{"id":14524,"date":"2025-06-07T16:31:34","date_gmt":"2025-06-07T11:01:34","guid":{"rendered":"https:\/\/ibexaviation.com\/pilot-training\/?p=14524"},"modified":"2026-07-26T09:55:44","modified_gmt":"2026-07-26T04:25:44","slug":"instrument-landing-system","status":"publish","type":"post","link":"https:\/\/ibexaviation.com\/pilot-training\/instrument-landing-system\/","title":{"rendered":"Instrument Landing System (ILS)"},"content":{"rendered":"<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-1.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<h2>Landing using Instrument Landing System (ILS)<\/h2>\n<p>ILS (Instrument Landing System) is a precision radio navigation system that guides aircraft during landing. It provides horizontal (localiser) and vertical (glide slope) guidance to align the aircraft with the runway. ILS enables safe and accurate landings, especially in poor visibility and bad weather.<\/p>\n<h3>Introduction to ILS<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-2.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>ILS provides horizontal and vertical guidance using the <strong>Localiser<\/strong> and <strong>Glide Path<\/strong>.<\/li>\n<li>The ILS reference datum is located on the runway centreline at <strong>50 ft above the threshold<\/strong>.<\/li>\n<li>Localiser frequencies range from <strong>108.00 to 111.975 MHz<\/strong> in the VHF band.<\/li>\n<li>Glide Slope frequencies range from <strong>329.15 to 335.00 MHz<\/strong> in the UHF band.<\/li>\n<li>Each Localiser is frequency-paired with its corresponding Glide Slope, so selecting the Localiser automatically selects the Glide Slope.<\/li>\n<li>Marker Beacons are low-power radio transmitters that provide distance alerts.<\/li>\n<li>A DME paired with the ILS provides distance information during final approach.<\/li>\n<\/ul>\n<h2>Localiser of ILS<\/h2>\n<h3>Principle of ILS Localiser<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-3.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The Localiser transmits two overlapping lobes along the runway approach path.<\/li>\n<li>Both lobes use the same carrier frequency but different audio modulation.<\/li>\n<li>The right-hand (blue) sector is modulated at <strong>150 Hz<\/strong>.<\/li>\n<li>The left-hand (yellow) sector is modulated at <strong>90 Hz<\/strong>.<\/li>\n<li>Equal 90 Hz and 150 Hz modulation indicates alignment with the runway centreline.<\/li>\n<li>The <strong>Difference in Depth of Modulation (DDM)<\/strong> is zero on the centreline.<\/li>\n<li>DDM increases as the aircraft deviates from the extended runway centreline.<\/li>\n<\/ul>\n<h3>Coverage of ILS Localiser (Normal Approach)<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-4.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Coverage depends on the glide path angle.<\/li>\n<li>Normal approach coverage is <strong>10\u00b0 up to 25 NM<\/strong>.<\/li>\n<li>Coverage increases to <strong>35\u00b0 up to 17 NM<\/strong>.<\/li>\n<li>Outside these limits, false beams make Localiser indications unreliable.<\/li>\n<\/ul>\n<h3>Coverage of ILS Localiser (Steep Approach)<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-5.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Coverage depends on the glide path angle.<\/li>\n<li>Steep approach coverage is <strong>10\u00b0 up to 18 NM<\/strong>.<\/li>\n<li>Coverage increases to <strong>35\u00b0 up to 10 NM<\/strong>.<\/li>\n<li>False beams make Localiser indications unreliable outside these limits.<\/li>\n<\/ul>\n<h3>Back Beam of ILS Localiser<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-6.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The Localiser antenna also radiates a back beam in the opposite direction.<\/li>\n<li>Back beam indications are reversed when approaching from the opposite runway.<\/li>\n<li>The indications effectively become a &#8220;what not to do&#8221; display.<\/li>\n<li>During a missed approach, back beam indications may provide useful guidance.<\/li>\n<li>Back beam approaches are approved only in certain countries.<\/li>\n<\/ul>\n<h3>ILS Localiser Indications<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-7.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Localiser indications on the CDI\/OBI provide command guidance toward the runway centreline.<\/li>\n<li>Needle Left \u2192 Fly Left.<\/li>\n<li>Needle Right \u2192 Fly Right.<\/li>\n<li>On a five-dot indicator, full-scale deflection equals <strong>2.5\u00b0<\/strong>.<\/li>\n<li>Each dot represents <strong>0.5\u00b0<\/strong>.<\/li>\n<li>The aircraft is considered established when within half-scale deflection.<\/li>\n<li>HSI displays Localiser guidance using an arrow or deviation needle.<\/li>\n<li>A warning flag appears when the carrier signal or one modulation signal is lost.<\/li>\n<\/ul>\n<h2>Glide Path of ILS<\/h2>\n<h3>Principle of ILS Glide Path<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-8.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The Glide Path transmits two overlapping lobes along the approach path.<\/li>\n<li>Both lobes share the same carrier frequency with different audio modulation.<\/li>\n<li>The lower (blue) sector is modulated at <strong>150 Hz<\/strong>.<\/li>\n<li>The upper (yellow) sector is modulated at <strong>90 Hz<\/strong>.<\/li>\n<li>Equal modulation indicates the correct glide path.<\/li>\n<li>DDM is zero on the correct glide path.<\/li>\n<li>DDM increases as the aircraft moves above or below the glide path.<\/li>\n<li>The receiver displays Fly Up or Fly Down guidance.<\/li>\n<\/ul>\n<h3>False Glide Path<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-9.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>A false glide path normally occurs at approximately twice the correct glide angle.<\/li>\n<li>It is caused by antenna characteristics and reflected radio waves.<\/li>\n<li>The Glide Slope should always be intercepted from below.<\/li>\n<li>The aircraft should first be established on the Localiser before intercepting the Glide Path.<\/li>\n<\/ul>\n<h3>Horizontal Coverage of Glide Path<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-10.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Coverage depends on the glide angle.<\/li>\n<li>For a normal approach, horizontal coverage extends approximately <strong>8\u00b0 up to 10 NM<\/strong>.<\/li>\n<li>For a steep approach, coverage is reduced to approximately <strong>8 NM<\/strong>.<\/li>\n<\/ul>\n<h3>Vertical Coverage of Glide Path<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-11.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Vertical coverage is measured relative to the published glide angle.<\/li>\n<li>Lower limit is <strong>0.45 \u00d7 glide angle<\/strong> below the nominal glide path.<\/li>\n<li>Upper limit is <strong>1.75 \u00d7 glide angle<\/strong> above the nominal glide path.<\/li>\n<li>For a standard 3\u00b0 glide slope:<\/li>\n<ul>\n<li>Coverage extends from <strong>1.65\u00b0 to 2.25\u00b0<\/strong> relative to the glide path.<\/li>\n<li>Equivalent elevation range is approximately <strong>1.35\u00b0 to 5.25\u00b0<\/strong> above the horizon.<\/li>\n<\/ul>\n<\/ul>\n<h3>Course Deviation Indicator (ILS Mode)<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-12.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Glide Path indications provide command guidance.<\/li>\n<li>Needle Up \u2192 Fly Up.<\/li>\n<li>Needle Down \u2192 Fly Down.<\/li>\n<li>Full-scale Glide Slope deflection equals approximately <strong>0.7\u00b0<\/strong>.<\/li>\n<li>Each dot on a five-dot indicator represents approximately <strong>0.14\u00b0<\/strong>.<\/li>\n<li>Maximum safe deviation is half-scale (<strong>0.35\u00b0<\/strong> above or below).<\/li>\n<\/ul>\n<h2>Measurement of Distance to Touchdown<\/h2>\n<h3>ILS Marker Beacons<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-13.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>Marker Beacons transmit on <strong>75 MHz<\/strong> using an elliptical or fan-shaped radiation pattern.<\/li>\n<li><strong>Outer Marker (OM)<\/strong><\/li>\n<ul>\n<li>Located 3.5\u20136 NM from touchdown.<\/li>\n<li>Blue light.<\/li>\n<li>400 Hz tone with two dashes per second.<\/li>\n<\/ul>\n<li><strong>Middle Marker (MM)<\/strong><\/li>\n<ul>\n<li>Located approximately 3,500 ft from touchdown.<\/li>\n<li>Amber light.<\/li>\n<li>1300 Hz alternating dots and dashes.<\/li>\n<\/ul>\n<li><strong>Inner Marker (IM)<\/strong><\/li>\n<ul>\n<li>Located 250\u20131,500 ft from touchdown.<\/li>\n<li>White light.<\/li>\n<li>3000 Hz tone with six dots per second.<\/li>\n<\/ul>\n<\/ul>\n<h2>Errors and categories of ILS<\/h2>\n<h3>ILS Categories<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-14.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>ILS Categories describe only the ground installation.<\/li>\n<li>Operational use also depends on:<\/li>\n<ul>\n<li>Airport procedures.<\/li>\n<li>Airborne equipment capability.<\/li>\n<li>Flight crew qualifications.<\/li>\n<\/ul>\n<\/ul>\n<h3>ILS Ground Monitoring<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-15.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>The ILS is continuously monitored within its protected coverage area.<\/li>\n<li>If limits are exceeded, transmissions cease and station identification is removed.<\/li>\n<li>Category II or III systems may automatically downgrade to a lower category.<\/li>\n<li>Monitoring activates when:<\/li>\n<ul>\n<li>Localiser shifts beyond allowable limits.<\/li>\n<li>Centreline error exceeds 35 ft (CAT I), 25 ft (CAT II), or 20 ft (CAT III).<\/li>\n<li>Glide Path angle changes by more than 0.075 \u00d7 the published glide angle.<\/li>\n<li>Transmitter power decreases by more than 50%.<\/li>\n<\/ul>\n<\/ul>\n<h3>Errors of ILS<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-16.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>ILS indications should always be cross-checked due to inherent system limitations.<\/li>\n<li>Beam bending caused by atmospheric conditions.<\/li>\n<li>Scalloping caused by reflected radio waves.<\/li>\n<li>Beam noise from transmitter or external interference.<\/li>\n<li>Restricting vehicles within critical and sensitive areas minimises interference.<\/li>\n<li>Aircraft should remain clear of sensitive areas during poor weather.<\/li>\n<li>Station identification must always be positively received before using the ILS.<\/li>\n<\/ul>\n<h3>Limitations of ILS<\/h3>\n<p> <img decoding=\"async\" src=\"https:\/\/ibexaviation.com\/page-show\/radio-navigation\/r8-instrument-landing-system-17.jpg\" alt=\"\" loading=\"lazy\"\/><\/p>\n<ul>\n<li>ILS is limited to approximately <strong>40 frequency channels<\/strong>.<\/li>\n<li>Requires a large, flat, obstacle-free installation area.<\/li>\n<li>Fixed Glide Path is unsuitable for some STOL aircraft.<\/li>\n<li>Only a limited number of aircraft can be accommodated simultaneously.<\/li>\n<li>Terrain may prevent installation in mountainous regions.<\/li>\n<li>The ILS critical area must be protected from interference.<\/li>\n<li>Nearby civilian FM broadcasts may cause interference.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Landing using Instrument Landing System (ILS) ILS (Instrument Landing System) is a precision radio navigation system that guides aircraft during landing. It provides horizontal (localiser) and vertical (glide slope) guidance to align the aircraft with the runway. ILS enables safe and accurate landings, especially in poor visibility and bad weather. Introduction to ILS ILS provides horizontal and vertical guidance using the Localiser and Glide Path. The ILS reference datum is located on the runway centreline at 50 ft above the threshold. Localiser frequencies range from 108.00 to 111.975 MHz in&hellip;<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"disable_featured_image":false,"footnotes":""},"categories":[326],"tags":[],"class_list":["post-14524","post","type-post","status-publish","format-standard","hentry","category-cpl-atpl-radio-navigation"],"_links":{"self":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14524","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/comments?post=14524"}],"version-history":[{"count":2,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14524\/revisions"}],"predecessor-version":[{"id":16602,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14524\/revisions\/16602"}],"wp:attachment":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/media?parent=14524"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/categories?post=14524"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/tags?post=14524"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}