The 19th Air Force, under Air Education and Training Command, issued a guidance Wednesday to stand down operations after a cluster of unexplained physiological events occurred at Columbus Air Force Base, Mississippi; Vance Air Force Base, Oklahoma; and Sheppard Air Force Base, Texas, within the last week, officials said in a release.
The pause began Feb. 1 to enable the service to examine "the root causes of the incidents, educate and listen to aircrew, develop and deliver mitigation solutions," the release said.
"The safety of our instructors and student pilots is paramount and has been our priority and focus," said Maj. Gen. Patrick Doherty, 19th Air Force commander.
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"We're acting swiftly, making temporary, but necessary, changes to everyone's training, general awareness, checklist procedures, and [may] possibly modify aircrew flying equipment to mitigate risk to the aircrew while we tackle this issue head-on to safeguard everyone flying T-6s," Doherty said in the release.
The news broke after the Facebook group, Air Force Amn/nco/snco -- which is popular within the Air Force but isn't officially run by the service -- obtained an email from Doherty, at first stating that solo flights for instructor and student pilots had been suspended "until further notice" over the growing concerns of unexplained physiological events.
The 19th Air Force is responsible for training more than 30,000 U.S. and allied students annually in various specialties, including pilot training.
According to the Air Force, instructor pilot training in the T-6A began at Randolph Air Force Base, Texas, in 2000, while Joint Primary Pilot Training, or JPPT, began in October 2001 at Moody Air Force Base, Georgia.
JPPT is currently held at Columbus AFB; Vance AFB; and Laughlin and Sheppard Air Force Bases in Texas, according to the service's T-6 factsheet.
This is just the latest in a series of hypoxia-related incidents for the Air Force.
The T-6A fleet at Vance's 71st Flying Training Wing was grounded in November after a handful of pilots observed hypoxia-like symptoms -- shortness of breath, confusion, wheezing -- in flight.
More than 100 of the trainers were put on operational pause between Nov. 15 and Dec. 5. However, officials could not pinpoint the origin of the incidents.
"No specific root cause for the physiological events was identified during two weeks of investigation by aviation, medical, functional and industry experts," according to an Air Force release at the time. "However, specific concerns were eliminated as possible causes, including maintenance and aircrew flight equipment procedures."
In January, the service announced it had created a new investigative team to research and record ongoing hypoxia episodes in hopes of minimizing future incidents.
Brig. Gen. Bobbi Jo Doorenbos, currently special assistant to the director of the Air National Guard, was named to head the "Unexplained Physiologic Events Integration Team."
The AETC on Wednesday said officials are relying on the team to pinpoint the problem.
"... Doorenbos is leading the team and will work closely with 19th Air Force, AETC, and other [major commands] to examine the causes of these incidents and ensure industry and enterprise-wide solutions are given high priority to find root causes and deliver solutions across all weapon systems," it said in a statement.
Aside from the T-6, more than two dozen A-10 Thunderbolt II attack aircraft at Davis-Monthan Air Force Base, Arizona, didn't fly in late November after two pilots reported they experienced hypoxia symptoms.
In June, the 56th Fighter Wing at Luke Air Force Base, Arizona, halted operations for all F-35As there after pilots complained of hypoxia-related issues. In succeeding days, the Air Force established initiatives to keep pilots safe and to avoid experiencing symptoms in flight, and the F-35s were back in the air that same month.
A root cause for all these incidents has not been determined.
While physiological events aren't common, the service has said it has seen an increase in pilots reporting the hazardous events. "The probability that a pilot will experience a physiological event is less than 1 percent per year," Doorenbos said last month.
"Still, we are aggressively addressing these events and communicating with aircrew so they remain confident in their aircraft and weapon systems," she said.