The Air Force is looking into recent accidents to determine if there might be a larger trend looming, Military.com reported Monday.

“What’s been trending up and was reported was our ‘Class C’ mishaps, or our lower category mishaps. Lower dollar figure, but still a concern to all of us,” Air Force Vice Chief of Staff Gen. Stephen Wilson said at a Future of War conference hosted by New America and Arizona State University, the military news outlet reported.

“We’ve got our safety professionals digging into it, and seeing if there is a noticeable trend that we have in our Class C mishaps.”

The news outlet reported its own accounting shows 133 service members have been killed in aircraft mishaps since 2013; the incidents include fatalities, severe damage totaling millions of dollars, or a complete loss of the aircraft.

The Air Force saw an increase in Class C mishaps, Military Times has reported, with the number increasing 16 percent between 2013 and 2017.

“Any Class A accident is one too many,” Wilson said, adding, “The safest year ever was 2014, and 2017 was our second safest year, so our Class A mishaps have been trending down.”

Over the last week, the Defense Department has seen a range of aviation accidents, the most recent occurring Friday when an Army Apache helicopter from Fort Campbell, Kentucky, crashed during a night-training mission, killing two.

An Air Force Thunderbird pilot was killed Wednesday after his F-16 crashed at the Nevada Test and Training Range.

Meanwhile, the Navy temporarily grounded military flights in Djibouti and canceled a major exercise days after it began, following two aviation accidents, Military.com reported.

Separately in the United States, a helicopter crashed March 2 during a training mission in Southern California, killing all four aboard.