The airline on Sunday declined to identify the pilot.
As per The Washington Post, the pilot, Saturday afternoon, boarded a flight at Austin-Bergstrom International Airport a bit late - wearing a ball cap and casual clothes.
Straight away, she appeared to get into an argument with crew members at the front of the plane.
From his seat near the back, passenger Chris Moore figured at first that she was just another San Francisco-bound flier.
"I thought she was a first-class passenger, complaining," he told The Washington Post. "Then she grabs the mic".
Passenger Randy Reiss tweeted, she "asked if we were ok with her flying like that? Then said `sorry I`m going through divorce`," CBS News reported
She then used the plane`s overhead public address system to rant about President Trump and former presidential hopeful Hillary Clinton - calling them both "a-holes" and then ranting about her divorce which led to passengers tweeting to United to get a new pilot.
About 20 passengers said they did not feel safe with her in the cockpit and got off the plane.
United Airlines spokesman Charlie Hobart said he did not know why the pilot was allowed on the plane in plain clothes. He said the company will discuss the incident with the pilot.
The flight from Austin, Texas, to San Francisco, then took off with a new pilot about 90 minutes late, passengers said.