[{"data":1,"prerenderedAt":490},["ShallowReactive",2],{"learn-\u002Flearn\u002Fthe-ils-explained":3,"learn-nav-\u002Flearn\u002Fthe-ils-explained":445},{"id":4,"title":5,"body":6,"date":377,"dateModified":378,"description":379,"draft":380,"extension":381,"faqs":382,"howTo":378,"keyTakeaways":392,"meta":398,"metaDescription":399,"navigation":400,"path":401,"quiz":402,"seo":429,"series":430,"seriesOrder":366,"sources":431,"stem":442,"topic":443,"__hash__":444},"learn\u002Flearn\u002Fthe-ils-explained.md","The ILS explained",{"type":7,"value":8,"toc":364},"minimark",[9,13,19,24,44,55,69,72,76,83,107,114,118,139,193,200,204,226,230,245,260,264,275,279,291,303,316,320,352,356],[10,11,12],"p",{},"The instrument landing system has guided aircraft onto runways in poor weather for the best part of a century, and it is still the benchmark precision approach. Under the jargon it is a simple idea: two radio beams, one for left and right, one for up and down, crossed in space to mark a single path down to the runway. Once you see it that way, the rest is detail.",[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},"two-beams-that-make-a-path","Two beams that make a path",[10,25,26,27,31,32,35,36,43],{},"An ILS is a ",[28,29,30],"strong",{},"precision"," approach because it gives guidance in ",[28,33,34],{},"both"," the lateral and the vertical plane, and it does so with two separate transmitters described in ",[37,38,42],"a",{"href":39,"rel":40},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fannex-10-aeronautical-telecommunications-volume-i-radio-navigational-aids",[41],"nofollow","ICAO Annex 10",".",[10,45,46,47,50,51,54],{},"The ",[28,48,49],{},"localiser"," provides ",[28,52,53],{},"lateral"," guidance. It is a VHF transmitter at the far end of the runway, radiating a beam aligned with the runway centreline, and your instrument shows whether you are left or right of that extended centreline. Fly to centre the needle and you track straight at the runway.",[10,56,46,57,60,61,64,65,68],{},[28,58,59],{},"glideslope"," (or glide path) provides ",[28,62,63],{},"vertical"," guidance. It is a UHF transmitter beside the touchdown zone, radiating a beam angled ",[28,66,67],{},"up from the ground, typically at about 3 degrees",", and your instrument shows whether you are above or below that descent path. Fly to centre this needle and you descend at the correct angle.",[10,70,71],{},"Cross the two and you have a single line in space, the right side-to-side position and the right height at every point, that leads down to the runway. The instrument that displays both, needles crossing at the centre when you are on the path, is the heart of flying an ILS.",[20,73,75],{"id":74},"the-markers-and-the-distance-cues","The markers and the distance cues",[10,77,78,79,82],{},"Older ILS installations added ",[28,80,81],{},"marker beacons",", low-power transmitters on the approach that flashed a light and sounded a tone as you passed over them, to tell you how far along the approach you were:",[84,85,86,94,100],"ul",{},[87,88,89,90,93],"li",{},"the ",[28,91,92],{},"outer marker",", several miles out, near where the glideslope is intercepted and the final descent begins;",[87,95,89,96,99],{},[28,97,98],{},"middle marker",", close to the CAT I decision height;",[87,101,102,103,106],{},"and, for lower categories, an ",[28,104,105],{},"inner marker"," nearer the threshold.",[10,108,109,110,113],{},"Many installations have replaced the markers with ",[28,111,112],{},"DME"," distance or named fixes, which give a continuous distance rather than a single beep, but the idea is the same: a way to cross-check your progress and your height against distance as you come down the beam. That cross-check is also your defence against following a false glideslope signal, by confirming you are at the right altitude for the distance.",[20,115,117],{"id":116},"the-categories-how-low-you-can-go","The categories: how low you can go",[10,119,120,121,124,125,128,129,132,133,138],{},"Not every ILS, and not every aircraft or crew, can be used down to the same minima. The ",[28,122,123],{},"categories"," describe how low the ",[28,126,127],{},"decision height"," and how poor the ",[28,130,131],{},"runway visual range (RVR)"," may be, and they sit in ",[37,134,137],{"href":135,"rel":136},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fprocedures-for-air-navigation-services-pans-aircraft-operations-volume-i-flight-procedures-doc-8168",[41],"ICAO Doc 8168"," and the operating rules:",[84,140,141,155,167],{},[87,142,143,146,147,150,151,154],{},[28,144,145],{},"Category I (CAT I):"," a decision height ",[28,148,149],{},"no lower than 200 ft",", and an RVR ",[28,152,153],{},"no less than about 550 m"," (or a prescribed visibility). This is the standard ILS most pilots fly.",[87,156,157,146,160,150,163,166],{},[28,158,159],{},"Category II (CAT II):",[28,161,162],{},"below 200 ft but not below 100 ft",[28,164,165],{},"no less than 300 m",". It needs more capable ground equipment, a radio altimeter and specific crew and aircraft approval.",[87,168,169,172,173,176,177,180,181,184,185,188,189,192],{},[28,170,171],{},"Category III (CAT III):"," lower again, subdivided by decision height and RVR. CAT IIIA reaches a decision height ",[28,174,175],{},"below 100 ft"," with RVR not less than ",[28,178,179],{},"200 m"," (the long-standing operational value; current ICAO gives 175 m); CAT IIIB goes to ",[28,182,183],{},"below 50 ft or no decision height"," with RVR down to ",[28,186,187],{},"50 m","; CAT IIIC has ",[28,190,191],{},"no decision height and no RVR limit",". These demand the most capable systems, including autoland, and full approvals.",[10,194,195,196,199],{},"The headline is that lower categories are not just about the weather; they require progressively more capable equipment ",[28,197,198],{},"on the ground, in the aircraft, and in the crew's training",". The category is the combination, not the cloud base alone.",[20,201,203],{"id":202},"when-part-of-it-fails","When part of it fails",[10,205,206,207,209,210,212,213,216,217,221,222,225],{},"The ILS degrades gracefully, which is part of why it has lasted. If the ",[28,208,59],{}," fails or is unavailable, you keep the ",[28,211,49],{},", so the approach becomes a ",[28,214,215],{},"localiser-only"," approach, a ",[37,218,220],{"href":219},"\u002Flearn\u002Fnon-precision-approaches","non-precision approach"," flown to a ",[28,223,224],{},"minimum descent altitude"," rather than a decision height. You stay aligned with the runway by the localiser and descend by reference to altitudes and distances instead of riding the beam down. If the localiser too is lost, the ILS is no longer usable and another procedure is needed. Knowing which guidance you still have, and therefore which minima apply, is a basic part of flying the approach.",[20,227,229],{"id":228},"intercepting-and-monitoring-the-beam","Intercepting and monitoring the beam",[10,231,232,233,236,237,240,241,244],{},"How you join the ILS matters as much as the beams themselves. The standard sequence is to intercept the ",[28,234,235],{},"localiser first",", in level flight, and then capture the ",[28,238,239],{},"glideslope from below"," as it descends to meet you. Intercepting the glideslope from above is discouraged, because the transmitter produces weaker ",[28,242,243],{},"false glideslope"," signals at steeper angles, and capturing one of those would send you down far too fast. Joining from below, at the published altitude, avoids the trap.",[10,246,247,248,251,252,255,256,259],{},"Two habits guard the approach. The first is to ",[28,249,250],{},"identify"," the facility by its Morse-code ident before relying on it, confirming you have tuned the right ILS and that it is serviceable. The second is a ",[28,253,254],{},"gross-error check",": as you pass the final approach fix or outer marker, your altimeter should read the published glideslope-crossing height, and if it does not, the glideslope picture is suspect and you should not follow it down. Even with the autopilot flying the approach, you keep monitoring the ",[28,257,258],{},"raw localiser and glideslope"," needles, because the displays are the truth and the automation is only as good as the signal it is tracking.",[20,261,263],{"id":262},"a-modern-relative","A modern relative",[10,265,266,267,270,271,274],{},"The ILS is no longer the only way to fly a guided, ILS-like descent. Satellite-based approaches, the ",[28,268,269],{},"LPV"," approaches flown with augmented GNSS, and ",[28,272,273],{},"GLS"," approaches flown with a ground-based augmentation system, present the pilot with the same picture, a lateral track and a glidepath to follow down to a decision altitude, without any localiser or glideslope antenna on the airfield. They are not an ILS, and their minima and equipment requirements are their own, but they are worth knowing as the family the ILS now belongs to: the precise, guided approach is increasingly delivered by satellites as well as by the two classic beams.",[20,276,278],{"id":277},"a-worked-example","A worked example",[10,280,281,282,284,285,287,288,290],{},"You are vectored toward the final approach for an ILS. First you intercept the ",[28,283,49],{},": the lateral needle comes alive and you turn to centre it, now tracking the extended runway centreline. You hold your altitude until the ",[28,286,59],{}," needle, which has been pinned at the top, begins to move down toward the centre. As it reaches the centre you start your descent to keep ",[28,289,34],{}," needles crossed, settling onto the roughly 3-degree path, and you cross-check your height against the DME or the marker as you pass it: at the right distance you should be at the right altitude.",[10,292,293,294,297,298,302],{},"You continue down the beam, both needles centred, to the ",[28,295,296],{},"CAT I decision height of 200 ft",". There you make the decision the whole approach has been building to, the subject of our guide to ",[37,299,301],{"href":300},"\u002Flearn\u002Fdecision-altitude-and-mda","decision altitude and the MDA",": if you have the required visual reference and the landing is assured, you land; if not, you fly the missed approach.",[10,304,305,306,309,310,312,313,315],{},"Now replay the approach with a ",[28,307,308],{},"glideslope failure"," as you intercept. The vertical needle flags as unreliable, so you switch to the ",[28,311,215],{}," minima: you keep the localiser centred for alignment, but you now descend by steps to a ",[28,314,224],{},", higher than the 200 ft decision height, and look for the runway from there. The lateral picture is unchanged; the vertical picture, and the minimum, are not.",[20,317,319],{"id":318},"common-pitfalls","Common pitfalls",[84,321,322,328,334,340,346],{},[87,323,324,327],{},[28,325,326],{},"Treating the ILS as one thing."," It is two independent beams; you can have one without the other, and the minima change accordingly.",[87,329,330,333],{},[28,331,332],{},"Forgetting the distance cross-check."," Confirming altitude against distance guards against following a false glideslope.",[87,335,336,339],{},[28,337,338],{},"Thinking the category is only about weather."," Lower categories require more capable ground equipment, aircraft and crew approval, not just a lower cloud base.",[87,341,342,345],{},[28,343,344],{},"Confusing a localiser-only approach with a full ILS."," Without the glideslope it is a non-precision approach to an MDA, not a decision height.",[87,347,348,351],{},[28,349,350],{},"Ignoring which minima apply."," The decision height or MDA depends on the category and on what guidance you actually have.",[20,353,355],{"id":354},"in-pilot-efb","In Pilot EFB",[10,357,358,359,363],{},"Pilot EFB is a study and planning companion for instrument procedures, helping you learn how an ILS is built and where its minima come from alongside the ",[37,360,362],{"href":361},"\u002Flearn\u002Freading-an-instrument-approach-chart","approach chart"," and the rest of your offline-first briefing. It does not fly the approach, display live guidance, or set your minima, and the procedure you fly comes from the current chart and your approved minima. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and fly the approach from your official source of record.",{"title":365,"searchDepth":366,"depth":366,"links":367},"",2,[368,369,370,371,372,373,374,375,376],{"id":22,"depth":366,"text":23},{"id":74,"depth":366,"text":75},{"id":116,"depth":366,"text":117},{"id":202,"depth":366,"text":203},{"id":228,"depth":366,"text":229},{"id":262,"depth":366,"text":263},{"id":277,"depth":366,"text":278},{"id":318,"depth":366,"text":319},{"id":354,"depth":366,"text":355},"2026-03-31",null,"How an instrument landing system works: the localiser, the glideslope, the marker beacons, and the CAT I, II and III categories that set how low you can go.",false,"md",[383,386,389],{"q":384,"a":385},"What are the two main parts of an ILS?","The localiser and the glideslope. The localiser gives lateral guidance, a radio beam aligned with the runway centreline that tells you whether you are left or right of the extended centreline. The glideslope gives vertical guidance, a beam angled up from the touchdown zone, usually at about 3 degrees, that tells you whether you are above or below the correct descent path. Flown together they guide you down a precise three-dimensional path to the runway.",{"q":387,"a":388},"What do the ILS categories mean?","The categories describe how low and in how poor a visibility an ILS approach may be flown, set against the equipment, the aircraft and the crew. A Category I (CAT I) approach has a decision height no lower than 200 ft and a runway visual range no less than about 550 m. CAT II goes lower, a decision height below 200 ft but not below 100 ft and an RVR of at least 300 m. CAT III is lower again, down to little or no decision height and very low RVR, requiring the most capable equipment on the ground and in the aircraft.",{"q":390,"a":391},"What happens if the glideslope fails?","You lose the vertical guidance but keep the lateral guidance from the localiser, so the approach becomes a localiser-only approach, which is a non-precision approach flown to a higher minimum descent altitude rather than a decision height. The localiser still keeps you aligned with the runway; you simply descend by reference to altitudes and distances instead of following the glideslope beam down.",[393,394,395,396,397],"An ILS is two beams: the localiser for lateral guidance and the glideslope for the roughly 3-degree vertical guidance.","The categories set how low you can go: CAT I to a decision height no lower than 200 ft, CAT II below 200 ft to 100 ft, CAT III lower still.","Lower categories need more capable ground equipment, aircraft and crew approval, not just a lower cloud base.","Lose the glideslope and the approach becomes a localiser-only non-precision approach to a minimum descent altitude.","Intercept the localiser first and the glideslope from below, and cross-check height against distance to guard against a false glideslope.",{},"How an ILS works: the localiser, the glideslope, the marker beacons, and the CAT I, II and III categories that set how low you can go.",true,"\u002Flearn\u002Fthe-ils-explained",[403,412,421],{"q":404,"options":405,"answer":410,"explanation":411},"Which ILS component provides vertical guidance?",[406,407,408,409],"The localiser","The glideslope","The outer marker","The approach lights",1,"The glideslope provides vertical guidance, a beam angled up from the touchdown zone at about 3 degrees. The localiser provides the lateral guidance aligned with the runway centreline.",{"q":413,"options":414,"answer":419,"explanation":420},"What is the lowest decision height for a Category I ILS approach?",[415,416,417,418],"No lower than 200 ft","No lower than 100 ft","No lower than 50 ft","There is no decision height",0,"A CAT I approach has a decision height no lower than 200 ft and an RVR no less than about 550 m. CAT II and CAT III go lower, with more capable equipment.",{"q":422,"options":423,"answer":410,"explanation":428},"If the glideslope fails, what does the approach become?",[424,425,426,427],"An immediate missed approach with no options","A localiser-only non-precision approach to a minimum descent altitude","A visual approach","A Category III approach","Losing the glideslope leaves the localiser, so the approach becomes a localiser-only non-precision approach flown to a minimum descent altitude rather than a decision height.",{"title":5,"description":379},"fly-an-instrument-approach",[432,434,436,439],{"label":433,"url":39},"ICAO Annex 10: Aeronautical Telecommunications, Volume I (Radio Navigation Aids)",{"label":435,"url":135},"ICAO Doc 8168: PANS-OPS, Volume I (Flight Procedures)",{"label":437,"url":438},"FAA Aeronautical Information Manual, Chapter 1 Section 1 (Navigation aids, ILS)","https:\u002F\u002Fwww.faa.gov\u002Fair_traffic\u002Fpublications\u002Fatpubs\u002Faim_html\u002Fchap1_section_1.html",{"label":440,"url":441},"SKYbrary: Instrument Landing System (ILS)","https:\u002F\u002Fskybrary.aero\u002Farticles\u002Finstrument-landing-system-ils","learn\u002Fthe-ils-explained","Operations","9kvgZYfc0xJ1OtySlwgaIstYdaS3fMGbcRr_O3ZlPU4",{"related":446,"newer":465,"older":472,"series":479},[447,453,459],{"path":448,"title":449,"description":450,"date":451,"topic":443,"draft":380,"minutes":452,"series":378,"seriesOrder":378},"\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,{"path":454,"title":455,"description":456,"date":457,"topic":443,"draft":380,"minutes":452,"series":430,"seriesOrder":458},"\u002Flearn\u002Fthe-missed-approach-and-go-around","The missed approach and the go-around","The difference between a go-around and a missed approach, when each is flown, the climb gradient that protects it, and why going around early is wise.","2026-06-19",7,{"path":460,"title":461,"description":462,"date":463,"topic":443,"draft":380,"minutes":464,"series":378,"seriesOrder":378},"\u002Flearn\u002Fthe-global-reporting-format-for-runway-conditions","The Global Reporting Format for runway conditions","How the Global Reporting Format describes a contaminated runway: the condition code from 6 to 0, the assessment matrix, and the report split into thirds.","2026-06-17",5,{"path":466,"title":467,"description":468,"date":469,"topic":470,"draft":380,"minutes":452,"series":471,"seriesOrder":366},"\u002Flearn\u002Fhow-to-read-a-taf","How to read a TAF","Decode a Terminal Aerodrome Forecast: the FM, BECMG, TEMPO and PROB change groups, the validity period, and how a forecast differs from a METAR.","2026-04-01","Weather","decode-the-weather",{"path":473,"title":474,"description":475,"date":476,"topic":477,"draft":380,"minutes":478,"series":378,"seriesOrder":378},"\u002Flearn\u002Facclimatisation-and-time-zones","Acclimatisation and crossing time zones","How your body clock changes the flight duty period you may fly, how EASA and the FAA define an acclimatised state, and what the circadian low is.","2026-03-30","Regulations",8,{"slug":430,"title":480,"part":366,"total":458,"prev":481,"next":485},"Fly an instrument approach",{"path":361,"title":482,"description":483,"date":484,"topic":443,"draft":380,"minutes":452,"series":430,"seriesOrder":410},"Reading an instrument approach chart","How an instrument approach chart is laid out: the plan view, profile, minimums box and missed approach, and how to read one in the order you will fly it.","2026-06-13",{"path":219,"title":486,"description":487,"date":488,"topic":443,"draft":380,"minutes":458,"series":430,"seriesOrder":489},"Non-precision approaches","What makes an approach non-precision, the VOR, NDB, localiser and RNAV types, why they use a minimum descent altitude, and how APV approaches sit in between.","2026-04-15",3,1782839406007]