15.3 Traffic Collision Avoidance and Terrain Awareness

15.3 Traffic Collision Avoidance and Terrain Awareness

Current Appendix I gives only the broad 5.15 heading. The pre-12 June 2024 detailed description named TCAS (Traffic Collision Avoidance System) among its typical systems, so this section retains TCAS as a level-1 study example. ICAO documents use the equivalent functional name ACAS (Airborne Collision Avoidance System); in European air-transport service the fitted equipment is TCAS II. This section treats TCAS as the historical syllabus example: general arrangement, Traffic Advisory (TA) versus Resolution Advisory (RA), the Mode S transponder relationship, and associated BITE.

Enhanced Ground Proximity Warning System (EGPWS) and Terrain Awareness and Warning System (TAWS) are taught only as related terrain-alert context. They are widely fitted on the same aeroplanes and appear in cockpit warning philosophy beside TCAS, but they were not a separate example in the former detailed 5.15 list. Study them as related terrain-warning context, not as a replacement for the TCAS collision-avoidance material.

TCAS general arrangement

A typical TCAS II installation comprises:

• a TCAS computer (processor and transmitter/receiver);
• upper and lower directional antennae so the system can interrogate traffic above and below the aircraft;
• a control interface (often combined with the transponder panel) with modes such as STBY, TA ONLY and TA/RA;
• traffic presentation on the EFIS navigation display (or a dedicated traffic display on older types);
• aural messages through the cockpit loudspeakers;
• a tight interface with the aircraft Mode S transponder.

TCAS interrogates nearby transponders on the Secondary Surveillance Radar (SSR) uplink frequency 1030 MHz and listens for replies on 1090 MHz. It builds a three-dimensional picture of range, relative altitude and closing trend. Mode A/C (ATCRBS) traffic can be tracked for range and Mode C altitude, but RA coordination between two TCAS II aircraft uses Mode S air-to-air data interchange. That is why a Mode S transponder is part of the TCAS II arrangement, not an optional extra.

Traffic symbols on the ND follow a conventional teaching set: other traffic as an open white diamond, proximate traffic as a filled white diamond, a TA as an amber circle, and an RA as a red square, with relative altitude and a climb/descend arrow. Exact glyph details are type-specific; the maintenance point is that the display is a TCAS output, not an FMS waypoint.

Traffic Advisory and Resolution Advisory

A Traffic Advisory is an awareness alert. The typical aural is 'Traffic, traffic'. The crew searches visually and prepares to respond; a TA does not command a climb or descent. TA-ONLY mode is used when an RA would be inappropriate (for example some engine-out or inoperative-equipment procedures, or as inhibited by the system near the ground).

A Resolution Advisory is a vertical manoeuvre command generated by TCAS II: climb, descend, increase climb, increase descent, level off, adjust vertical speed, or a reversal, depending on the encounter and the software version. European airspace has required TCAS II version 7.1 behaviour, which includes the 'Level off, level off' RA and revised reversal logic. Two equipped aircraft coordinate complementary RAs via Mode S so that one climbs and the other descends rather than both choosing the same sense. The aural 'Clear of conflict' ends the RA.

Sensitivity and inhibits vary with radio altitude. Teaching figures used in maintenance familiarisation (always confirm the type AMM) include inhibition of descent RAs at low height AGL and inhibition of increase-descent RAs somewhat higher, with all RAs inhibited close to the ground so that a landing aircraft is not commanded into the terrain. TA aurals are also inhibited at very low height. These inhibits are why a hangar self-test must be performed to the AMM procedure: you are proving lamps, speakers and computer BITE, not flying a live coordinated RA against another aircraft.

Crew doctrine that matters to technicians reading defect reports: the crew follows the RA even if it disagrees with an ATC instruction during the conflict, then returns to ATC clearance when 'Clear of conflict' is heard. Opposite manoeuvres against an RA have caused accidents; that is an operational rule, but it explains why a TCAS computer or transponder fault that produces nuisance RAs is a serious defect, not a nuisance light.

TCAS I, still met on some lighter types, provides TAs only. If the exam names TCAS without a mark, assume the air-transport TA + RA system (TCAS II) unless the question states TA-only.

Related context: EGPWS / TAWS (terrain, not traffic)

Related context — not a separate example in the former detailed 5.15 list. Ground-proximity alerting protects against controlled flight into terrain, which is a different hazard from mid-air collision. Classic GPWS uses radio altitude, barometric altitude rate, airspeed/configuration and ILS glideslope to generate modes that are still the teaching backbone:

EGPWS (a widely used enhanced implementation) and TAWS (the generic / certification name used in TSO/ETSO language) add a terrain database and aircraft position (FMS/GPS/IRS) so the system can look ahead, not only straight down the radio altimeter. The ND may show a terrain picture. Aurals such as 'Caution terrain' or 'Terrain, terrain, pull up' can occur before classic Mode 1/2 envelopes would fire. Geometric altitude from GPS is often used to improve the model. A terrain-database load has effectivity in the same software-control spirit as an FMS navigation database, but it is a terrain file for the warning computer, not the FMS route file, and it is still related context rather than an extra 5.15 title.

Do not mix the two alerting families:

• TCAS = transponder-equipped traffic, TAs and RAs, Mode S coordination;
• EGPWS/TAWS = terrain and sink-rate (and look-ahead terrain when enhanced), radio altimeter plus database.

A red pull-up from terrain is not a TCAS climb RA, even though both may shout in the same cockpit. Inhibit switches (GPWS/G/S, FLAP, TERR) and TCAS TA-ONLY are different controls; pulling the wrong circuit breaker during troubleshooting is a common hangar error.

Associated BITE

TCAS BITE includes power-up self-test, continuous monitoring of antenna VSWR, transponder interface and computer memory, and a crew/maintenance self-test that typically sounds a test aural ('TCAS test' or similar), paints a test pattern on the ND, and reports via the CMC/CFDS. The AMM procedure exists so that you do not radiate a real RA sequence into a crowded ramp. Antenna bonding, coaxial connectors and water ingress in the upper/lower antennae are frequent physical faults. A TCAS fault with a failed Mode S transponder is often the transponder, not the TCAS computer — isolate with BITE before swapping the expensive box.

For the related terrain system, BITE covers the EGPWS/TAWS computer, radio-altimeter inputs, position inputs, loudspeaker path and terrain-database identity. A database mismatch or an invalid position (IRS/GPS) can produce a terrain-display inoperative message without a TCAS fault.

Level 1 familiarisation is this separation of duties: TCAS talks to transponders and may command a vertical RA; the related terrain computer talks to the radio altimeter and a terrain database and may command a pull-up from the ground. BITE proves each computer, its antennae or probes, and its database or transponder interface.