When the agency approved moving Newark air traffic controllers to Philadelphia, it said the odds of a dangerous communications breakdown was about 1 in 11 million.

Prompted by a radar failure a week ago at Newark Liberty Airport, and a midair collision that killed 67 people in Washington D.C., President Donald Trump‘s administration announced an unprecedented program to rebuild the nation’s air traffic control system and recruit more air traffic controllers.

A deadly midair collision in January between a military helicopter and a commercial airliner, and several crashes since, have prompted officials to focus on fixing the nation’s outdated air traffic control system and hiring more controllers.