Jonathan Stewart was badly shaken by a recent close call in airspace around Newark, N.J. “I don’t want to be responsible for killing 400 people,” he said in an interview.

Air traffic controllers in Denver had to scramble to use backup frequencies after their main communications lines went down for 90 seconds earlier this week in the latest Federal Aviation Administration equipment failure.

US aviation safety regulators are investigating a communications outage affecting flights approaching Denver International Airport earlier this week, the latest in a series of glitches that have put a spotlight on the nation’s aging air traffic control system.