As the plane got closer to landing, a tower supervisor used a red light gun to alert the crew to abort the landing. A flashing light gun is “standard protocol” when an air crew is not responding to radio instructions, (FAA spokesman) Gregor said.
However, despite all those attempts to abort the landing, the Air Canada plane still landed on Runway 28R at 9:26 p.m., Gregor said.
http://www.eastbaytimes.com/2017/10/...mix-up-at-sfo/