ILS

 

CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001 II. Technical Subject Areas A. Aircraft Flight Instruments and Navigation Equipment 2. Flight instrument systems and their operating characteristics c. Instrument Landing Systems (ILS) (includes SDF and LDA) 1) ILS is an electronic system that provides horizontal and vertical guidance to a specific runway, used to execute a precision instrument approach. 2) Three types based on equipment at airport and pilot experience level a) Category I - approach height not less than 200 feet above touchdown b) Category II Approach height not less than 100 feet above touchdown Requires special pilot certification c) Category III No decision height Requires special pilot certification 3) Ground components a) Localizer (LOC) equipment i) Antenna array located beyond opposite end of runway that radiates a field pattern providing left/right guidance along the centerline of the runway (in both directions) ii) Transmits on odd tenths from 108.1 to 111.95 MHz out to 18 NM away from and 4,500 feet above the antenna site iii) LOC course width normally 5° (full-scale deflection when aircraft is 2.5° to either side of centerline) iv) Audibly identified by "I" plus three-letter designator and includes a voice feature for use by ATC b) Glide slope (GS) equipment i) Located in a building 750-1,250 feet from approach end of runway, 400-600 feet to one side of centerline ii) Projection angle normally adjusted to 2.5-3.5° above horizontal to intersect the middle marker (MM) at about 200 feet and the outer marker (OM) at about 1,400 feet above runway elevation iii) GS signal is radiated on front course only (NOT on back course) iv) GS normally 1.4° thick (about 1,500 feet at 10 NM from touchdown, narrowing to just a few feet at touchdown) c) Marker beacons i) Low-powered transmitters direct signal upward in a small fan-shaped pattern ii) Outer marker (OM) located 4-7 miles from airport near position where aircraft at appropriate altitude will intercept glidepath iii) Middle marker (MM) located approximately 3,500 feet from landing threshold at position where glide-slope centerline is about 200 feet above touchdown zone elevation iv) Inner marker (IM) located at decision height on glidepath for Category II approach v) There may be a back-course marker to indicate the back-course FAF d) Compass locator i) Supplementary, optional component that may be employed to increase safety and utility ii) A low-powered NDB colocated with the OM and/or MM facilities iii) Outer marker locator ID code consists of the first two letters of the LOC 3-letter identifier iv) Middle marker locator ID code consists of the second two letters of the 3-letter LOC identifier e) Approach lighting systems (ALS) i) Provide directional, distance and glidepath information for safe transition to runway threshold ii) Common ALS configurations Approach light sytem with sequenced flashing lights (ALSF) Simplified short approach light system with runway alignment indicator lights (SSALR) Medium intensity approach light system with runway alignment indicator lights (MALSR) Runway end identification lights (REIL) Medium intensity approach light system with sequenced flashing lights (and runway alignment) (MALSF) Omnidirectional approach light system (ODALS) iii) High-intensity flasher system ("the rabbit") lights flash in sequence towards the runway (2 times per second) iv) REIL are a pair of synchronized flashing lights one on each side of threshold facing approach area v) Visual approach slope indicator (VASI) projects a visual glidepath providing safe obstruction clearance within the approach zone 4) ILS airborne components include receivers and indicator instruments for the following a) Localizer i) Typically VOR receiver is also a localizer (LOC) receiver that switches to LOC automatically when sensing odd tenths between 108 and 112 MHz ii) Warning flag shows when unstable signal or receiver malfunction occurs b) Glideslope (GS) i) Usually tuned automatically with LOC ii) Cross-pointer indicator is both a LOC and GS indictor iii) GS needle is quite sensitive since glidepath is relatively narrow (approximately 1.4° from full up to full down deflection) iv) GS warning flag shows if signal is unstable or receiver malfunctions c) Marker beacons i) OM - low-pitched tone, continuous dashes, two per second and purple or blue light ------ ii) MM - intermediate tone, alternate dots and dashes, 95 .- per minute and amber light .-.-.- iii) IM - high-pitched coninuous dots, six per second and white light ...... iv) BCM (back-course marker) - high-pitched pairs of dots, 72-75 pairs per minute and white light .. .. .. v) Many units allow selection of 2 sensitivities Low gives sharpest indication of position and should be used during an approach High provides an earlier indication that aircraft is approaching the beacon 5) ILS function a) Localizer needle i) Indicates whether aircraft is right or left of localizer centerline, regardless of aircraft heading ii) OBS has no effect on localizer needle; but it is recommended to set LOC inbound course under course index iii) Remains directional inbound on front course or out bound on back course b) Flying inbound on back course or outbound on front course i) Unless reverse sensing is in use, heading corrections to on-course are opposite to needle deflection ("Fly away from the needle." or "You ARE the needle.") ii) Do not use back course signals for approach unless procedure is published and authorized by ATC c) Maintain LOC centerline with decreasingly small drift corrections, ideally no more than 2° inside the OM d) Adjust pitch to maintain proper rate of descent and power to maintain proper airspeed along glidepath from OM to MM making small corrections based on glideslope needle indications 6) ILS errors a) LOC and GS signals i) Reflection from surface vehicles and low-flying (<5,000 feet AGL) aircraft ii) False courses from GS facility at higher vertical angles (e.g. 9-12°) (avoid by flying approach at altitudes specified on approach chart) b) Marker beacons i) Inadequate receiver power supply (may utilize power from another source, e.g. ADF, which must be turned on) ii) "Test" position of switch usually tests only the light bulb, not the receiver c) Operational errors i) Failure to understand differences between VOR and ILS; that LOC CDI sensing is sharper and faster ii) Disorientation due to poor planning and failure to utilize all available equipment iii) Disorientation on LOC course due to i) above iv) Incorrect localizer interception angles. Helpful to turn on first indication of needle movement (when needle "comes alive"), and use ADF if there is a locator or NDB on inbound course v) Chasing CDI needle and glidepath needles (study the approach before the flight) 7) Simplified directional facility (SDF) a) Similar to ILS, but course may be wider than standard ILS, resulting in less precision b) SDF antenna may be offset from runway centerline so that inbound course is not aligned with runway centerline (difference is usually not more than 3°) c) SDF signal course width is fixed at either 6° or 12° d) Three-letter SDF identifier is transmitted on SDF frequency 8) Localizer type directional aid (LDA) a) Accuracy and utility comparable to that of a localizer, but not part of a complete ILS b) Course width is between 3 and 6° c) Some LDAs have glideslopes d) LDA course is not aligned with runway i) If angle between LDA course and runway is not greater than 30°, then straight-in minimums may be published ii) Angle greater than 30°: only circling minimums are published e) Identifier is "I" plus 3 letters References: Instrument Flying Handbook, FAA-H-8083-15, 1999 AIM 1-1-9