On 13 December 2020, an Air France Boeing 777-300ER (registered F-GZNN) – operating flight AF480 between Paris CDG, France, and Lima, Peru, – was climbing from flight level 360 to flight level 380 when the aircraft’s TCAS warning system was activated. The aircraft was coming too close to an American Airlines Boeing 737-800 (registered N879NN) that was flying at level 380 on flight AA1546 between St. George, Grenada, and Miami, Florida, United States.
The Traffic Alert and Collision Avoidance Systems (TCAS) on both aircraft were then activated, website The Aviation Herald writes. The American Airlines Boeing 737 climbed, and the Air France Boeing 777 descended to flight-level 374.
After being clear of conflict both aircraft continued to destinations without further incident.
A word of explanation: the traffic collision avoidance system (abbreviated as TCAS, and pronounced tee-kas) is an aircraft collision avoidance system designed to reduce the incidence of mid-air collisions between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control (ATC), and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision (MAC).
The next step beyond identifying potential collisions is automatically negotiating a mutual avoidance manoeuvre (currently, manoeuvres are restricted to changes in altitude and modification of climb/sink rates) between the two (or more) conflicting aircraft. These avoidance manoeuvres are communicated to the flight crew by a cockpit display and by synthesised voice instructions.
The airborne collision avoidance system is mandated by the International Civil Aviation Organization (ICAO) to be fitted to all aircraft with a maximum take-off mass (MTOM) of over 5,700 kg or authorised to carry more than 19 passengers.