AvS-3 Figure 3_1 3_2 Amended1 PDF

Chapter 3 INSTRUMENT LANDING SYSTEM (ILS) & VERY HIGH FREQUENCY OMNI RANGE SYSTEM (VOR) Learning Outcomes  Working of ILS System  ILS Aircraft Equipment  ILS Airborne Loading  Working of VOR System  VOR Aircraft Equipment  VOR Airborne Loading  Principle of Automat...

Chapter 3 INSTRUMENT LANDING SYSTEM (ILS) & VERY HIGH FREQUENCY OMNI RANGE SYSTEM (VOR) Learning Outcomes  Working of ILS System  ILS Aircraft Equipment  ILS Airborne Loading  Working of VOR System  VOR Aircraft Equipment  VOR Airborne Loading  Principle of Automatic Flight Control AVIONIC SYSTEMS INSTRUMENT LANDING SYSTEM (ILS) & VHF OMNI RANGE SYSTEM (VOR) 3.1 Introduction In order to be able to land the aircraft frequently in bad weather where pilot can’t see runway of the airport by any means requires electronic aid to the pilot. This electronic aid is nothing but Instrument Landing System (ILS) was developed at the end of the Second World War, which provides greater precision guidance for the aircraft to land on the selected run way. An early directional transmitting navigation aid, radio range, operated in the MF/LF band and received on early ADF equipment. It had its limitations inherent with lower frequencies and at best it could produce only four fixed tracks to or from the beacon. A need for a more flexible and reliable aid soon became apparent, and VHF omni range emerged as its successor. 3.2 Working Principle of Instrument Landing System (ILS) ILS system consists, pair of antennas transmitting and Localizer (LOC) Glideslope (G/S) beams, directed close to either side of the ideal approach path with their emission code designated as A8W. Glideslope provides a fixed descent rate path to the touchdown point and localizer is aligned to centreline of the runway. The International Civil Aviation Organisation (ICAO) has defined three categories of visibility in terms of Runway Visual Range (RVR), given in Table 3-1. Table 3-1: ICAO Visibility Categories CATEGORY RVR I 800 m (2600 ft) II 400 m (1200 ft) III 200 m (700 ft) ET0171/Chaganti Page 3-1 AVIONIC SYSTEMS Localizer, glideslope operates in aeronautical VHF and UHF bands, given in Table 3-2. Table 3-2: Localizer/Glideslope frequency pairing in MHz Localiser Glide Localiser Glide Localiser Glide Localiser Glide slope slope slope slope 108.10 334.70 109.10 331.40 110.10 334.40 111.10 331.70 108.15 334.55 109.15 331.25 110.15 334.25 111.15 331.55 108.30 334.10 109.30 332.00 110.30 335.00 111.30 332.30 108.35 333.95 109.35 331.85 110.35 334.85 111.35 332.15 108.50 329.90 109.50 332.60 110.50 329.60 111.50 332.90 108.55 329.75 109.55 332.45 110.55 329.45 111.55 332.75 108.70 330.50 109.70 333.20 110.70 330.20 111.70 333.50 108.75 330.35 109.75 333.05 110.75 330.05 111.75 333.35 108.90 329.30 109.90 333.80 110.90 330.80 111.90 331.10 108.95 329.15 109.95 333.65 110.95 330.65 111.95 330.95 3.2.1 Localizer Localizer transmits signals in the VHF band between 108.10 to 111.95 MHz, using the first odd decimals with a channel spacing of 50 KHz. Localizer produces two directional beams on the left and right sides of airport runway centre line (Figure 3-1). Both these left, right beams are modulated with 90Hz and 150Hz signals respectively. Examine two cases, where in case one, if an airplane is located on the left side of course path, the 90 Hz signal dominates and localizer indicators on board deflected to the right indicating the runway centreline is to the right. In case two, if an airplane is located on the right side of course path, the 150 Hz signal dominates and localizer indicators on board deflected to the left indicating the runway centreline is to the left. ILS receivers measure the Difference in Depth of Modulation (DDM) of the carrier waves modulated with 90Hz, 150Hz tones and this quantity is expressed as % Modulationof l arg er signal  % Modulationof smaller signal DDM  100 ET0171/Chaganti Page 3-2 AVIONIC SYSTEMS Coverage of localizer beams are within 10º of runway centre line out to 25 nm, within 35º of runway centre line out to 17nm shown in Figure 3-3a. Localizer deviation 1º from the course path produces 75 µAmps receiver output current. To confirm the aircraft’s position from the runway, marker beacons are provided under the ideal approach path which transmits an independent signal on a frequency of 75 MHz. When a marker signal is received, a light appears on the instrument panel, and an audio tone is heard. 3.2.2 Glideslope Glideslope transmits signals in the UHF band between 329.5 to 335.0 MHz, with a channel spacing of 150 KHz. Pilot selection of required localizer frequency on the controller will cause both localizer and glideslope receiver’s tune to the appropriate paired frequencies. Glideslope produces two directional beams one above the other, upper beam is modulated with 90Hz and lower beam is modulated with 150Hz signals respectively (Figure 3-2). Examine two cases, where in case one, if an airplane is located above the glideslope, the 90 Hz signal dominates and glideslope indicators on board deflected down indicating glideslope is below the aircraft. In case two, if an airplane is located below the glideslope, the 150 Hz signal dominates and glideslope indicators on board deflected up indicating glideslope is above the aircraft. The glideslope beams covers over a vertical angles of between 1.35º and 5.25º out to 10nm shown in Figure 3-3b. Glideslope deviation 0.35º from the course path produces 75 µAmps receiver output current. Glideslope transmit antenna system at the airports will not produce back beam like localizer transmit antenna system. Example 3-1: Find the harmonics present in 90 Hz and 150 Hz tones 150 + 90 = 240 Hz 150 – 90 = 60 Hz ET0171/Chaganti Page 3-3 AVIONIC SYSTEMS LOCALIZER CARRIER FREQUENCIES: 108.10 MHz TO 111.95 MHz 1º LOCALIZER DEVIATION = 75 µAmps RECEIVER OUTPUT 90Hz AMPLITUDE A RUNWAY LOCALIZER MODULATION 90Hz AMPLITUDE TRANSMITTER MODULATION D B 150Hz AMPLITUDE MODULATION C 150Hz AMPLITUDE MODULATION A FLY RIGHT FLY LEFT C B ON DOT LINE FLY LEFT D Figure 3-1 Localizer Principle ET0171/Chaganti Page 3-4 AVIONIC SYSTEMS A GLIDE SLOPE CARRIER FREQUENCIES: 329.5MHz TO 335.0MHz 0.35º GLIDE SLOPE DEVIATION = 75 µAmps RECEIVER OUTPUT B 90Hz AMPLITUDE C D MODULATION GLIDESLOPE TRANSMITTER A FLY DOWN C FLY UP B ON DOT LINE 150Hz AMPLITUDE MODULATION RUNWAY Figure 3-2 Glideslope Principle ET0171/Chaganti Page 3-5 AVIONIC SYSTEMS 17 nm 25º 5.25º 10º 3º R/W- C/L 1.35º 25 nm 10º 17 nm 25º Figure 3-3a Localizer Coverage Sector Figure 3-3b Glideslope Coverage Sector ET0171/Chaganti Page 3-6 AVIONIC SYSTEMS Example 3-2: What rate of descent should an aircraft use on 30 ILS glideslope if it has a ground speed of 120 knots? 1 in 60 rule 3.3 ILS Aircraft Equipment ILS aircraft equipment (Figure 3-4) consists of  Receivers  Control unit  ILS indicators  Aerials 3.3.1 Receivers In ILS operation there will be two receivers, a VHF localizer receiver and a UHF glideslope receiver. Both these receivers are operated from the same unit for easier functionality. These receivers consists of all necessary circuits for receiving, decoding and computing the localizer and glideslope signals transmitted by airport antennas. Self calibration is provided for these receivers to monitor the reliability of the computed signal before displayed to the pilot. 3.3.2 Control unit A standard communication/navigation box, a modern example of which is illustrated in Figure 3-5, is often fitted in most aircraft’s. When control unit selects VHF localizer frequency which automatically selects paired UHF glideslope frequency. The pilot should first tune the required ILS localizer frequency and identify the station from the audio signal, then localizer signal will give an indication of the aircraft position in relation to the run way. The illustrated unit controls electronic screens to display ILS commands and information. ET0171/Chaganti Page 3-7 AVIONIC SYSTEMS 3.3.3 ILS indicators The ILS signals are often displayed to a conventional or electronic indicators, most modern aircrafts uses Attitude Direction Indicator (ADI) for ILS landing as shown in Figure 3-6. In ADI the localizer drives a rising runway laterally to display deviation (vertical movement representing radio altitude) while glideslope drives a pointer over a scale, again on the left- hand side of the instrument. The localizer deviation will be used to supply the appropriate demand signal to roll and yaw channels. The pitch channel will respond to glideslope. 3.3.4 Aerials Being in the VHF band, the localizer signals are often received by whip antennas on either side of the tail, which may also be used for VOR signal reception. The UHF glideslope signals can be received by folded dipole antenna that is located at nose of the aircraft. Lower frequency marker signals require a larger antenna, which is usually mounted below the fuselage. 3.4 ILS Airborne Loading With the aid of ILS, electronic approach path for the aircraft in three dimensional planes can be set accurately, to land safely on the selected runway. The aircraft is guided with received localizer and glideslope signal levels of modulation tones. The Localizer course path is aligned to the centre of the runway means signal level of 90 Hz tone is equal to the signal level of 150 Hz tone. Similarly, glideslope descent path is set at 3º to the runway means signal level of 90 Hz tone is equal to the signal level of 150 Hz tone for glideslope signals. Outer marker, middle marker and inner marker signals along the runway gives third dimensional range from the runway. To receive and process the localizer signals in aeronautical VHF band, a conventional AM- superhetrodyne receiver is most suitable (Figure 3-7). The received localizer signal consists of a carrier frequency at VHF band modulated with 90 Hz and 150 Hz tones. Once ground station transmits the localiser signal, it will be intercepted by the aircraft localizer antenna and reaches to the preselector. ET0171/Chaganti Page 3-8 AVIONIC SYSTEMS G/S ANTENNA RECEIVER VOR/LOC INDICATOR ANTENNA CONTROL UNIT Figure 3-4 ILS Aircraft Equipment ET0171/Chaganti Page 3-9 AVIONIC SYSTEMS COMM NAV 118.00 108.00 VOR Figure 3-5 Control Unit ET0171/Chaganti Page 3-10 AVIONIC SYSTEMS 356 20 20 G 10 10 S 163 20 20 LOC Figure 3-6 Attitude Director Indicator ET0171/Chaganti Page 3-11 AVIONIC SYSTEMS Preselector selects required localiser frequency and filter out all other frequency components for high Carrier to Noise ratio (C/N). After the preselector, signal goes to the mixer where it mixes with synthesiser frequency to produce Intermediate Frequency (IF) signal. This IF signal is sent through the detector stage, where 90 Hz and 150 Hz tones are separated from IF carrier signal. The detected 90 Hz and 150 Hz tones are fed into two different circuits. Monitor circuit checks reliability and quality, deviation instrumentation circuit compares the signal levels of 90 Hz and 150 Hz tones, which produces a difference signal (or Δ signal) between these tones. This Δ signal is applied to the Attitude Director Indicator to drive the horizontal deviation course pointers. Similarly, glideslope receiver works on the same principle as localizer receiver. In glideslope receiver the operating frequency automatically changed to UHF band for glideslope signal requirement. The glideslope deviation instrumentation compares the signal levels of 90 Hz and 150 Hz tones as localizer deviation instrumentation but the produced difference signal is applied to drive the vertical deviation course pointers of Attitude Director Indicator. If the rough track to the aerodrome is known, the pilot can intercept the localizer some distance along it, in a similar fashion to VOR radial intercept. Control unit displays the selected localizer frequency by the pilot based on the airport information. Each component of ILS ground equipment contains its own monitoring and self checking system, which will switch off either the carrier waves or modulating tones if they are outside the required parameters, or indeed if the monitoring system fails. In such cases airborne ILS receiver system shows total depth of modulation of composite 90/150 Hz signal is less than 28 percent and can be termed as warning signal, which leads to manual landing of the aircraft on the runway by pilot. ET0171/Chaganti Page 3-12 AVIONIC SYSTEMS GLIDESLOPE LOC/VOR ANTENNA ANTENNA PRESELECTOR PRESELECTOR MIXER GLIDESLOPE LOCALIZER MIXER SYNTHESISER SYNTHESISER GLIDESLOPE TUNE LOCALIZER DETECTOR CONTROL DETECTOR GLIDESLOPE LOCALIZER DEVIATION DEVIATION INSTRUMENTATION INSTRUMENTATION GLIDESLOPE LOCALIZER MONITOR MONITOR CONTROL UNIT LOC LOC FLAG DEV GS FLAG GS DEV ATTITUDE DIRECTOR INDICATOR Figure 3-7 ILS Airborne Loading ET0171/Chaganti Page 3-13 AVIONIC SYSTEMS 3.5 Working Principle of VHF Omni Range (VOR) The VOR system operates in 108-118 MHz band with channels spaced at 50 KHz. This band shares with ILS localizer frequency with first even decimals. VOR produces an infinite number of tracks (360 possible radials) from a beacon, refer to Figure 3-8. It is practically free from static and does not suffer from night effect, consequently it can be used with confidence at any time throughout the 24 hours. VOR BEACON RADIALS Figure 3-8 VOR Radial Concept A simple analogy to VOR is given by imagining a lighthouse which emits an omnidirectional pulse of light every time the beam is pointing due north. If the speed of rotation of the beam is known, a distant observer could record the time interval between seeing the omnidirectional flash and seeing the beam, and hence calculate the bearing of the lighthouse. In reality a VOR station radiates VHF energy modulated with reference phase signal- the omnidirectional light – and a variable phase signal similar to the rotating beam. The bearing of the aircraft depends on the phase difference between reference and variable phases- time difference between light and beam. ET0171/Chaganti Page 3-14 AVIONIC SYSTEMS The radiation from a Conventional VOR (CVOR) station is horizontally polarized VHF wave modulated as follows (Figure 3-9)  30 Hz AM : the variable phase signal (achieved by physical or electronic rotation of dipole)  9960 Hz AM: this is a subcarrier frequency Modulated at 30 Hz with a deviation of ± 480Hz. The 30 Hz signal is the reference phase. 3.5.1 Reference Phase Signal The station transmits an omnidirectional horizontally polarised continuous wave signal on its allocated frequency between 108.0 and 117.95 MHz refer Figure 3-10. The emission code is A9W. That signal is amplitude modulated by a 9960 Hz sub-carrier which itself is frequency modulated at 30Hz. This omnidirectional radiation has circle as its horizontal polar diagram. At a given range from the transmitter, an aircraft’s receiver will detect the same phase on all bearings around it. As can be seen in Figure 3-10, the phase pattern produced is independent of the receiver’s bearing from the station. In the receiver, the 30 Hz component of the transmission is used as a reference for the phase comparison which will provide the bearing. 3.5.2 Variable Phase signal This is also transmitted on the station frequency, by an aerial which rotates either physically or electronically 30 times per second (30 Hz). The signal is less strong than the omnidirectional signal, so that together the signals appear to be a single carrier wave, amplitude modulated at 30Hz to a depth of 30%. This resultant signal is called a limacon shown in Figure 3-11. An aircraft in certain position will receive this rotating pattern as a signal with a 30Hz amplitude modulation. The phase of this received AM signal will vary as the position of the receiver around the circle of rotation. This phase can be compared with the phase of the FM omnidirectional signal, and displayed to the pilot. ET0171/Chaganti Page 3-15 AVIONIC SYSTEMS VARIABLE 30 Hz (AM) REFERENCE 30 Hz (FM) fc 30 Hz 9960 Hz ±480 Hz Figure 3-9 VOR Frequency Spectrum ET0171/Chaganti Page 3-16 AVIONIC SYSTEMS The transmitter is arranged so that the two signals are in phase when the receiver is in the direction of magnetic north from the transmitter. As the bearing from the transmitter changes, so does the phase of the variable signal. The phase difference from the reference signal equates to the magnetic bearing of the aircraft from the station. This is shown in Figure 3-11. 3.5.3 Station Identification The carrier wave has keyed AM audio frequency (1020 Hz) signal to provide station identification at least once every 10 seconds, as recommended by International Civil Aviation Organisation (ICAO). This identification consists of three Morse letters transmitted at a rate of about seven words per minute. The carrier can also be modulated at audio frequency (300-3000 Hz) by a voice signal. This can give  Station identification  Weather reports  Aerodrome flight information service (AFIS) 3.6 VOR Aircraft Equipment VOR aircraft equipment (Figure 3-12) consists of  Receivers  Control unit  VOR indicators  Aerials ET0171/Chaganti Page 3-17 AVIONIC SYSTEMS Rx N Rx Rx Rx Figure 3-10 Reference Signal- Same Phase in All Directions ET0171/Chaganti Page 3-18 AVIONIC SYSTEMS PHASE DIFFERENCE 0º N PHASE DIFFERENCE 270º PHASE DIFFERENCE 90º LIMACON PHASE DIFFERENCE 180º Figure 3-11 Bearing by Phase Comparison ET0171/Chaganti Page 3-19 AVIONIC SYSTEMS 3.6.1 Receivers This airborne receivers picks up omnidirectional signals from ground station VOR antennas at the selected frequency and process the bearing information, which will be displayed on Radio Magnetic Indicator (RMI). Self calibration is provided for these receivers to monitor the reliability of the computed bearing information before displayed to the pilot. VOR navigational receivers will also process ILS guidance information from airport ground facility. In order to receive VOR signal, pilot has to fly on the minimum altitude of 1000 feet above ground level (AGL). 3.6.2 Control unit A standard navigation control box as illustrated in Figure 3-5, with 160 frequency channels. This control unit provides all control circuits for VHF-VOR navigation system. The pilot should first tune the required VOR frequency and identify the station from the audio signal, at the same time he can also switch to VHF radio communication. This control unit can support both Localizer frequency selection as well as VOR frequency selection. Which one is selected depends on the location of aircraft. 3.6.3 VOR indicators The magnetic bearing from the station is displayed (Figure 3-13) generally on Omni Bearing Selector (OBS). Most VOR stations are positioned along airways, and airway centreline is defined as a particular radial from one station until it meets the reciprocal radial from the next station along the airway, change over comes at halfway point. The pilot selects the radial along which he wises to fly, or it’s reciprocal, on the OBS as an intended track. 3.6.4 Aerials Bat-wing type antennas are most suitable for VOR navigational receivers at VHF band, as these antennas produce horizontally polarized omnidirectional radiation pattern. Being in the VHF band, the localizer signals are often received by VEE-dipole antennas on either side of the tail, which may also be used for VOR signal reception. ET0171/Chaganti Page 3-20 AVIONIC SYSTEMS RECEIVER VOR/LOC ANTENNA O B S INDICATOR V C N O O A R M CONTROL V UNIT M 1 1 0 Equipment Figure 3-12 VOR Aircraft 1 8 8. ET0171/Chaganti. 0 Page 3-21 0 0 0 AVIONIC SYSTEMS Omni Bearing Selector A C ROTATING TO-FROM COURSE INDICATOR CARD B D OMNI OBS COURSE BEARING DEVIATION SELECTOR INDICATOR Figure 3-13 A: Rotating Course Card is calibrated from 0 to 360 degrees, which indicates the VOR bearing chosen as the reference to fly by pilot. B: Omni Bearing Selector (OBS Knob) used to manually rotate the course card to where the point to fly to. C: TO-FROM indicator, the triangle arrow will point UP when flying to VOR station. The arrow will point DOWN when flying from the VOR station. A red flag replaces these TO- FROM arrows when the VOR is beyond reception range or the station is out. D: Course Deviation Indicator (CDI) needle moves left or right indicating the direction to turn the aircraft to return to course. Dot: The horizontal dots at centre represent the aircraft away from the course. Each dot represents 2 degree deviation from desired course. ET0171/Chaganti Page 3-22 AVIONIC SYSTEMS 3.7 VOR Station TO-FROM Aircraft VOR navigation receiver detects and compares the phase difference between reference and variable signals. This phase difference is proportional to the radial angle from the VOR ground station, from which bearing of VOR station is determined. VOR station bearing and aircraft heading are displayed in Radio Magnetic Indicator (RMI); magnetic north bearing is derived from compass to the RMI indicator. Course Deviation Indication (CDI) and TO-FROM indicator gives the aircraft position relative to the selected VOR station radial. Figure 3-14 shows, TO-FROM division line with aircraft course path set to 330º, deviation of course path for different aircraft positions are also indicated. Example 3-3: Bearing Calculation Refer to Figure 3-15, an aircraft is heading on 0º represented with triangle on Radio Magnetic Indicator (RMI). Arrow pointer on bottom RMI shows a bearing of 0º-To the station and aircraft is on the 180º radial-From the station. Once the aircraft crosses over the VOR station the RMI arrow pointer will rotate and indicates a bearing of 180º-To the station on top RMI which is 0º radial-From the station. 3.8 VOR Airborne Loading Once the aircraft leaves the airport, its time for the pilot to tune the VOR receiver and select frequency of a VOR ground station. The received signal is a combination of carrier frequency (AM: 108-117.95 MHz) and sub-carrier frequency (FM: 9960 Hz) modulated with 30Hz signals. VOR receiver separates variable signal (30 Hz) and reference signal (30 Hz) from the respective carriers for comparing the difference in phase. Receiver (Figure 3-16) sends the signal to preselector for selecting required VOR frequency and filter out all other frequency components for good carrier to nose ratio (C/N). After the preselector, signal goes to the mixer where it mixes with synthesiser frequency to produce intermediate frequency (IF) signal. ET0171/Chaganti Page 3-23 AVIONIC SYSTEMS 330º CDI FROM 330º TO 150º FROM 330º 330º FROM TO-FROM LINE 150º TO 150º-330º VOR 330º TO 150º FROM TO 330º FROM 150º TO 330º 150º Figure 3-14 TO-FROM and CDI Indications ET0171/Chaganti Page 3-24 AVIONIC SYSTEMS RM I N 0º RADIAL 0º RADIAL 0º HEADING 180º BEARING 270º RADIAL 90º VOR RADIAL RM I 180º RADIAL 0º HEADING 0º BEARING S 180º RADIAL Figure 3-15 Bearing Calculation ET0171/Chaganti Page 3-25 AVIONIC SYSTEMS Detector demodulates the IF signal and separate it into the original signal. The audio identification tone of VOR station is filtered, amplified and sent to cockpit speaker. The detected signal is applied to two separate circuits. 30 Hz variable signal is filtered, amplified and detected by one circuit, while other circuit detects 30 Hz reference signal. The variable 30 Hz and reference 30 Hz signals are compared for phase difference in phase comparator. The detected phase difference is converted into voltage in Sync format, which can be applied to Radio Magnetic Indicator (RMI). Course deviation left, right parameters and TO-FROM indications are also determined from the phase relationship between 30 Hz reference and 30 Hz variable signals. The Left/Right course deviation compares the angle computed from VOR signal and the course selector on Omni Bearing Selector (OBS). The TO-FROM indication on course direction is determined by shifting one of these signal phase by 90 degrees. VOR equipment can detect many possible problems. A warning flag, usually red, will appear on the face of the instrument if any of the following is detected.  No power or low power to the aircraft equipment  Failure of the aircraft’s equipment  Failure of the ground station equipment  Failure of the indicator  Weak signal from either the reference or variable signal The flag will also appear during tuning. The Transmitted VOR station signal is subject to errors, but for 95% of the time it must be at least better than ± 3º. The errors due to the airborne equipment and interpretation are similar, but when all errors are combined, the accuracy of the indication will be within ± 5º for that 95% of the time. ET0171/Chaganti Page 3-26 AVIONIC SYSTEMS LOC/VOR ANTENNA PRESELECTOR TUNE CONTROL MIXER VOR SYNTHESISER CONTROL UNIT AUDIO COCKPIT DETECTOR FILTER AMPLIFIER SPEAKER VARIABLE FILTER 30-Hz DETECTOR MONITOR CIRCUITS FILTER FM-9960 Hz PHASE DETECTOR COMPARATOR FLAG RMI SYNCHRO VOR DRIVER RADI MAGNETIC INDICATOR Figure 3-16 VOR Airborne Loading ET0171/Chaganti Page 3-27 AVIONIC SYSTEMS 3.9 Auto Pilot 3.9.1 ILS Interface with Automatic Flight Control System (AFCS) ILS interface consists of Flight Control Computer (FCC), which monitors ILS deviation data and applied to autopilot inside the FCC which converts deviation data into flying commands during automatic landing by aircraft control. Figure 3-17 shows two ILS receivers with flight course displays one for captain and other for first officer. Same LOC and G/S antennas are shared by these two receivers for receiving ILS signals by tuning on to selected localizer frequency from a common control unit. Localizer and glideslope deviations are displayed on electronic flight display instrument system. Once the aircraft nears the airport, pilot will switch from navigational VOR mode to ILS mode by changing the frequency of the control unit. This LOC frequency by default paired with glideslope frequency. The outer marker beacon allows the aircraft to start holding onto ILS course path. Once aircraft crosses all marker beacons and maintains localizer and glideslope deviation at the centre of the course deviation indicator, aircraft will approach with in the specified visual range from the run way, which can land safely. Flight Warning Computer (FWC) monitors the conditions of the ILS system and issue a warning SIGNAL to the pilot in case of any failure. 3.9.2 VOR Interface with Automatic Flight Control System (AFCS) VOR interface consists of Flight Control Computer (FCC) with autopilot. VOR bearing and course deviation from a selected track is monitored and converted into flying commands for flying along the selected track. Figure 3-18 shows two VOR receivers with flight course displays, marker system and Distance Measuring Equipment (DME). Control unit selects both VOR as well as DME stations; selected station audio monitoring is done with audio integration system. ET0171/Chaganti Page 3-28 AVIONIC SYSTEMS CAP F/O FLIGHT DISPLAY 1 FLIGHT DISPLAY 2 2 2 2 2 10 10 10 10 0 0 0 0 2 2 VOR VOR 2 2 0 0 NAV NAV 0 0 NAVIGATION NAVIGATION DISPLAY 1 ILS ILS DISPLAY 2 CONTROL UNIT FWC1 FWC2 ILS LOC ANTENNA ILS RECEIVER 1 G/S ANTENNA RECEIVER 2 INTERPHONE FCC1 FCC2 Figure 3-17 ILS Automatic Flight Control System ET0171/Chaganti Page 3-29 AVIONIC SYSTEMS CAP F/O FLIGHT DISPLAY 1 FLIGHT DISPLAY 2 VOR 1 VOR2 VOR VOR NAV NAV NAV 1 NAV 2 ILS ILS VOR FCC1 VOR RECEIVER 1 FCC2 RECEIVER 2 CONTROL UNIT 1 CONTROL UNIT 2 INTERPHONE LOC/VOR ANTENNA Figure 3-18 VOR Automatic Flight Control System ET0171/Chaganti Page 3-30 AVIONIC SYSTEMS VOR station bearing data and course deviation paths are displayed on electronic flight display instrument system. Once the aircraft is above 1000 feet, pilot can pick up VOR station frequency and fly along the radial TO or FROM that station. The frequencies and airway paths are determined from navigation charts provided to the pilot. VOR station bearing is determined by phase difference between received reference and variable signals, that difference is applied to the indicator and display. TO-FROM indication is also available for the pilots to show them which side of VOR station the aircraft is flying. Pilot continue to fly the aircraft on selected radial until he reaches next VOR station at that point he need to change station frequency. Once the aircraft reaches destination, pilot will switch over to LOC frequency for ILS landing. Flight Warning Computer (FWC) monitors the reliability of VOR system and issue a warning FLAG to the pilot in case of any failure. Notes ET0171/Chaganti Page 3-31