Radarspotting

Under regulations introduced in 2012, passengers on flights held at the gate or on the tarmac at US airports must be allowed to disembark where practical.

A flight from Boston on November 15 last year with 329 passengers was held at the gate for more than four hours. The rules say that if the aircraft is at the gate with the door open, passengers must be allowed to disembark.
http://www.ttgdigital.com/news/air-travel/ba-hit-with-225k-fine-for-tarmac-delays/4691077.article