Whenever aircraft lights are not required to be illuminated, they may always be turned on at any time at the discretion of the pilot. Usually this is done when the pilot wishes to make the aircraft more visible to other nearby aircraft. At cruise altitude in a passenger jet this is uncommon, since air traffic control (ATC) is assigning altitudes and headings, and should be spacing aircraft a minimum distance from one another. Also, these jets are equipped with a radar-based traffic warning system (TCAS) which should let them know of the whereabouts of nearby aircraft. But anything can happen.