"Belligerent" United passenger faces hefty fine. Here's how much.

Unruly behavior on airplanes can lead to hefty fines for passengers.

Just ask Alexander Michael Dominic MacDonald, from Chelmsford, England, who this week was ordered to pay $20,638 to United Airlines for his outbursts on a flight from London to Newark, New Jersey, in March.

The incident kicked off when MacDonald, 30, was having a loud argument with his girlfriend, according to an affidavit. The situation soon escalated and he started yelling at a flight attendant. He was both verbally and physically aggressive, according to court documents.

"When flight attendants asked MacDonald to be quiet and attempted to calm him, he became belligerent, threatening, and intimidating towards them," the Transportation Security Administration said in a statement announcing the fine. He also told the cabin crew lead that he would "mess up the plane," court documents show.

MacDonald was eventually restrained with flex cuffs, and the flight, with 160 people on board, was diverted to Bangor, Maine. MacDonald pleaded guilty on March 22 to one count of interfering with a flight crew and was also sentenced to time already served.

At the time of the incident, United said in a statement that the plane had landed in Bangor "after two passengers, who appeared intoxicated, became disruptive." Law enforcement officials removed the passengers, who were not identified, from the flight, which took off again to land at Newark airport.

Bad behavior on flights surged during the pandemic, with tensions running high among passengers and flight crew over issues like mask-wearing.

In 2022, the FAA announced it was making permanent a zero-tolerance policy against unruly passengers.

"Behaving dangerously on a plane will cost you; that's a promise," said Acting FAA Administrator Billy Nolen at the time. "Unsafe behavior simply does not fly and keeping our Zero Tolerance policy will help us continue making progress to prevent and punish this behavior."