The driver, operator or attendant of any tank vehicle shall be in attendance with the vehicle at a time when the vehicle is fueling or refueling an aircraft. Such person shall not leave the tank vehicle to handle hose for overwing refueling or fuel from a work stand in excess of six feet in height or in a position more than 50 feet from the tank vehicle unless a firewatch, familiar with the operation, is on hand on the ground. A firewatch shall not serve more than two vehicles or two aircraft under fuel servicing simultaneously and must be within 75 feet of any operation under his surveillance.