The Traffic Alert and Collision Avoidance System (TCAS) alerts the crew to traffic that may present a collision threat, and provides the crew with a vertical avoidance manoeuvre. TCAS is independent of, but does not replace, the ground based ATC system.

TCAS equipment uses the Mode S transponder to interrogate the transponders of other aircraft in the vicinity to determine their range, bearing and altitude. TCAS generates a Traffic Advisory (TA) when another aircraft becomes a potential threat and is approximately 40 seconds from the closest point of approach. No manoeuvres are required for a TA. If the intruder continues to close and becomes an imminent threat, a Resolution Advisory (RA) is generated when the other aircraft is approximately 25 seconds from the closest point of approach. The RA provides a vertical restriction or manoeuvre to maintain or increase separation from the traffic.

Non-transponder equipped aircraft are invisible to TCAS, and no RA is generated for aircraft operating in Mode A only, or without serviceable Mode C (altitude reporting). TCAS has the ability to resolve multiple aircraft encounters.

Annunciations associated with TCAS are displayed either in the middle of a flat panel improved VSI (IVSI), or on the ADI and HSI, according to the aircraft fit.

The table below shows the symbology used and the aural warnings generated:

Symbol	Name	Remarks
	Other Traffic Symbol	Displayed when TFC selected on the control panel and traffic greater than 1,200ft vertical or 6 miles lateral from present position. Example shows traffic in level flight 2,000ft below.
	Other Traffic Symbol	Second example of the Other Traffic Symbol, showing an aircraft 1,300ft above and descending at 500ft/min or greater.
	Proximate Traffic Symbol	Displayed when TFC selected on the control panel and traffic within 1,200ft vertical and 6 miles lateral from present position. Example shows traffic 900ft above climbing at 500ft/min or greater.
	TA Traffic Symbol	Displayed during TCAS Traffic Advisory when TFC selected on the control panel. Example shows traffic 700ft below climbing at 500ft/min or greater.
	RA Traffic Symbol	Displayed during TCAS Resolution Advisory when TFC selected on the control panel. Example shows traffic 400ft below climbing at 500ft/min or greater.

A TA is indicated by the aural "TRAFFIC, TRAFFIC" which sounds once, and is then reset until the next TA occurs. On aircraft with the EFIS fit, the TRAFFIC message appears on the HSI, and pressing the TFC switch on the HSI range selector displays the range and relative bearing of the other aircraft. Intruder aircraft are automatically displayed on aircraft with the IVSI fit. If the other aircraft's transponder is operating in Mode C or S, altitude information and vertical motion, if applicable, are also displayed.

RAs are indicated by one or more of the aurals shown in the table below:

Aural	Meaning
"MONITOR VERTICAL SPEED, MONITOR VERTICAL SPEED"	Avoid deviations from the current vertical speed.
"CLIMB, CLIMB, CLIMB"	Climb at the pitch required.
"DESCEND, DESCEND, DESCEND"	Descend at the pitch required.
"REDUCE CLIMB, REDUCE CLIMB"	Reduce climb pitch as required.
"REDUCE DESCENT, REDUCE DESCENT"	Reduce descent pitch as required.
"CLIMB, CROSSING CLIMB, CLIMB, CROSSING CLIMB"	Directs a climb and informs the pilot that safe vertical separation will result in climbing through the intruders altitude.
"DESCEND, CROSSING DESCEND, DESCEND, CROSSING DESCEND"	Directs a descent and informs the pilot that safe vertical separation will result in descecnding through the intruders altitude.
"INCREASE CLIMB, INCREASE CLIMB"	Increase climb rate after an initial "CLIMB" RA is received.
"INCREASE DESCENT, INCREASE DESCENT"	Increase descent rate after an initial "DESCEND" RA is received.
"CLIMB - CLIMB NOW, CLIMB - CLIMB NOW"	Reversal manoeuvre after an initial "DESCEND" RA is received.
"DESCEND - DESCEND NOW, DESCEND - DESCEND NOW"	Reversal manoeuvre after an initial "CLIMB" RA is received.
"CLEAR OF CONFLICT"	The RA encounter is terminated.

The TRAFFIC message appears as before, and pressing the TFC switch on the HSI range selector displays the other aircraft's range, relative bearing and altitude on the HSI. An RA pitch indication or manoeuvre appears on the ADI. A manoeuvre is required if any portion of the aircraft symbol is within the red region on the ADI. On those aircraft with the IVSI fit, red and green arcs appear around the outside of the IVSI scale, and manoeuvring is required to position the vertical speed pointer out of the red region and into the green.

An OFFSCALE message appears during a TA or RA if the traffic's position is outside the selected Traffic Display's range.

A TA or RA message followed by the traffic's range, altitude and (if applicable) vertical motion arrow appear on the display if TCAS cannot determine the other aircraft's bearing.

TCAS alerts are inhibited by GPWS, and Windshear warnings, and at low altitudes where the Traffic Avoidance Manoeuvre would be inappropriate.

If an inhibit occurs during an RA, the RA aural is silenced, and the traffic symbol reverts to a TA symbol. If an inhibt occurs during a TA, only the TA aural is silenced.

The TCAS operating mode is controlled from the transponder panel. TCAS is normally operated in the TA/RA (TA and RA) mode. Sometimes it may be necessary to operate in the TA Only mode to prevent undesired RAs.

The TA Only mode is used during engine out operation to prevent RAs when inadequate thrust is available to follow the RA commands. Also, it may be used when intentionally operating near other aircraft that may cause RAs, such as during parallel approaches and VFR operations.