[{"data":1,"prerenderedAt":417},["ShallowReactive",2],{"learn-\u002Flearn\u002Ftcas-and-airborne-collision-avoidance":3,"learn-nav-\u002Flearn\u002Ftcas-and-airborne-collision-avoidance":397},{"id":4,"title":5,"body":6,"date":317,"dateModified":318,"description":319,"draft":320,"extension":321,"faqs":322,"howTo":332,"keyTakeaways":341,"meta":348,"metaDescription":349,"navigation":350,"path":351,"quiz":352,"seo":379,"series":318,"seriesOrder":318,"sources":380,"stem":394,"topic":395,"__hash__":396},"learn\u002Flearn\u002Ftcas-and-airborne-collision-avoidance.md","TCAS and airborne collision avoidance",{"type":7,"value":8,"toc":306},"minimark",[9,13,19,24,61,73,77,90,113,135,148,152,155,186,202,206,237,241,254,258,292,296],[10,11,12],"p",{},"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.",[14,15,16],"blockquote",{},[10,17,18],{},"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.",[20,21,23],"h2",{"id":22},"a-safety-net-that-talks-to-transponders","A safety net that talks to transponders",[10,25,26,27,31,32,39,40,43,44,47,48,51,52,56,57,60],{},"The international standard is ",[28,29,30],"strong",{},"ACAS"," (airborne collision avoidance system), specified in ",[33,34,38],"a",{"href":35,"rel":36},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fannex-10-aeronautical-telecommunications-volume-iv-surveillance-radar-and-collision-avoidance-systems",[37],"nofollow","ICAO Annex 10, Volume IV","; ",[28,41,42],{},"TCAS II"," is the equipment that implements it, and ",[28,45,46],{},"version 7.1"," is the current standard. The principle is elegantly self-contained: your TCAS ",[28,49,50],{},"interrogates the transponders"," of aircraft around you, exactly as a ground radar would, times the replies to measure range, and reads the altitude the ",[33,53,55],{"href":54},"\u002Flearn\u002Fsquawk-codes-and-transponders","Mode C or Mode S transponder"," reports. From successive replies it builds each intruder's track and computes the ",[28,58,59],{},"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.",[10,62,63,64,67,68,72],{},"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 ",[28,65,66],{},"no operating transponder does not exist"," as far as TCAS is concerned, which is one more reason the ",[33,69,71],{"href":70},"\u002Flearn\u002Fright-of-way-rules","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.",[20,74,76],{"id":75},"ta-then-ra-two-rings-of-alarm","TA then RA: two rings of alarm",[10,78,79,80,83,84,89],{},"TCAS thinks in ",[28,81,82],{},"time, not distance",". Alerts fire on tau, the projected time to the closest point of approach, with thresholds that vary with altitude, per the ",[33,85,88],{"href":86,"rel":87},"https:\u002F\u002Fwww.eurocontrol.int\u002Fpublication\u002Fairborne-collision-avoidance-system-acas-guide",[37],"EUROCONTROL ACAS Guide",".",[10,91,92,93,96,97,100,101,104,105,108,109,112],{},"The outer ring is the ",[28,94,95],{},"traffic advisory (TA)",", announced as ",[28,98,99],{},"\"Traffic, traffic\"",", typically ",[28,102,103],{},"20 to 48 seconds"," before the predicted closest approach. The intruder turns amber on the traffic display. A TA asks three things of you: ",[28,106,107],{},"look for the traffic",", tighten up the flying, and ",[28,110,111],{},"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.",[10,114,115,116,100,119,122,123,126,127,130,131,134],{},"The inner ring is the ",[28,117,118],{},"resolution advisory (RA)",[28,120,121],{},"15 to 35 seconds"," from closest approach, and it is a command. TCAS II resolves conflicts ",[28,124,125],{},"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 ",[28,128,129],{},"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 ",[28,132,133],{},"\"Clear of conflict\"",", then return to your clearance.",[10,136,137,138,147],{},"When both aircraft carry TCAS II, the two units ",[28,139,140,141],{},"coordinate through ",[33,142,146],{"href":143,"className":144},"\u002Flearn\u002Fglossary#gt-mode-s",[145],"glossary-link","Mode S",": they negotiate complementary manoeuvres, one climbing while the other descends. That coordination is the deep reason for the iron rule that follows.",[20,149,151],{"id":150},"the-ra-outranks-the-controller","The RA outranks the controller",[10,153,154],{},"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.",[10,156,157,158,167,168,173,174,177,178,181,182,185],{},"The doctrine that hardened worldwide after Uberlingen is now written into ",[33,159,162,163,166],{"href":160,"rel":161},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fprocedures-for-air-navigation-services-pans-aircraft-operations-volume-iii-aircraft-operating-procedures-doc-8168",[37],"ICAO PANS-OPS ",[28,164,165],{},"Volume III",", Section 4, Chapter 3",", which is where the ACAS operating procedures live (Volume I contains none), and into the ",[33,169,172],{"href":170,"rel":171},"https:\u002F\u002Fwww.faa.gov\u002Fair_traffic\u002Fpublications\u002Fatpubs\u002Faim_html\u002Fchap4_section_4.html",[37],"FAA AIM",": ",[28,175,176],{},"once an RA is issued, follow it",", even when it contradicts an ATC instruction, and ",[28,179,180],{},"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 ",[28,183,184],{},"\"TCAS RA\"","; the controller then stops issuing instructions to the involved aircraft until you report returning to the clearance after \"Clear of conflict\".",[10,187,188,189,192,193,197,198,201],{},"The safety net has its own edges. Close to the ground the escape options shrink, so ",[28,190,191],{},"RAs are progressively inhibited, and below roughly 1000 ft above ground level TCAS stops issuing them"," altogether. And a genuine stall warning or a ",[33,194,196],{"href":195},"\u002Flearn\u002Fwind-shear-explained","wind shear"," or ground proximity warning ",[28,199,200],{},"takes priority over an RA",": the aircraft must be kept flying before it is kept separated.",[20,203,205],{"id":204},"who-must-carry-it","Who must carry it",[10,207,208,209,214,215,218,219,222,223,226,227,173,231,236],{},"Equipage is a matter of each authority's rules. In European airspace, ",[33,210,213],{"href":211,"rel":212},"https:\u002F\u002Feur-lex.europa.eu\u002Flegal-content\u002FEN\u002FTXT\u002F?uri=CELEX%3A32011R1332",[37],"Regulation (EU) No 1332\u002F2011"," requires ",[28,216,217],{},"ACAS II version 7.1"," aboard turbine-powered aeroplanes with a maximum certificated take-off mass over ",[28,220,221],{},"5700 kg"," or more than ",[28,224,225],{},"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 ",[33,228,230],{"href":229},"\u002Flearn\u002Fmode-s-and-ads-b-explained","Mode S and ADS-B",[33,232,235],{"href":233,"className":234},"\u002Flearn\u002Fglossary#gt-ads-b",[145],"ADS-B"," broadcasts your position to anyone listening, while TCAS actively interrogates and, uniquely, fights back.",[20,238,240],{"id":239},"a-worked-example","A worked example",[10,242,243,244,246,247,250,251,253],{},"In the cruise, the traffic display paints an amber intruder and the flight deck announces ",[28,245,99],{},": 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 ",[28,248,249],{},"\"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 ",[28,252,133],{},". 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.",[20,255,257],{"id":256},"common-pitfalls","Common pitfalls",[259,260,261,268,274,280,286],"ul",{},[262,263,264,267],"li",{},[28,265,266],{},"Manoeuvring on a TA."," \"Traffic, traffic\" means look and prepare; uncommanded avoidance this early can create the conflict.",[262,269,270,273],{},[28,271,272],{},"Following ATC against an active RA."," The RA is coordinated with the other aircraft; the controller's instruction is not.",[262,275,276,279],{},[28,277,278],{},"Manoeuvring opposite the RA, or overshooting it."," Fly the commanded band, promptly and smoothly, nothing more.",[262,281,282,285],{},[28,283,284],{},"Expecting protection from every aircraft."," No transponder means no TCAS protection; altitude-less targets can trigger only TAs.",[262,287,288,291],{},[28,289,290],{},"Forgetting the aftermath."," After \"Clear of conflict\", return to the clearance, tell ATC, and make the required reports.",[20,293,295],{"id":294},"in-pilot-efb","In Pilot EFB",[10,297,298,299,301,302,305],{},"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 ",[33,300,230],{"href":229}," and ",[33,303,304],{"href":54},"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.",{"title":307,"searchDepth":308,"depth":308,"links":309},"",2,[310,311,312,313,314,315,316],{"id":22,"depth":308,"text":23},{"id":75,"depth":308,"text":76},{"id":150,"depth":308,"text":151},{"id":204,"depth":308,"text":205},{"id":239,"depth":308,"text":240},{"id":256,"depth":308,"text":257},{"id":294,"depth":308,"text":295},"2026-07-10",null,"How TCAS watches the sky through other aircraft's transponders, what a traffic advisory and a resolution advisory demand of you, and why the RA outranks ATC.",false,"md",[323,326,329],{"q":324,"a":325},"What is the difference between a TA and an RA?","A traffic advisory (TA), announced as 'Traffic, traffic', is an alert: it tells the crew a nearby aircraft may become a threat, so they look for it and prepare to act, but they do not manoeuvre on a TA alone. A resolution advisory (RA) is a command: the system has judged a collision risk and issues vertical guidance, such as 'Climb, climb' or 'Descend, descend', which the crew must follow promptly and smoothly. The TA typically comes roughly 20 to 48 seconds before the predicted closest point of approach and the RA roughly 15 to 35 seconds, depending on altitude.",{"q":327,"a":328},"Can TCAS see every aircraft?","No. TCAS works by interrogating other aircraft's transponders, so an aircraft with no transponder, or with its transponder off or failed, is invisible to it. Against a target whose transponder does not report altitude, TCAS can only issue a traffic advisory, never a resolution advisory, because it cannot compute safe vertical guidance without knowing the intruder's level. See and avoid still matters.",{"q":330,"a":331},"What should the crew do if an ATC instruction contradicts a TCAS RA?","Follow the RA. Once a resolution advisory is issued, its guidance takes precedence over ATC instructions, because TCAS II units on both aircraft coordinate complementary manoeuvres with each other through Mode S and a controller cannot see that negotiation. The crew flies the RA, tells ATC as soon as workload permits using the phrase 'TCAS RA', and returns to the clearance once 'Clear of conflict' is announced. This doctrine was hardened worldwide after the 2002 Uberlingen midair collision.",{"name":333,"steps":334},"How to respond to a TCAS resolution advisory",[335,336,337,338,339,340],"Respond promptly and smoothly: adjust the flight path to follow the RA's vertical guidance, without aggressive manoeuvring.","Never manoeuvre in the opposite sense to the RA: on TCAS II equipped traffic the advisories on the two aircraft are coordinated, and reversing yours defeats both.","Do not follow an ATC instruction that contradicts the RA while the RA is active; the advisory takes precedence.","Inform ATC as soon as workload permits, using the standard phrase 'TCAS RA'.","Follow any strengthening, weakening or reversal the system announces until it declares 'Clear of conflict'.","After 'Clear of conflict', return promptly to the last ATC clearance and tell the controller you are doing so.",[342,343,344,345,346,347],"TCAS is independent of ATC: it interrogates nearby transponders directly and times the replies, so a non-transponder aircraft is invisible to it.","A TA ('Traffic, traffic') means look and prepare, not manoeuvre; an RA is a command to fly the indicated vertical guidance promptly and smoothly.","TCAS II resolves conflicts vertically only, and paired units coordinate complementary RAs with each other through Mode S.","An RA takes precedence over ATC instructions; report 'TCAS RA' when able and return to the clearance after 'Clear of conflict'.","Alerts are driven by time to closest approach, roughly 20 to 48 seconds for a TA and 15 to 35 seconds for an RA, and RAs are inhibited close to the ground.","In European airspace, ACAS II version 7.1 is required for turbine aeroplanes over 5700 kg MTOM or with more than 19 passenger seats.",{},"How TCAS works through transponders, what a TA and an RA each demand of the crew, and why following the resolution advisory outranks ATC.",true,"\u002Flearn\u002Ftcas-and-airborne-collision-avoidance",[353,362,370],{"q":354,"options":355,"answer":360,"explanation":361},"A TCAS resolution advisory tells you to climb, but ATC has just instructed you to descend. What do you do?",[356,357,358,359],"Follow ATC, because controllers see the whole picture","Follow the RA, and tell ATC 'TCAS RA' when workload permits","Split the difference and hold level flight","Ask ATC to confirm before manoeuvring",1,"Once an RA is issued it takes precedence over ATC instructions, because paired TCAS II units coordinate complementary manoeuvres that the controller cannot see. Fly the RA, report 'TCAS RA', and return to the clearance after 'Clear of conflict'.",{"q":363,"options":364,"answer":308,"explanation":369},"What does TCAS need from another aircraft to generate a resolution advisory against it?",[365,366,367,368],"An ADS-B position report","A flight plan in the ATC system","An operating transponder that reports altitude","Radio contact on the same frequency","TCAS interrogates transponders. Without altitude reporting it can issue at most a traffic advisory, and an aircraft with no operating transponder is invisible to TCAS entirely.",{"q":371,"options":372,"answer":377,"explanation":378},"What manoeuvres can a TCAS II resolution advisory command?",[373,374,375,376],"Vertical manoeuvres only, such as climb or descend","Turns only","Combined turns and climbs","Speed changes only",0,"TCAS II resolves conflicts in the vertical plane only: climb, descend, level off, or adjust vertical speed. It never commands a turn.",{"title":5,"description":319},[381,383,385,387,389,391],{"label":382,"url":35},"ICAO Annex 10: Aeronautical Telecommunications, Volume IV (Surveillance and Collision Avoidance Systems)",{"label":384,"url":86},"EUROCONTROL: Airborne Collision Avoidance System (ACAS) Guide",{"label":386,"url":170},"FAA Aeronautical Information Manual, Chapter 4 Section 4 (ATC clearances; TCAS and RA response)",{"label":388,"url":160},"ICAO Doc 8168: PANS-OPS, Volume III (Aircraft Operating Procedures), Section 4 Chapter 3: ACAS operating procedures",{"label":390,"url":211},"Regulation (EU) No 1332\u002F2011: ACAS II equipage and version 7.1",{"label":392,"url":393},"SKYbrary: Airborne Collision Avoidance System (ACAS)","https:\u002F\u002Fskybrary.aero\u002Farticles\u002Fairborne-collision-avoidance-system-acas","learn\u002Ftcas-and-airborne-collision-avoidance","Operations","J37CAXoN_N8ubfhDrtTw2QXa1_rmHs0rRvWIn9_ZroQ",{"related":398,"newer":318,"older":399,"series":318},[399,405,411],{"path":400,"title":401,"description":402,"date":403,"topic":395,"draft":320,"minutes":404,"series":318,"seriesOrder":318},"\u002Flearn\u002Frnav-rnp-and-gnss-explained","RNAV, RNP and GNSS explained","What performance-based navigation means: how GNSS fixes a position, what RAIM and SBAS add, RNAV versus RNP, and how to read the minima on an RNP approach.","2026-07-09",8,{"path":406,"title":407,"description":408,"date":409,"topic":395,"draft":320,"minutes":410,"series":318,"seriesOrder":318},"\u002Flearn\u002Fvor-dme-and-ndb-explained","VOR, DME and NDB explained","How the classic ground-based navaids work: VOR radials and the CDI, DME slant range, the NDB and ADF, IFR accuracy checks, and where each fits in a GNSS world.","2026-07-08",9,{"path":412,"title":413,"description":414,"date":415,"topic":395,"draft":320,"minutes":416,"series":318,"seriesOrder":318},"\u002Flearn\u002Fv-speeds-explained","V-speeds explained","What the V in V-speeds means, the difference between a design limit and an operating speed, and a tour of the speeds a pilot lives by, from stall to Vne.","2026-06-20",4,1783767261224]