A passenger who appeared to be experiencing a mental health crisis punched out the plexiglass of a window mid-flight.

A Frontier Airlines passenger was restrained on a flight from Denver to Houston after he allegedly began screaming and punching the aircraft window

"He was scary," a witness said. "He started kicking profusely and started punching the window." Passengers on board restrained the man with zip ties and a shoelace.

An unruly passenger on a Frontier Airlines flight was seen on video yelling and being taken down by fellow passengers.