Every day, somewhere, a quiet electronic negotiation between two aircraft prevents a midair collision. TCAS, the traffic alert and collision avoidance system, is the last technical safety net when separation has failed: it watches the sky through other aircraft's transponders, decides in seconds whether a collision threatens, and if necessary commands an escape manoeuvre that outranks the controller. Knowing exactly what it is telling you, and what it expects back, is not optional knowledge in a transponder-equipped sky.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
A safety net that talks to transponders
The international standard is ACAS (airborne collision avoidance system), specified in ICAO Annex 10, Volume IV; TCAS II is the equipment that implements it, and version 7.1 is the current standard. The principle is elegantly self-contained: your TCAS interrogates the transponders of aircraft around you, exactly as a ground radar would, times the replies to measure range, and reads the altitude the Mode C or Mode S transponder reports. From successive replies it builds each intruder's track and computes the time to the closest point of approach. Everything happens aircraft to aircraft: no ground station, no controller, no flight plan is involved, which is precisely why it still works when the rest of the system has failed.
The corollary deserves emphasis. TCAS can only protect you from aircraft it can hear. A target whose transponder reports no altitude can trigger at most a traffic advisory. An aircraft with no operating transponder does not exist as far as TCAS is concerned, which is one more reason the see-and-avoid rules never lapse, however sophisticated the equipment. TCAS I, fitted to some smaller transport aircraft, gives traffic advisories only; this guide describes TCAS II, which adds resolution advisories.
TA then RA: two rings of alarm
TCAS thinks in time, not distance. Alerts fire on tau, the projected time to the closest point of approach, with thresholds that vary with altitude, per the EUROCONTROL ACAS Guide.
The outer ring is the traffic advisory (TA), announced as "Traffic, traffic", typically 20 to 48 seconds before the predicted closest approach. The intruder turns amber on the traffic display. A TA asks three things of you: look for the traffic, tighten up the flying, and prepare for a possible RA. What it does not ask is a manoeuvre: crews do not climb, descend or turn on a TA alone, because the system has not yet judged the geometry dangerous and an uncommanded excursion can create the very conflict it fears.
The inner ring is the resolution advisory (RA), typically 15 to 35 seconds from closest approach, and it is a command. TCAS II resolves conflicts in the vertical plane only: "Climb, climb", "Descend, descend", "Level off, level off", or an instruction to adjust or maintain vertical speed, shown on the vertical speed indication and spoken aloud. The required response is prompt and smooth, an ordinary-firm pitch change flown within seconds, not a violent escape. The system may strengthen ("Increase climb"), weaken, or even reverse the advisory as the geometry evolves; you follow it until it announces "Clear of conflict", then return to your clearance.
When both aircraft carry TCAS II, the two units coordinate through Mode S: they negotiate complementary manoeuvres, one climbing while the other descends. That coordination is the deep reason for the iron rule that follows.
The RA outranks the controller
On 1 July 2002, over Uberlingen in southern Germany, a Tu-154 and a Boeing 757 collided at night. Both aircraft carried TCAS, and TCAS had resolved the conflict, commanding one to climb and the other to descend. But a controller, unaware of the RAs, instructed the Tu-154 to descend; its crew followed the controller while the 757 followed its RA, and both aircraft descended into each other. Seventy-one people died.
The doctrine that hardened worldwide after Uberlingen is now written into ICAO PANS-OPS Volume III, Section 4, Chapter 3, which is where the ACAS operating procedures live (Volume I contains none), and into the FAA AIM: once an RA is issued, follow it, even when it contradicts an ATC instruction, and never manoeuvre opposite to it. The controller cannot see the coordination running between the two aircraft, so overriding the RA breaks the one plan both machines have agreed on. Tell ATC as soon as workload permits, with the standard phrase "TCAS RA"; the controller then stops issuing instructions to the involved aircraft until you report returning to the clearance after "Clear of conflict".
The safety net has its own edges. Close to the ground the escape options shrink, so RAs are progressively inhibited, and below roughly 1000 ft above ground level TCAS stops issuing them altogether. And a genuine stall warning or a wind shear or ground proximity warning takes priority over an RA: the aircraft must be kept flying before it is kept separated.
Who must carry it
Equipage is a matter of each authority's rules. In European airspace, Regulation (EU) No 1332/2011 requires ACAS II version 7.1 aboard turbine-powered aeroplanes with a maximum certificated take-off mass over 5700 kg or more than 19 passenger seats; US airline rules impose equivalent TCAS II requirements on passenger transport aircraft. TCAS also complements, rather than replaces, the wider surveillance picture built by Mode S and ADS-B: ADS-B broadcasts your position to anyone listening, while TCAS actively interrogates and, uniquely, fights back.
A worked example
In the cruise, the traffic display paints an amber intruder and the flight deck announces "Traffic, traffic": converging traffic 1000 ft below, climbing. You and the other pilot look, find nothing yet in the haze, and stop the non-essential tasks. Fifteen seconds later the display turns red and the system commands "Climb, climb". The pilot flying pitches up smoothly to put the vertical speed needle into the green band, the pilot monitoring transmits "climbing, TCAS RA", and ATC acknowledges and goes quiet. The intruder flashes past below, the system softens the advisory, then announces "Clear of conflict". You report returning to the assigned level, settle back into the cruise, and file the report your operator requires. Total elapsed time: under a minute. That is TCAS working exactly as designed, and the crew's whole contribution was to believe it promptly.
Common pitfalls
- Manoeuvring on a TA. "Traffic, traffic" means look and prepare; uncommanded avoidance this early can create the conflict.
- Following ATC against an active RA. The RA is coordinated with the other aircraft; the controller's instruction is not.
- Manoeuvring opposite the RA, or overshooting it. Fly the commanded band, promptly and smoothly, nothing more.
- Expecting protection from every aircraft. No transponder means no TCAS protection; altitude-less targets can trigger only TAs.
- Forgetting the aftermath. After "Clear of conflict", return to the clearance, tell ATC, and make the required reports.
In Pilot EFB
Pilot EFB is a study and planning companion, and collision avoidance sits entirely with your aircraft's certified equipment and your eyes: the app has no traffic display, receives no transponder or TCAS data, and plays no role in the air. Where it helps is beforehand and afterwards: understanding the system here in Learn alongside the Mode S and ADS-B and squawk code guides, and keeping your own records and notes organised in one offline-first place. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and rely on your certified avionics, your training and ATC in the air.