Refer to figure.

A TCAS II provides the intruder relative position (Traffic Advisory and proximate traffic) and possibly an indication of a collision avoidance maneuver (Resolution Advisory) within the vertical plane only, by comparing the mode C pressure altitudes of the aircraft. The instrument determines the bearing of an intruder by using two specific antennas. The TCAS II can only operate normally with a fully operational transponder since it is using the replies from the transponders of other aircraft. Therefore, if other aircraft do not have a serviceable transponder no protection will be provided against them. In the same way, if any of the concerned aircraft does not have the altitude reporting capability from the mode C transponder, the TCAS II will only issue Traffic Advisories (TA).

The TCAS computer uses the aircraft’s configuration data (gear/flaps), pressure altitude from the mode C transponder, and the radio altimeter’s height. The TCAS II has inputs from the radio altimeter in order to gradually inhibit the resolution advisories (RAs) when getting closer to the ground such as during the approach phase. The instrument itself can have a specifically dedicated screen situated on the cockpit’s instruments panel or it can be a variometer on a liquid crystal screen displaying the TAs and RAs. The TCAS II can also be integrated into an EFIS screen or be combined with the weather radar.

The resolution advisories issued by the TCAS are either preventive or corrective:

Preventive – advise the flight crew to monitor the vertical speed.

Corrective – the vertical speed must be corrected without delay.

The color and symbols code of the TCAS II is the following:

Proximate traffic – white or cyan solid lozenge.

Traffic Advisory – amber solid circle.

Resolution Advisory – red full square.