With flight reductions beginning Friday at 40 major U.S. airports, Transportation Secretary Sean Duffy said safety is his top priority — but he declined to share information about the data that led to the decision to cut capacity.

Airlines were ordered to reduce flights as air traffic control staffing grew increasingly strained during the government shutdown, leading to intermittent ground stops.

"We had seen the rolling staffing triggers that come from air traffic controllers — one day it was L.A., another day it was in New York, Minneapolis, Atlanta — and so as we were looking at data, we were seeing increased numbers in some of the categories we don't like," Duffy told "CBS Mornings" on Friday.

"As that data came in we decided to make the decisions we thought were appropriate," Duffy said.

He would not give specifics about what the data showed, but said it will be provided to airlines and Congress at a future time. 

Later in the day, speaking at Reagan National Airport in Washington, Duffy indicated that some of those concerns included "more breaches" of the minimum distance kept between planes in the air and an increase in tarmac incursions.

Airlines are expected to cut at least 4% of Friday's flights and ramp up to 10% by the end of next week if the shutdown continues. The cuts will impact everything from commercial travel to cargo and private jets — even space launches.

"My job is to be preemptive"

As the shutdown drags into its second month, Duffy said some air traffic controllers have been working 10-hour days, six days per week.

"That pressure on the controllers were giving us numbers that were not troubling, but were concerning. So my job is to be preemptive," he said.

The air traffic controllers' union said the nation was already thousands of controllers short of what's needed before the shutdown. As employees continue to go without pay, there's been an increase in sick calls.

Rep. Rick Larsen of Washington, the top Democrat on the House Committee on Transportation and Infrastructure, has called for more transparency as flights are slashed.

"If people want to question us, I would throw it back at them: open up the government," Duffy said. "You know, we have to take unprecedented action because we're in an unprecedented situation with the shutdown." 

He said he worked with the FAA administrator and safety team on the plans.

"I have done all I can to minimize disruption in the airspace. I'm trying to get people where they want to go and get them there safely," he said.

In its flight reduction order, the FAA said it would impose a $75,000 fine for every flight a carrier operates above government limits.