Invisible Aircraft   Pilots and Air Traffic Controllers have lodged Mandatory Occurrence Reports to the UK Civil Aviation Authority complaining of unlit airliners being towed along taxi ways during darkness.
This results from British Airways being allowed a 12 months exemption to evaluate the latest tugs ( aircraft towing tractors ).  The tugs are a desperate cost cutting scheme by British Airways, they are not equipped with Auxiliary Power Units which would normally provide electrical power to aircraft when engines are not running.  The tugs are equipped with a light for the tug drivers benefit but this only illuminates the underside of the aircraft nose and is invisible to most other observers especially the high viewpoints common to pilots and controllers.