The rash of hypoxia-like problems in the Air Force’s fleet of T-6 Texan II trainers was primarily caused by fluctuating concentrations of oxygen in the cockpit, the service said Thursday.

Air Education and Training Command said in a release that the Air Force will start putting a series of fixes in place to correct the problems. These fixes will include redesigning the oxygen system in the T-6, adjusting oxygen levels in flight, and increasing maintenance on the On-Board Oxygen Generating System, or OBOGS.

A six-month study conducted by AETC and Air Force Materiel Command, uncovered the problem with varying oxygen concentration levels, which AETC described as the “major factor in unexplained physiological events” affecting T-6 pilots, AETC said. Experts from the Navy and NASA also assisted with the study.

“So far, technical efforts to date and analysis of data collected have determined that pilots have been exposed to significantly changing levels of oxygen concentration,” AETC Commander Lt. Gen. Steve Kwast said in the release. “The varying levels of oxygen concentration, even though in excess of what the body typically needs, has caused physiological stress that most pilots, on most days, actually adapt to without noticing.”

While most pilots may adapt, some develop symptoms similar to hypoxia, or lack of oxygen, or hypocapnia, a lack of carbon dioxide, as a result, AETC said. This can cause problems such as shortness of breath, dizziness or disorientation in pilots, and even cause them to lose consciousness.

In a July 23 interview with Air Force Times, Kwast said that fluctuating oxygen proportions in the air was one issue the board had discovered, though he stressed at the time it was too soon to say whether it was causing or contributing to the problem.

“So the question is, what does that do to the human body, when you have a fluctuation of oxygen?” Kwast said in July. “That’s the kind of work they’re doing as they discover something that is a little ... different than we thought.”

The T-6 fleet has been plagued by a series of hypoxia- or hypoxia-like events that resulted in several groundings and precautionary measures over the past year. Last November, the 71st Flying Training Wing at Vance Air Force Base in Oklahoma grounded its T-6s for nearly three weeks after five pilots reported hypoxia-like symptoms during four flights.

In January, the 19th Air Force suspended all solo flights in T-6s, and authorized pilots to fly with their masks down to breath cockpit air in an attempt to lessen the risks of hypoxia-related problems. The following month, Maj. Gen. Patrick Doherty, the 19th Air Force commander, grounded the entire T-6 fleet for about a month after a cluster of unexplained physiological events at three bases in the last week of January.

During that so-called “operational pause,” Materiel Command formed an independent team to work with AETC and look into the potential causes of the problem, including multiple inspections of the OBOGS system.

The inspectors found that the OBOGS filter and drain valves were failing at a much higher rate than anticipated, AETC said, and those parts were replaced when necessary.

But the fact that oxygen system components were failing more frequently than expected worried officials. At that point, the T-6 Program Office at Tinker Air Force Base in Oklahoma ordered the systems be inspected “on a more aggressive timeline.”

The Air Force will also put new maintenance procedures in place that were learned from several Air Force and Navy bases that fly T-6s, such as purging excess moisture from the OBOGs system. This appears to keep the OBOGS operating more efficiently over time, Doherty said in the release.

In the July interview, Kwast said that moisture in the system also might be contributing to hypoxia-like problems.

AETC and AFMC’s redesign of the OBOGS system, intended to stabilize the proportion of oxygen in the air pilots breathe, will take between two and four years, the release said. In the meantime, AETC and AFMC are also working with T-6 manufacturer Beechcraft to adjust the OBOGS software algorithm to stabilize the plane’s oxygen concentrations and reduce physiological events.

Brig. Gen. Edward Vaughan, head of the newly renamed Air Force Physiological Episodes Action Team, plans to keep working with officials from the Air Force and other services to find out if the T-6 OBOGS fixes will also help with other aircraft using OBOGS, the release said.

And AETC is adding study materials on unexplained physiological events for T-6 pilots to help them learn how to identify symptoms of all such events, responses and corrective procedures.

“Since our T-6 operational pause, we have made every effort to communicate with every instructor and every student exactly what we’ve found,” Doherty said. “Transparency remains of utmost importance to use as we all work together to ensure that our pilots are safe and know the way ahead.”

In the July interview, Kwast told Air Force Times that the unexplained physiological events were continuing to happen episodically, and that investigators were looking for patterns. The 559th Flying Training Squadron at Joint Base San Antonio-Randolph also paused flying operations for one day in June after pilots reported issues with the OBOGS.

But the groundings had an immediate and noticeable effect on the Air Force’s pilot production. In March, AETC said the service would likely graduate 10 percent fewer new pilots than originally anticipated in 2018 due to the groundings.