The FAA said it is investigating after a Delta Air Lines flight and United Airlines flight experienced 'a loss of required separation' while heading inbound to Phoenix Sky Harbor International Airport.

The planes, carrying more than 350 people, experienced "a loss of required separation" while preparing to land at the same Phoenix airport, the FAA said.

A United flight from San Francisco and a Delta flight from Detroit came within 425 feet of each other in the sky, according to flight radar data. NTSB guidelines say planes should always stay at least five miles apart.

The two planes experienced ‘a loss of required separation’ when they neared Phoenix Sky Harbor International Airport, the FAA said