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.

Pilots flying into Denver International Airport on Monday couldn’t communicate with air traffic controllers for about six minutes after multiple radio transmitters failed, according to reporting

Sen. Ted Cruz asked whether a hotline between military and civilian air traffic controllers in Washington, D.C., that hasn’t worked for more than three years may have contributed to another near miss shortly after the U.