Vance Air Force Base in Oklahoma has grounded its T-6A Texan II trainer planes while it investigates a series of recent physiological events, the Air Force said last week.

Flights of the turboprops were suspended Nov. 15 after instructor and student pilots reported four physiological incidents since Nov. 1, the Air Force said. While the Air Force did not disclose details about the incidents, it indicated that in each case, the aircraft’s backup oxygen system worked properly, allowing the plane to land safely.

The Air Force has more than 440 T-6As at several bases. Raytheon [RTN] Aircraft, now Textron [TXT] Aviation, finished building those planes in 2010.

Several other types of military aircraft, including the Navy’s T-45C trainer jets, have experienced a surge in physiological episodes in recent years, triggering alarm on Capitol Hill. The Senate version of the fiscal year 2018 defense appropriations bill, which lawmakers unveiled last week, would add more than $129 million to the Trump administration’s budget request to try to fix the problem.

In a report explaining the bill, the Senate Appropriations Committee wrote that it “is extremely concerned about the numerous cases of physiological episodes among military pilots of T-45, F/A-18, F-22 and F-35 aircraft. In some instances, these episodes have grounded entire fleets of aircraft, adding to an already strained pilot training pipeline.”

The FY 2018 defense authorization conference report, which Congress passed and sent to the president earlier this month, would authorize the defense secretary to hold a prize competition to speed up the search for the causes of, and solutions to, such episodes.