Refer to figure.
Airborne Collision and Avoidance Systems (ACAS) provide pilots with an additional means of detection, independent of visual observation and Air Traffic Control (ATC), to assess the likelihood of colliding with another aircraft.

TCAS (T stands for Traffic) is the system developed by the FAA, while ACAS is the generic name used by ICAO.

There are three (3) types of TCAS :

• TCAS I, providing traffic warnings without guidance. TCAS I relies on time-based anti-collision logic.

• TCAS II, providing advisory information to aircraft pilots in the vertical plane about nearby aircraft, as a:

1. Traffic Advisory (TA), or a warning, informing where nearby transponding aircraft are or

2. Resolution Advisory (RA), which suggests avoiding action in the pitch plane only.

• TCAS III (under development), capable of providing horizontal and vertical guidance.

Concerning its operation, the system is independent of ground facilities. It interrogates other aircraft using:

• four antennas;

• a computer and

• a transponder (SSR)

and continually survey the helicopter's airspace predicting likely intruders' flight paths based on Mode C (for TCAS I) or S (for TCAS II) transponder signals from other traffic.

In addition, TCAS II can display on its own screen, on weather radar, EFIS, a variometer (VSI) with an LCD display, etc.

When both aircraft are equipped with TCAS II and a “threat” materialises, the mode S data link between the two aircraft provides coordinated and complementary RAs. The first aircraft to make the interrogation sends an intent message to which the receiving aircraft reacts to in the opposite sense.

According to the full range of “Commands and Advisories” for TCAS II version 7 the preventive "resolution advisory" (RA) advises the pilot to avoid certain deviations from the current vertical rate but not requiring any change to be made to that rate.