TRAFFIC ALERT AND COLLISION AVOIDANCE SYSTEMS

(TCAS)

The general term for collision avoidance systems operating from aircraft is Airborne Collision Avoidance Systems (ACAS). The system now in use is TCAS (pronounced Tee-Kass), which is sophisticated method by which a pilot is alerted to the presence of another aircraft which represents (or may become) a hazard due to its proximity.

TCAS operates by using the transponder of one aircraft to interrogate the transponders of other aircraft in the vicinity; by doing so, the flight profiles of the aircraft are compared to determine whether a possible conflict exists. Depending upon the level of sophistication of the equipment it can also advise the pilot what action is required to resolve the situation. The system operates entirely independently of any ground-based equipment.

There are three types of TCAS (although the third type is still under development).

TCAS I warns the pilot of the presence of another aircraft but does not give any other information concerning avoiding action.

TCAS II also warns the pilot of the presence of other aircraft (only those operating transponders, of course) and also gives advice on avoiding action in the vertical plane.

TCAS III provides the pilot with two kinds of advisories: Traffic Advisories are indications to show the approximate position of neighbouring transponding aircraft which may become a threat.

Resolution Advisories advise the pilot on climbs or descents in order to resolve a conflict with another aircraft.

A warning appears on a small cathode ray tube showing the conflicting aircraft both in plan and elevation, while a synthetic voice also warns the crew. The system uses a Mode C transponder which can also be used as a datalink to send and receive data to the ground. The warning indicates the time to the ‘closest point of approach’ (CPA) between the two aircraft.

Forty seconds before CPA the pilot is given a Traffic Advisory warning (TA). The voice messages says the words ‘Traffic, traffic’.

Twenty to thirty seconds before CPA the pilot is given a Resolution Advisory (RA) warning with the advice to climb, descend, or remain level. In these cases the voice warning says the words ‘Climb’, ‘Descend’ or ‘Maintain Level’ as appropriate. In each case the words are repeated three times.

If a pilot receives a Resolution warning the appropriate action is to be taken without informing Air Traffic Control, but the controller is to be told of the situation as soon as possible.