US safety officials are investigating a Southwest Airlines pilot who was recorded making an expletive-filled rant about northern California liberals as his plane readied for take-off in San Jose, California.

A spokesman for the Federal Aviation Administration said on Friday the agency also reported the incident to the airline.

Southwest said it was handling the matter internally.

The incident, first reported by SFGate.com, happened earlier this month at Norman Y Mineta San Jose International Airport and was captured by the Live ATC website.

“F— this place, Goddamn liberal f—-,” the unidentified pilot says on the recording. “F—— weirdos, probably driving around in f—— Hyundais … go slow as f—.”

The pilot also worked in an admiring reference to “coal rolling”, which is modifying a truck to spew out more exhaust. A few seconds later, he can be heard telling air traffic controllers, “Southwest 531 is ready to go.”

It was not clear what, if anything, set off the pilot.

“FAA regulations prohibit airline pilots from talking about subjects that are unrelated to safely conducting their flight while taxiing and while flying below 10,000 feet altitude,” FAA spokesman Ian Gregor said.

Dallas-based Southwest declined to identify the pilot. Spokesman Chris Mainz said his comments “are inconsistent with professional behaviour and overall respect that we require of our employees”.

“We are fully addressing the situation internally”, he said, and the airline did not publicly discuss employee matters.