The U.S. Federal Aviation Administration is investigating how an Air Canada pilot lined up to land last week on a taxiway at San Francisco International Airport where four planes were waiting to depart, the agency said on Tuesday.

The Air Canada A320 from Toronto was cleared to land on a runway at the airport just before midnight on Friday, the FAA said in a statement.

But instead, the pilot of flight AC759 inadvertently lined up to land on a parallel taxiway where four planes were waiting to depart.

The Air Canada plane was estimated to have flown over the planes on the taxiway by as little as 100 feet (30 meters), according to a preliminary summary published on Tuesday by the Transportation Safety Board of Canada.

Along with the FAA, the U.S. National Transportation Safety Board is investigating the incident.

According to an audio recording of the incident, a United Airlines pilot is heard saying that the Air Canada plane "flew directly over us," alerting the air traffic controller that the plane was headed toward the taxiway.

The controller took charge, averting what could have been the worst aviation disaster since the collision of two B747 planes in 1977 in Tenerife in the Canary Islands, U.S. pilot and Aero Consulting Experts Chief Executive Ross Aimer said by phone.

"An air traffic controller sent the Air Canada jet around," the FAA said. "The plane made another approach and landed without incident."

The FAA is now investigating the distance between the Air Canada aircraft and the jets that were lined up on the taxiway.

In separate statements, both Air Canada and its pilots' union said they could not comment further since the incident is under investigation.