The pilot of Flight 1980 from San Francisco International Airport reported the sighting to air traffic controllers at Lindbergh Field after the aircraft landed safely.
"Okay over to spot 1 and contact ramp. And we hit a drone probably at around 3,000 feet. About uh.. Yeah around on our base leg between KEEDG and SAIEE," the pilot said. "Red drone heading west."
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The FAA said the sighting occurred at 4,000 feet, which is significantly higher than the legal maximum altitude for drones.
United Airlines said a postflight inspection found no damage to the Boeing 737.
According to the FAA, air traffic control alerted other pilots in the area but did not receive any additional dronesighting reports.