Airborne Collision Avoidance System (ACAS / TCAS)

The Airborne Collision Avoidance System (ACAS) is ICAO's generic name for equipment that detects other aircraft through their transponders and warns the crew of a risk of collision without any help from the ground. The equipment actually fitted to airliners is TCAS (Traffic Alert and Collision Avoidance System), developed in the United States. TCAS II version 7.0 was the first to meet the ICAO ACAS II standard, which now requires version 7.1. ICAO and EASA documents say ACAS, FAA documents and most cockpit panels say TCAS, and exam questions use both.

ACAS is a safety net, not a separation tool. ICAO states that the carriage of ACAS in an area must not be a factor in deciding which air traffic services are provided there. It acts in roughly the last minute before a possible collision, and it works only if crews trust it and fly what it commands. The 2002 mid-air collision over Überlingen, where one crew followed its resolution advisory and the other followed a conflicting ATC instruction, is why today's rule is unambiguous: the RA is flown, even against ATC.

ACAS overview

ICAO standards cover two levels of ACAS in service. ACAS I (in practice TCAS I) gives traffic advisories only. ACAS II (TCAS II) adds resolution advisories in the vertical plane and coordinates them with the other aircraft when both carry TCAS II.

The EU rule, Regulation (EU) No 1332/2011, applies in EU airspace, so third-country operators flying there also need version 7.1. A newer logic, ACAS X, has been standardised as RTCA DO-385 and EUROCAE ED-256; ICAO's Annex 10 standards have accepted ACAS Xa as an alternative to version 7.1 since November 2022. It presents TAs and RAs to the crew in the same way, but replaces the hand-written threat logic with an optimised probabilistic model.

Surveillance, CPA and tau

TCAS II works like airborne secondary radar: it interrogates on 1030 MHz and receives replies on 1090 MHz. Mode S transponders are addressed individually by their 24-bit address; Mode A and C transponders are interrogated in a whisper-shout sequence that limits how many reply at once. Range comes from reply timing, relative altitude from the intruder's own pressure-altitude report, and bearing from a directional antenna. Bearing is the weak measurement, which is why RAs are purely vertical and why the traffic display is never used as a basis for turning.

An aircraft with no working transponder is invisible to TCAS. One replying without altitude (Mode A only, or altitude reporting switched off) can produce a TA but never an RA. Two TCAS II aircraft exchange coordination messages over the Mode S link so that their RAs are complementary, for example one climbing while the other descends.

The Closest Point of Approach (CPA) is the predicted point of minimum range between the two aircraft. TCAS alerts on time rather than distance. TCAS tau is the time to CPA, approximately range divided by closure rate horizontally and relative altitude divided by vertical closure rate vertically. An advisory needs both horizontal and vertical criteria to be met. Because a slowly converging aircraft could come very close while its tau stays large, the logic adds a minimum protected range (DMOD) and a fixed altitude threshold.

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Traffic display and proximate traffic

Traffic appears on the navigation display or a dedicated screen with standard symbols:

Proximate traffic is shown only to help visual acquisition; it generates no alert. Each symbol carries a data tag giving relative altitude in hundreds of feet, with a plus sign above and a minus sign below, and an arrow when the intruder climbs or descends at 500 ft/min or more.

TCAS altitude range selection (ABOVE / NORMAL / BELOW) changes only what is drawn. NORMAL shows traffic within 2,700 ft above and below; ABOVE extends the band upwards for climbs and BELOW downwards for descents, to 7,000 ft on some Boeing installations and 9,900 ft on the A320. The range selector likewise changes display scale, not alerting: TAs and RAs are always shown.

If TCAS cannot determine a threat's bearing it gives a TCAS no-bearing advisory: a text box under the TRAFFIC message showing range, relative altitude and vertical trend, so the crew must search without an azimuth cue.

Traffic advisories

A Traffic Advisory (TA) is issued when an intruder is roughly 20 to 48 seconds from CPA, depending on altitude. The aural message is "TRAFFIC, TRAFFIC" and the symbol turns amber. The TA prepares the crew for a possible RA and aids visual search.

Pilots must not manoeuvre on a TA alone. Bearing is coarse, closure in the vertical is hard to judge from a display, and an improvised climb or turn can create a new conflict or cancel out the coordinated RA that follows. Airbus lists it among its memory items: look out, prepare, do not manoeuvre.

Resolution advisories

A Resolution Advisory (RA) comes roughly 15 to 35 seconds from CPA. The symbol turns red and the vertical speed scale or PFD pitch cue shows a red zone to avoid and, where a rate is required, a green zone to fly. A corrective RA demands a change of vertical speed; a preventive RA ("MONITOR VERTICAL SPEED") tells the crew that the present rate is safe but certain changes are not. Exam questions on the preventive vs. corrective resolution advisory distinction turn on this: the preventive RA needs no manoeuvre if the aircraft is already outside the red.

The logic assumes the pilot starts the manoeuvre within about 5 seconds, with a pitch change of about 0.25 g, and responds to a strengthened or reversed RA within about 2.5 seconds with about 0.35 g. Overreacting is also an error: excessive rates can conflict with other traffic and injure cabin occupants. Crews normally disconnect the autopilot and fly the RA by hand; some Airbus types have an AP/FD TCAS mode that flies it, and Airbus procedures call for a go-around on a climb RA in landing configuration.

PANS-OPS, EASA rules and the FAA AIM (4-4-16) and 14 CFR 91.123 converge on the same pilot duties:

• follow the RA immediately unless doing so would jeopardise the safety of the aircraft, even if it conflicts with an ATC instruction;
• never manoeuvre in the opposite sense to an RA;
• do not rely on visual acquisition: the aircraft in sight may not be the one causing the RA;
• stall, windshear and GPWS or TAWS warnings take precedence;
• report "TCAS RA" as soon as workload permits, "UNABLE, TCAS RA" if ATC then issues a contrary clearance, and "CLEAR OF CONFLICT, RETURNING TO (clearance)" followed by "CLEAR OF CONFLICT, (clearance) RESUMED".

Once the aircraft departs from its clearance to comply with an RA, the controller ceases to be responsible for separation between it and any aircraft affected as a direct consequence, must not try to modify its flight path, and resumes responsibility when the crew reports the clearance resumed. An RA is a reportable occurrence under EASA occurrence reporting rules; in the United States an RA that must be complied with on an IFR flight to avert a substantial risk of collision is also notifiable to the NTSB.

Crossing and reversal RAs

A crossing RA directs the aircraft through the intruder's altitude, for example "CLIMB, CROSSING CLIMB" while the intruder is still above. TCAS issues it only when crossing gives more vertical separation at CPA than staying on the same side. It feels wrong, but it must be flown.

An RA reversal changes the sense mid-encounter, from climb to descend or the reverse, with "CLIMB, CLIMB NOW" or "DESCEND, DESCEND NOW". It usually means that the other aircraft is not following its own RA or is manoeuvring the same way. The response window shrinks to about 2.5 seconds.

At Überlingen on 1 July 2002 a Tupolev Tu-154M and a Boeing 757 freighter collided at night; all 71 people on board died. The 757 crew followed a descend RA. The Tu-154M crew had been told to descend by the controller, received a climb RA seconds later and kept descending on the ATC instruction. The German investigator, the BFU, recommended resolving any ambiguity in favour of TCAS, and ICAO's procedures were strengthened accordingly.

Version 7.1 made two changes to the logic. It corrected the reversal logic (change proposal CP112E) so that a reversal is issued when the other aircraft moves against its RA, the weakness exposed by the Überlingen geometry. It also replaced "ADJUST VERTICAL SPEED, ADJUST" with "LEVEL OFF, LEVEL OFF" (CP115), because crews too often responded to the old message by increasing their rate instead of reducing it.

Sensitivity levels and TA-only mode

The TCAS sensitivity level sets the tau thresholds and the target vertical miss distance (ALIM). It is chosen automatically from own altitude, using radio altitude low down. Level 1 is standby and level 2 gives TAs only.

The larger values at altitude reflect higher closure speeds and larger altimetry errors. The figures of about 40 and 25 seconds quoted in Airbus and Boeing manuals are the level 5 values.

TA only mode, selected on the transponder panel, forces sensitivity level 2 at any altitude: all RAs are suppressed and the TA threshold becomes 20 seconds. Operators specify when to use it, typically after an engine failure, when the aircraft could not follow a climb RA, or for procedures such as closely spaced parallel approaches. TA/RA is otherwise the normal mode from line-up to vacating the runway. An aircraft in TA ONLY still replies to other TCAS aircraft, which receive normal RAs.

RA inhibition and reduced surveillance

TCAS RA inhibition near the ground stops the system commanding a descent into terrain:

• Increase Descent RAs are inhibited below about 1,550 ft AGL (1,450 ft when descending, 1,650 ft when climbing);
• Descend RAs are inhibited below about 1,100 ft AGL;
• all RAs are inhibited below about 1,000 ft AGL, where the system reverts to TA only;
• all TCAS aural messages are inhibited below about 500 ft AGL.

Each threshold has about 100 ft of hysteresis. Climb and Increase Climb RAs are also inhibited where the aircraft cannot achieve them, for instance near its performance ceiling or in some configurations, and TCAS then picks another solution. On most installations a GPWS or windshear warning also inhibits TCAS alerts.

TCAS reduced surveillance limits interrogation load. Distant Mode S intruders that are not predicted to become threats are interrogated once every 5 seconds instead of every second. Interference limiting lowers interrogation power as the number of nearby TCAS aircraft rises, which can shorten effective range in very dense traffic. Hybrid surveillance, an option in versions 7.0 and 7.1, tracks non-threatening aircraft passively from their ADS-B squitters and switches back to active interrogation as they close.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.