A traffic alert and collision avoidance system, is an aircraft collision avoidance system designed to reduce the incidence of mid-air collision (MAC) between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of MAC. It is a type of airborne collision avoidance system mandated by the International Civil Aviation Organization to be fitted to all aircraft with a maximum take-off mass (MTOM) of over 5,700 kg (12,600 lb) or authorized to carry more than 19 passengers. CFR 14, Ch I, part 135 requires that TCAS I be installed for aircraft with 10-30 passengers and TCAS II for aircraft with more than 30 passengers. ACAS/TCAS is based on secondary surveillance radar (SSR) transponder signals, but operates independently of ground-based equipment to provide advice to the pilot on potentially conflicting aircraft.

On 12 November 1996, Saudia Flight 763, a Boeing 747 en route from Delhi, India, to Dhahran, Saudi Arabia, and Kazakhstan Airlines Flight 1907, an Ilyushin Il-76 en route from Chimkent, Kazakhstan, to Delhi, collided over the city of Charkhi Dadri, around 100 km west of Delhi. The crash killed all 349 people on board both planes, making it the world's deadliest mid-air collision and the deadliest aviation accident ever in India. The crash was caused by failure of the Kazakh crew to maintain the correct altitude, because of confused dialogue with the tower communicated via the radio operator. In addition, departures and arrivals both shared a single corridor within the civilian airspace around New Delhi.