Traffic alert and Collision Avoidance

System (TCAS)

What is TCAS?

TCAS is a special system designed to reduce the incidence of mid-air collisions between aircraft.

A mid-air collision is an aviation accident in which two or more aircraft come into contact during flight.

Basic Features• TCAS monitors the airspace around an aircraft for other aircraft

equipped with a corresponding active transponder(independent of ATC). TCAS warns pilots of the presence of other aircraft (transponder-equipped) which may present a threat of mid-air collision (MAC).

• TCAS is mandatory for all aircraft with MTOM (maximum take-off mass) over 5700 kg or carry more than 19 passengers

TCAS Display

System components• TCAS computer unit:

Performs airspace surveillance, intruder tracking, its own aircraft altitude tracking, threat detection, RA manoeuvre determination and selection, and generation of advisories.

• Antennas:The antennas used by TCAS include a directional antenna that is mounted on the top of the aircraft and either an omnidirectional or a directional antenna mounted on the bottom of the aircraft. In addition to the two TCAS antennas, two antennas are also required for the transponder. These antennas enable the transponder to receive interrogations at 1030 MHz and reply to the received interrogations at 1090 MHz.

• Cockpit presentation:The TCAS interface with the pilots is provided by two displays: the traffic display and the RA display.

System DescriptionTCAS involves communication between all aircraft equipped with an appropriate transponder. Each TCAS-equipped aircraft interrogates all other aircraft in a determined range about their position (via the 1,030 MHz radio frequency), and all other craft reply to other interrogations (via 1,090 MHz).

TCAS Alerts• Traffic Advisory (TA) : When a TA is issued, pilots are instructed to initiate a visual search for the traffic causing the TA.• Resolution Advisory (RA) : When a RA is issued, pilots are expected to respond immediately to the RA else it would jeopardize the safe operation of the flight.

TCAS Operation Modes• Stand-By : Power is applied to the TCAS Processor and the

transponder but TCAS does not issue any interrogation and the transponder will reply to only discrete interrogations.

• Transponder : The transponder is fully operational and will reply to all ground and TCAS interrogations. TCAS remains in Stand-by.

• TA Only : TCAS will operate normally and issue the appropriate interrogations and perform all tracking functions. However, TCAS will only issue Traffic Advisories (TA), and the Resolution Advisories (RA) will be inhibited.

• Automatic or TA/RA : The transponder is fully operational . TCAS will issue Traffic Advisories (TA), and the Resolution Advisories (RA) , when appropriate.

Types of traffic and resolution advisoriesType Text Meaning Required action

TA “Traffic; traffic“ Intruder ahead beyond 5 miles of altitude. Attempt visual contact, and be prepared if an RA occurs.

RA “Climb; climb” Intruder too close below. Begin climbing.

RA “Descend; Descend” Intruder too close above. Begin descending.

RA “Increase climb” Intruder slowly climbing toward level Climb at a faster rate.

RA “Increase descent” Intruder slowly descending toward level Descend at a faster rate.

RA “Reduce climb” Intruder slowly climbing away from level. Climb at a slower rate.

RA Reduce descent. Intruder slowly descending away from level. Descend at a slower rate

RA “Climb; climb now” Intruder rapidly descending toward level Change from a descent to climb.

RA “Descend; descend Intruder rapidly climbing toward level Change from a climb to descent. now“

CC Clear of conflict. Intruder is no longer a threat. Return to previous ATC clearance.

TYPICAL TCAS DISPLAY

TCAS DisplayIn modern glass cockpit aircraft, the TCAS display may be integrated in the Navigation

Display

TCAS Advantages

• Highly accurate distance measurements, with a one-second refresh rate.

• All threats taken into account• Detection of all transponding aircraft, including those

which are not displayed on the air traffic controller’s screen

• Independent system, which acts as a last resort measure to avoid mid-air collision when other safety precautions fail.

• TCAS reduced the risk of mid-air collision

TCAS Limitations

• No detection of aircraft without or not operating transponders (e.g. VFRs, military flights)

• No knowledge of the pilot’s intentions and of the ATC separation minima

• Basic display: no identification, no past positions, no speed vector

• Sometime generate unnecessary alerts.