The 19th Air Force has indefinitely suspended all solo flights in T-6 Texan trainer aircraft ― both for students and instructor pilots ― over concerns about hypoxia and other so-called unexplained physiological events.

A screenshot of a message posted early Wednesday on the unofficial Facebook page Air Force amn/nco/snco also said that Maj. Gen. Patrick Doherty, commander of the 19th Air Force, has authorized all T-6 instructor and student pilots to fly with their masks down ― that is, with one bayonet, or connector, unlatched. This is so they can breathe cockpit air to lessen the risk against hypoxia and related problems, and will be allowed both on the ground and in flight.

“Due to the nature of T-6 Unexplained Physiological Events (UPE) symptoms and ability to recognize the situation, all solo sorties (both student and IP) are suspended until further notice,” the message said.

Air Education and Training Command spokesman Capt. Beau Downey confirmed the guidance was sent by Doherty. The 19th Air Force oversees the training of more than 32,000 U.S. and allied aircrew each year, flying 28 different aircraft.

Hypoxia is a physiological symptom caused by too little oxygen in the body.

RANDOLPH AIR FORCE BASE, Texas -- The T-6A Texan II is phasing out the aging T-37 fleet throughout Air Education and Training Command. (Air Force photo by Master Sgt. David Richards)
RANDOLPH AIR FORCE BASE, Texas -- The T-6A Texan II is phasing out the aging T-37 fleet throughout Air Education and Training Command. (Air Force photo by Master Sgt. David Richards)

The message also said that if a pilot encounters cabin pressure issues, smoke, or fumes, or if a pilot needs to eject, he should first put his mask up and latch the other bayonet before following checklist procedures.

The entire fleet of T-6A Texans at the 71st Flying Training Wing at Vance Air Force Base in Oklahoma was grounded last November after five pilots in four different flights experienced hypoxia-like symptoms.

But after two weeks of trying to find the cause of the hypoxia problems, investigators were stumped. Experts found no specific cause for them, though they were able to rule out maintenance and aircrew flight equipment procedure problems. The 71st resumed T-6A flights on Dec. 5.

The Air Force on Jan. 22 announced it had created an Unexplained Physiologic Events Integration Team, headed by Brig. Gen. Bobbi Jo Doorenbos, to try to find ways to eliminate events such as hypoxia, or minimize their impact.