TCAS: Traffic Collision Avoidance

TCAS is the last autonomous line of defence in the air: separation broken, no visual pickup, and it still has 25 seconds to pull two aircraft apart in the vertical. Its philosophy is plain — never mind who is right, just separate: the two aircraft's TCAS coordinate over Mode S, one climbs and one descends, neither fighting for the same side. This configuration carries the AP/FD TCAS mode: the moment an RA sounds, the autopilot flies the escape itself — a comfort upgrade, but more importantly a safety upgrade against reverse manoeuvring (pulling the wrong way when startled is not a rare accident cause).

1. Detection principle — "seconds remaining," not "miles remaining"

Per FCOM DSC-34-20-60-10:

The Traffic Collision Avoidance System (TCAS): ‐ Detects and displays surrounding aircraft that have a transponder ‐ Calculates and display possible collision threats ‐ Triggers vertical speed orders, in order to avoid collisions.

Three sentences, three capabilities: see → assess → command. The range seen. Per FCOM DSC-34-20-60-10:

The TCAS detection capability is limited to intruders flying within a maximum range of 30 NM on either sides and approximately 30 NM to 100 NM longitudinally (depending on aircraft configuration and external conditions), and within a maximum altitude range of 9 900 ft above and below the aircraft.

It measures three things — bearing (antenna direction-finding), range (interrogate-reply, the transponder principle), and relative altitude (the other's Mode C/S report). Per FCOM DSC-34-20-60-10:

The TCAS then calculates the intruder trajectory, the Closest Point of Approach (CPA), and the estimated time (TAU) before reaching the CPA.

The TAU is the ratio between the distance that separates both aircraft, and the sum of their speed.

TAU is the soul of the whole TCAS: it alerts not by "miles" but by "seconds remaining" — a head-on 10 NM at 900 kt closure is only 40 s, while a same-direction 10 NM overtake may be ten minutes; distance lies, time does not. From TAU come the four zones. Per FCOM DSC-34-20-60-10:

The TCAS divides the space surrounding the aircraft into the following four zones, in order to evaluate and categorize possible collision threats: ‐ Resolution Advisory (RA) ‐ Traffic Advisory (TA) ‐ Proximate intruders ‐ Other intruders.

Thresholds: TA ≈ TAU 40 s (potential threat), RA ≈ TAU 25 s (real threat); proximate = within 6 NM laterally and ±1200 ft vertically but no threat; other = background traffic within the surveillance ring. The hardware list confirms the transponder "message office" relationship. Per FCOM DSC-34-20-60-10:

The system includes: ‐ A single channel TCAS computer ‐ Two TCAS antennas ‐ Two mode S ATC transponders, one active the other in standby.

There is only one TCAS computer (part of why the MEL allows a three-leg dispatch); its eyes, ears, and mouth are all the transponder's — dual transponder death = TCAS death.

2. Three modes — TA/RA is normal, TA ONLY is "one arm behind the back"

TA/RA = normal; STBY = fully silent (surveillance and advisory functions all off, no TCAS information on ND/PFD); the middle setting TA ONLY is the most nuanced. Per FCOM DSC-34-20-60-10:

The TA ONLY mode can be selected: ‐ Manually in case of aircraft degraded performance (engine failure, landing gear extended), or in specific airports, and for specific procedures (identified by operators) that may provide RA that are neither wanted nor appropriate (e.g. closely-spaced parallel or converging runways). ‐ Automatically, if TA/RA mode is previously selected and: • The windshear alert is triggered • The stall warning is triggered • GPWS alerts are triggered • Aircraft is below 1 000 ft AGL.

The manual logic: performance is crippled (single-engine cannot climb at the RA's required V/S), or the environment guarantees false alerts (parallel approach always has an "intruder" opposite) — rather than give an order you cannot or should not fly, only advise, do not command. The automatic logic: a stall/windshear/terrain warning is already sounding, and any "CLIMB/DESCEND" could clash with a life-saving warning — TCAS yields (the mode-level expression of the audio pecking order). Once in TA ONLY. Per FCOM DSC-34-20-60-10:

When the TCAS is operating in TA ONLY mode: ‐ All RAs are inhibited and converted into TAs (TCAS flight guidance mode is inhibited) ‐ TA threshold is set to TAU ≤20 s, irrespective of the aircraft’s altitude ‐ No vertical speed advisories are indicated on the PFDs

Note the TA threshold tightens from 40 s to 20 s — with no second RA warning to come, the TA must wait until it is truly close, to avoid crying wolf.

3. AP/FD TCAS — letting the autopilot fly the escape

The configuration's signature. Per FCOM DSC-34-20-60-10:

The AP/FD TCAS mode is a vertical guidance mode of the AP/FD.

The AP/FD TCAS mode optimizes the vertical speed for a rapid and appropriate response to an RA, and minimizes the deviations from the latest ATC clearance.

"Minimises the deviation" is the underrated half: the common human error in flying an RA is over-response (1500 fpm required, 3000 fpm given), turning 300 m of separation into crossing two flight levels — the auto mode rides just along the lower edge of the green band, so the controller sees the least deviation. A three-act life cycle.

Act 1 — TA arm. Per FCOM DSC-34-20-60-10:

In that case, the AP/FD TCAS mode automatically arms: TCAS appears on the FMA to inform the flight crew that the AP/FD TCAS mode will be available in the case a Resolution Advisory (RA) is subsequently triggered.

The FCTM callout: the PF announces "TCAS blue"; if the A/THR is off, have the PM turn it on. Per FCOM PRO-ABN-SURV:

It is recommended to set the A/THR to ON in order to avoid the AUTO FLT A/THR LIMITED alert at the automatic A/THR activation that occurs in the case of a RA.

Do not perform a maneuver based on a TA alone.

(At the TA stage your only "manoeuvre" is your eyes and fingers: read the FMA, ready the A/THR.) If the blue TCAS does not appear = the guidance mode is unavailable, the PF announces "TCAS, I have control," ready to fly the RA manually.

Act 2 — RA engage. Per FCOM DSC-34-20-60-10:

The AP/FD TCAS mode automatically engages: TCAS appears on the FMA. The flight crew has vertical guidance to fly the RA orders, automatically with the AP/FD, or manually with the FDs only (if AP was not engaged).

Three thoughtful automations. Per FCTM AS-TCAS:

If the FDs are disengaged, they automatically engage. The FD pitch bar does not flash, and the triple click aural alert does not sound, in order to avoid to disturb the PF during the evasive maneuver.

If APs and FDs are OFF when the RA is triggered, HDG automatically engages.

Per FCOM DSC-34-20-60-10:

‐ If the A/THR is disconnected, it automatically becomes armed or active, depending on the thrust lever position. When active, the speed/Mach mode engages, and the speed/Mach control becomes selected.

Two underlying boosts: In AP/FD TCAS mode, the load factor authority of the guidance law is increased (the escape wants to be crisp); and the speed has a bodyguard. Per FCTM AS-TCAS:

The AP/FD TCAS mode is speed protected, i.e. it ensures that the aircraft speed remains between VLS -5 kt and VMAX. Therefore, in the case the RA is triggered when the aircraft is close to its performance limits, the RA vertical speed target may not be reached with the AP/FD TCAS mode.

The protection is double-edged: it will not let you stall, but that may keep you from reaching the green band — so human monitoring is always on. Per FCTM AS-TCAS:

If for any reason during an RA, the aircraft vertical speed does not reach the green area of the vertical speed scale, the PF should disconnect the AP, and override the FD orders, in order to lead the aircraft vertical speed out of the red area of the vertical speed scale.

If necessary, the PF must use the full speed range between VαMAX and VMAX.

(After taking over, the speed boundary is wider than the auto mode — VαMAX is the AOA-protection ceiling; here altitude is bought with speed.) The PM's rule in one line: The PM monitors the evasive maneuver. The PM does not try to see the reported traffic.

Act 3 — clear of conflict. Per FCOM DSC-34-20-60-10:

In most of the cases, the AP/FD TCAS mode automatically reverts to V/S mode. The vertical speed target leads the aircraft toward the FCU selected altitude.

On «CLEAR OF CONFLICT»: the PM notifies ATC, the PF adjusts vertical mode and speed per the latest clearance. A "spring-back" design along the whole ATC clearance: least deviation, fastest return.

4. Display and voice — red is a wall, green is a door

The ND side. Per FCOM DSC-34-20-60-20:

The traffic is displayed in all ROSE modes and ARC mode whatever NM range is selected. Only the eight most threatening intruders are displayed.

A four-tier symbol threat spectrum: hollow white diamond (other) → filled white diamond (proximate) → amber circle (TA, with «TRAFFIC TRAFFIC») → red square (RA, with the PFD orders); relative altitude (hundreds of feet) plus a vertical arrow. Per FCOM DSC-34-20-60-20:

displayed only if the intruder vertical speed is greater than ± 500 ft/min

When bearing cannot be measured, the target is not plotted but shown as a digital readout at the bottom of the ND (range + relative altitude, coloured by threat) — do not assume all is well because nothing is on the map. The PFD side. Per FCOM DSC-34-20-60-20:

In case of RA detection, the vertical speed scale becomes rectangular and the PFD presents vertical orders on the vertical speed scale.

Indicates the vertical speed where the risk of conflict is high.

Green = recommended V/S (the "FLY TO" sector, only for a corrective RA); a preventive RA paints only the red — stay anywhere in grey, just out of the red. The voice family (corrective): CLIMB/DESCEND ×2 (fly into the green), the CROSSING set (you will cross the other's altitude — do not panic, the crossing is computed), the INCREASE set (the current amount is not enough), the REVERSAL set (CLIMB CLIMB NOW / DESCEND DESCEND NOW — the other changed its mind, direction reversed, the fiercest), LEVEL OFF (V/S to zero); preventive: MONITOR VERTICAL SPEED (once only), MAINTAIN VERTICAL SPEED. The close-out. Per FCOM DSC-34-20-60-20:

The range increases, and separation is adequate. Return to assigned clearance.

5. The inhibition table and the "mute" intruder

Low altitude is TCAS's self-restraint zone. Per FCOM DSC-34-20-60-10:

Some advisories are inhibited depending on the aircraft altitude: ‐ All intruders flying below 380 ft AGL when the own aircraft altitude is below 1 750 ft AGL in climb or 1 650 ft AGL in descent ‐ All TA aural messages below 600 ft AGL in climb or below 400 ft AGL in descent ‐ All RA aural messages below 1 100 ft AGL in climb or 900 ft AGL in descent. In this case, the RA are converted into TA

Finer still: DESCEND RA inhibited below 1200/1000 ft, INCREASE DESCENT below 1650/1450 ft (it will never point you into the ground — TCAS does not touch GPWS's turf); AP/FD TCAS guidance inhibited below 900 ft. The offset climb/descent thresholds are a hysteresis, preventing the inhibition state from flickering at a critical height. The other "incomplete opponent" — an intruder that does not report altitude. Per FCOM DSC-34-20-60-10:

For intruders that do not report their altitude: ‐ The relative altitude does not appear on ND ‐ The TCAS never triggers any RA ‐ The TCAS inhibits the TA when own aircraft altitude is above 15500 ft.

Mechanism: with no other-aircraft altitude, the vertical-escape equation has no solution — no RA; above 15500 ft it assumes "a non-reporting light aircraft cannot be up here," so it saves the TA too. But below 15500 ft the TA it gives you is "lateral threat, vertical unknown" — how to handle it, where to look visually, and how to coordinate with ATC is an operational topic in its own right.

6. Panel and memo — the consequences of cohabiting with the transponder

This group's panel (1241-1912 text) TA/RA position has two implicit prerequisites. Per FCOM DSC-34-20-60-20:

Proximate and other intruders, TAs and RAs are displayed provided: ‐ The ALT RPTG switch is at ON, and ‐ The ATC mode selector is at ON or AUTO.

— transponder not reporting altitude, TCAS on strike (the origin of the TCAS STBY memo in the QNH > 1050 procedure). The TRAFFIC selector governs only "how much background traffic to display," not the alert logic: THRT (proximate/other only with a TA/RA present, ±2700 ft) / ALL (always ±2700) / ABV (up 9900 / down 2700, for climb) / BLW (the reverse, for descent). The FCTM sets the default: ALL. Memo summary (four triggers, amber phase 6 / green phases 2, 7). Per FCOM DSC-34-20-60-20:

TCAS STBY : This memo appears in amber in flight phase 6 when: ‐ The flight crew selects TCAS STBY or ATC STBY on the ATC/TCAS panel, or ‐ Both ATCs or both RAs fail, or ‐ The flight crew turns OFF the ALT RPTG sw, or ‐ In the case of a triple ADR failure.

The four triggers correspond to its four survival dependencies: the panel, the mouth (XPDR), the ruler (RA), the altitude source (ADR) — the fault chapters meet this memo repeatedly.

7. Handling discipline — before an RA there is no controller, and no visual

The FCOM's overarching rule, first in the procedure. Per FCOM PRO-ABN-SURV:

Always follow RA orders, even if this results in crossing the intruder altitude, because these orders ensure the best altitude separation.

Be aware that the intruder may have a TCAS, and may maneuver in response to a coordinated RA order. Therefore, not following an RA order could compromise safe separation.

The FCTM lines up three "even"s: follow though it contradicts ATC (the blood lesson of Überlingen — RA > ATC); follow a CLIMB at maximum ceiling (do not reverse even if performance cannot climb — buy altitude with speed); follow though you will cross the other's altitude. Reverse manoeuvring is an absolute no. Per FCTM AS-TCAS:

‐ The flight crew should never maneuver in the opposite direction of the RA, because TCAS maneuvers are coordinated.

The manual-flying response calibration (FCTM response table): initial corrective — 0.25 g, within 5 s into the green (about a quarter of a "comfortable turn," not a yank); increase/reversal (INCREASE / NOW set) — 0.35 g, within 2.5 s (far more decisive). Per FCTM AS-TCAS:

Smoothly and firmly (0.25 g) follow the green area of the V/S scale within 5 s.

Smoothly and firmly (0.35 g) follow the green area of the V/S scale within 2.5 s.

Without AP/FD TCAS guidance (mode failed / below 900 ft) the old-style procedure: AP OFF + both FDs OFF + hand-fly the green. Per FCOM PRO-ABN-SURV:

Both FDs must be disconnected when APs are disconnected: ‐ To ensure autothrust speed mode ‐ To avoid possible confusion between FD bar orders, and TCAS aural, and VSI orders.

The last special case, a CLIMB RA on final in full configuration. Per FCTM AS-TCAS:

If any CLIMB RA (For more information Refer to FCOM/DSC-34-20-60-20 Aural Messages) is generated when the aircraft is in approach in CONF 3 or FULL: ‐ The flight crew perform a go-around and follow the SRS orders ‐ The AP and FD can be kept engaged during the go-around

Per FCOM PRO-ABN-SURV:

The AP/FD TCAS mode disengages (the AP/FD does no longer follows the RA orders).

Why not simply follow the RA? Landing configuration is high-drag, low-energy, and the RA's climb rate may not be achievable; a TOGA go-around's climb is itself ≥ the RA's required separation, and it solves the "no guidance mode at low altitude" problem too — but watch the V/S out of the red, and. Per FCOM PRO-ABN-SURV:

Respect stall, GPWS or windshear warnings.

Key numbers

Self-test

[!note]- Q1. Head-on at 900 kt closure, how far apart at the TA? At the RA? (use TAU) TA ≈ 40 s → 900 kt × 40 s ≈ 10 NM; RA ≈ 25 s → ≈ 6.25 NM. Time, not distance, drives it.

[!note]- Q2. The four automatic TA ONLY triggers, and the shared design logic in one line? Windshear, stall, GPWS, below 1000 ft AGL. TCAS yields to a life-saving warning or a regime where an RA cannot/should not be flown.

[!note]- Q3. An RA triggers while you hand-fly with FDs off — the FMA vertical and lateral? Which two sounds are deliberately absent? Vertical: TCAS engages (FD auto-engages); lateral: HDG auto-engages. The FD pitch-bar flash and the triple-click are deliberately absent.

[!note]- Q4. The AP/FD TCAS speed-protection range, why it can leave the green "unreachable," and what you do? VLS−5 to VMAX. Near performance limits it may not reach the RA target; then disconnect AP, override the FD, and use VαMAX–VMAX to get out of the red.

[!note]- Q5. On final at 800 ft AGL, an intruder closes fast from below — the most you will get, and why? A TA at most: below 1100/900 ft all RA aural is inhibited and RA converts to TA; below 900 ft the guidance mode is inhibited too.

[!note]- Q6. CONF FULL on final, «CLIMB CLIMB» — action, mode change, monitoring point? Go around on the SRS (AP/FD may stay). The AP/FD TCAS mode disengages. Monitor V/S out of the red, and respect stall/GPWS/windshear.

Key takeaways

References

• FCOM DSC-34-20-60-10 — principle, TAU, four zones, components, three modes, AP/FD TCAS, inhibition table, mute target.
• FCOM DSC-34-20-60-20 — panel, ND/PFD indications, voice table, memo.
• FCOM PRO-ABN-SURV — TCAS memory items (TA/RA/CONF3-FULL/no-guidance).
• FCTM AS-TCAS — response table 0.25–0.35 g, operating technique, speed protection, go-around management.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.