, two-seat turboprop aircraft launched in 2000.