AP) — Three of the five close-calls involving aircraft reported in the past 16 months at San Francisco International Airport were caused by pilots, federal authorities said Wednesday.

The Federal Aviation Administration found that in three instances planes lined up to use the wrong runway and taxiways due to pilot error.

Another plane was mistakenly cleared to land on the wrong runway by a tower controller, the FAA said.

In the most recent incident on Jan. 9, an Aeromexico passenger jet was ordered to abort a landing as it descended toward a runway occupied by another commercial jet.

The FAA determined the mistake was pilot error and said Aeromexico provided more training to its pilots for flying into San Francisco and distributed a safety alert to its pilots about the incident.

On Oct. 22, 2017, an Air Canada flight was cleared to land but a tower controller then instructed the crew multiple times to circle because he was not certain that a preceding arrival would be clear of the runway.

The Air Canada crew did not acknowledge the controller’s instructions. The FAA’s investigation found the crew inadvertently switched from the San Francisco tower frequency to a ground frequency after receiving landing clearance.

The FAA determined that on Feb. 15, 2017, a traffic controller mistakenly cleared a Compass passenger jet lo land on a runway where a Virgin America plane was waiting for take-off.

Two months earlier, on Dec. 14, 2016, a SkyWest pilot was given taxiing instructions and correctly read them back but then turned onto the wrong taxiway. The aircraft stopped 65 feet from the runway edge, where a United Airlines Boeing 737 was taking off.

The airport, which many pilots say is notoriously difficult for landings, has been under more scrutiny since July 8, when an Air Canada jet was descending toward a taxiway holding four other loaded planes rather than the assigned runway and narrowly avoided disaster.

The investigation led by the National Transportation Safety Board is ongoing.

Since then, the airport has reinforced its ground radar system and shut down a confusing taxiway, the FAA said.

The report findings were first reported by the Mercury News in San Jose.