Police said the man walked into the aircraft around 4:45 a.m. as a crew was cleaning it. When the crew saw him, they held him down until officers arrived to make an arrest.
The suspect allegedly used a pipe to open the bottom of a perimeter fence and squeezed underneath it to trespass onto the airfield.
The man, who police described as homeless, was arrested for trespassing and was taken in for a psychiatric evaluation.
American Airlines said in a statement the plane has since been re-inspected by security teams.
Airport officials say operations were not impacted by the incident.
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