Nav Topic 10 instrument landing systems

FOR TRAINING PURPOSE ONLY

Subject Code: AFD 31202

Malaysian Institute of Aviation Technology

Issue No : 000

Introduction

The instrument landing system (ILS) is designed to allow pilots the opportunity The instrument landing system (ILS) is designed to allow pilots the opportunity to land their aircraft with the aid of instrument references. to land their aircraft with the aid of instrument references.

A typical ILS system will allow the pilot to bring an aircraft to within ½ M of the A typical ILS system will allow the pilot to bring an aircraft to within ½ M of the runway and less than 200 ft above the runway without any external visual runway and less than 200 ft above the runway without any external visual references. references.

At these minimums (the decision height) the pilot must identify the runway At these minimums (the decision height) the pilot must identify the runway environment in order to continue the landing process. environment in order to continue the landing process.

If the runway environment cannot be identified, the pilot must execute a missed approach procedure.

An extremely accurate ILS, called the Category IIIC approach, will allow for aircraft landings with near zero visibility.

Both the airport facilities and the aircraft must be correctly equipped and certified for a Category IIIC ILS approach.




Issue No : 000

The ILS provides a The ILS provides a horizontal directional referencehorizontal directional reference and a and a vertical referencevertical reference called the called the glide slopeglide slope. .

The The directional referencedirectional reference signal is produced by the runway signal is produced by the runway localizerlocalizer transmitter.transmitter.

It is installed approximately 1000 ft [305 m] from the far end of the runway It is installed approximately 1000 ft [305 m] from the far end of the runway

It operates at frequencies of 108 to 112 MHz. It operates at frequencies of 108 to 112 MHz.

The glide slope signal is produced by the glide slope transmitter, The glide slope signal is produced by the glide slope transmitter,

It is located near the side of the runway on a line perpendicular to the It is located near the side of the runway on a line perpendicular to the runway centerline at the point where airplane touchdown occurs. runway centerline at the point where airplane touchdown occurs.

This point is generally about 15 percent of the runway length from the This point is generally about 15 percent of the runway length from the approach end of the runway. approach end of the runway.

The glide slope transmitter operates at a frequency of 328.6 to 335.4 MHz. The glide slope transmitter operates at a frequency of 328.6 to 335.4 MHz.




Issue No : 000

Instrument landing systems were developed in 1942 in response to the need for pilots to locate and fly to an airport during times of poor visibility.

With the use of the ILS, the pilot is able to fly the aircraft, without outside visual reference, along the approach path to a pre-determined decision height (usually a minimum of 200 feet altitude) before establishing visual contact with the runway approach lights.

If visual contact is not established at this point, the pilot must: If visual contact is not established at this point, the pilot must:

select an alternative airport in which to land the aircraft, or select an alternative airport in which to land the aircraft, or

circle the airport until such time that visibility improves.circle the airport until such time that visibility improves.

The ILS transmitters located at or near the airport facility, The ILS transmitters located at or near the airport facility,

radiate directional signals which are received by the airborne receiver. radiate directional signals which are received by the airborne receiver.




Issue No : 000

The directional signals are then processed to provide the pilot with: The directional signals are then processed to provide the pilot with: a visual horizontal directional reference, a visual horizontal directional reference, vertical reference, and vertical reference, and distance reference along the approach path. distance reference along the approach path.

The The localizer transmitterlocalizer transmitter provides the provides the horizontal reference signalhorizontal reference signal, , displayed by the "displayed by the "left/rightleft/right" needle on the course deviation indicator " needle on the course deviation indicator (CDI). (CDI).

The The vertical reference signalvertical reference signal, obtained from the , obtained from the glideslope transmitterglideslope transmitter, ,

results in an "results in an "up/downup/down" needle indication on the CDI." needle indication on the CDI.

The pilot flies an ILS approach by centering the The pilot flies an ILS approach by centering the left/rightleft/right and and up/downup/down needles on his CDI needles on his CDI

while monitoring the marker cues to determine progress along his or her while monitoring the marker cues to determine progress along his or her descent. descent.




Issue No : 000

In order to provide pilots with an indication of their distance from the runway, In order to provide pilots with an indication of their distance from the runway, Marker BeaconMarker Beacon transmitters are installed. transmitters are installed.

Marker BeaconMarker Beacon transmitters provide an transmitters provide an audioaudio ( (tonetone) and ) and visualvisual ( (lightlight) indication ) indication along the approach path. along the approach path.

The marker-beacon transmitter operates at a frequency of 75 MHz and The marker-beacon transmitter operates at a frequency of 75 MHz and produces both aural and visual signals. produces both aural and visual signals.

Marker Beacon located at pre-determined distances from the end of the runway.

The Outer-marker transmitter

The Mid-marker transmitter

The Inner-marker transmitter




Issue No : 000

The outer-marker transmitter: Located at approximately 5 mi [8 km] from the runwayProduces a 400 Hz intermittent signal Illuminate a blue indicator light on the instrument panel.

The The mid-marker transmittermid-marker transmitter : : Located at approximately Located at approximately 1/31/3 mimi [ [11 kmkm] from the runway] from the runwayProduces a signal modulated at Produces a signal modulated at 13001300 HzHz Illuminate an Illuminate an amberamber marker-beacon light marker-beacon light on the instrument panel. on the instrument panel.

The inner-marker transmitter: Located approximately 1500 ft from the end of the runway. Produced a signal modulated at 3000 Hz Illuminate a white panel light when the aircraft is over the appropriate position.

The white lamp is usually labeled "FM/Z" because the 3000-Hz signal is also produced by en route airway, or "Z," markers.




Issue No : 000

Localizer

A typical VHF localizer station:

Located 1,000 feet from the end of the runway and

Offset 300 feet from the runway centerline.

It has a horizontally-polarized antenna array

Radiates approximately 100 watts of RF power on one of 40 available channels spaced alternately at 50 and 150 kHz apart

VHF frequency range of 108.10 MHz to 111.95 MHz.

Identified by the transmission of a four-letter identification code modulated at 1,020 Hz and also by voice identification.

Principles of Localizer (LOC)




Issue No : 000

The localizer station radiates a beam of information indicating the horizontal The localizer station radiates a beam of information indicating the horizontal centerline of the runway. centerline of the runway.

This beam is produced by two transmitters: This beam is produced by two transmitters:

Operating on the same channel frequency Operating on the same channel frequency

Amplitude-modulated with different audio signals, 90 Hz and 150 HzAmplitude-modulated with different audio signals, 90 Hz and 150 Hz

The transmitting antennas are aligned to radiate two lobes that intersect the The transmitting antennas are aligned to radiate two lobes that intersect the runway centerline. runway centerline.

Viewed from the approach end of the runway: Viewed from the approach end of the runway:

The 150-Hz modulated lobe is on the rightThe 150-Hz modulated lobe is on the right

The 90-Hz lobe is on the leftThe 90-Hz lobe is on the left

Principles of Localizer (cont’d)




Issue No : 000

When the aircraft approaches the runway: When the aircraft approaches the runway:

The navigation receiver demodulates the received signal and The navigation receiver demodulates the received signal and

The localizer instrumentation unit compares the amplitude of the 90-Hz The localizer instrumentation unit compares the amplitude of the 90-Hz and 150-Hz signal levels and 150-Hz signal levels

provide a deviation signal output to the CDI which is proportional to the provide a deviation signal output to the CDI which is proportional to the balance or unbalance of the 90-Hz and 150-Hz signals. balance or unbalance of the 90-Hz and 150-Hz signals.

The aircraft is centered with the runway when the 90-Hz and 150-Hz localizer The aircraft is centered with the runway when the 90-Hz and 150-Hz localizer signals are equal in amplitude as indicated by zero deflection of the left/right signals are equal in amplitude as indicated by zero deflection of the left/right needle on the CDI. needle on the CDI.





Issue No : 000 Flying a localizer course




Issue No : 000

The course width is the amount of off-course distance for a given needle The course width is the amount of off-course distance for a given needle indication, indication,

This is set at 2.50 off-center for full-scale needle deflection. This is set at 2.50 off-center for full-scale needle deflection.

For example, if the aircraft is five miles from the runway: For example, if the aircraft is five miles from the runway:

A full left or right needle deflection on the CDI would represent a A full left or right needle deflection on the CDI would represent a distance of approximately 1,500 feet from the left or right of the runway distance of approximately 1,500 feet from the left or right of the runway centerline. centerline.

Naturally, as the aircraft flies closer to the runway, the sensitivity of the Naturally, as the aircraft flies closer to the runway, the sensitivity of the left/right deflection increases. left/right deflection increases.





Issue No : 000

Typical airborne localizer instrumentation circuits utilize the same VHF Typical airborne localizer instrumentation circuits utilize the same VHF receiver that is used for VOR navigation. receiver that is used for VOR navigation.

In some cases, a common VHF receiver operating in the frequency range of In some cases, a common VHF receiver operating in the frequency range of 108.000 to 135.975 MHz is used for: 108.000 to 135.975 MHz is used for:

VHF communication (118.000 to 135.975 MHz)VHF communication (118.000 to 135.975 MHz)VOR navigation (108.000 to 117.950 MHz)VOR navigation (108.000 to 117.950 MHz)LOC operation (108.100 to 111.950 MHz). LOC operation (108.100 to 111.950 MHz).

The 40 localizer channels are located on odd-tenths frequencies to distinguish The 40 localizer channels are located on odd-tenths frequencies to distinguish them from the even-tenths VOR frequencies. them from the even-tenths VOR frequencies.

The navigation control head is used by the pilot to select the desired VHF The navigation control head is used by the pilot to select the desired VHF frequency. frequency.

The navigation control head outputs a control signal when an odd-tenth The navigation control head outputs a control signal when an odd-tenth frequency below 112 MHz is selected. frequency below 112 MHz is selected.

Localizer Circuit Theory




Issue No : 000

The control signal switches The control signal switches the instrumentation unit from the instrumentation unit from the VOR mode to the LOC the VOR mode to the LOC modemode

The LOC signal is The LOC signal is horizontally-polarized. horizontally-polarized.

Allowing the VOR Allowing the VOR antenna to also be used antenna to also be used for localizer reception. for localizer reception.

In typical localizer In typical localizer instrumentation, the audio instrumentation, the audio output from the VHF output from the VHF navigation receiver is fed to navigation receiver is fed to two filters. two filters.

Localizer Circuit Theory (cont’d)

Typical localizer instrumentation block diagram




Issue No : 000

One filter passes the 90-Hz One filter passes the 90-Hz modulation component, and modulation component, and the other separates the 150-the other separates the 150-Hz component from the Hz component from the detected localizer signal. detected localizer signal.

The filtered signals are then The filtered signals are then rectified to produce a DC rectified to produce a DC signal signal

It energizes a It energizes a galvanometer movement galvanometer movement to form a left/right needle to form a left/right needle indication on the CDI indication on the CDI relative to the localizer relative to the localizer course. course.

Typical localizer instrumentation block diagram





Issue No : 000

LOC steering information is obtained by comparing the difference in output LOC steering information is obtained by comparing the difference in output levels from the two bandpass filters. levels from the two bandpass filters.

In ILS mode, the output of these filters are summed together to provide the In ILS mode, the output of these filters are summed together to provide the voltage necessary to pull the "NAV" warning flag on the CDI out-of-view. voltage necessary to pull the "NAV" warning flag on the CDI out-of-view.

The NAV flag appears from beneath a cutout on the face of the indicator The NAV flag appears from beneath a cutout on the face of the indicator when the output from either one or both of the filters falls below a useable when the output from either one or both of the filters falls below a useable level. level.

The “The “to/fromto/from” indication on the CDI is intended only for VOR operation” indication on the CDI is intended only for VOR operation

It is biased out-of-view in LOC mode and disappears beneath the metal It is biased out-of-view in LOC mode and disappears beneath the metal cutout on the indicator. cutout on the indicator.





Issue No : 000

The VIR-351 tunes a frequency range from 108.00 MHz to 117.95 MHz with 50-kHz spacing between channels.

This spacing establishes 200 channels in which 40 are used for localizer guidance on ILS equipped runways.

The VIR-351 navigation receiver processes the VOR and localizer signals separately while using common video and deviation amplifiers.

Localizer Receiver, S-TEC VIR-351

S-TEC VIR-351 localizer converter block diagram




Issue No : 000

ILS logic, supplied by the front panel frequency selection switches, ILS logic, supplied by the front panel frequency selection switches, automatically controls the receiver mode of operation. automatically controls the receiver mode of operation.

Selection of an ILS frequency automatically disables operation in the VOR Selection of an ILS frequency automatically disables operation in the VOR mode. mode.

During operation in the ILS mode, the localizer converter: During operation in the ILS mode, the localizer converter:

Receives the detected signal from the receiver section,Receives the detected signal from the receiver section,

Amplifies the composite signal in the same video amplifier that is used for Amplifies the composite signal in the same video amplifier that is used for VOR operationVOR operation

Isolates the 90-Hz and 150-Hz components of the input signal in two Isolates the 90-Hz and 150-Hz components of the input signal in two bandpass filters.bandpass filters.

The isolated 90-Hz and 150-Hz signals are then rectified and compared by a The isolated 90-Hz and 150-Hz signals are then rectified and compared by a matched resistive differential network to determine the amplitude difference matched resistive differential network to determine the amplitude difference between the two signals. between the two signals.

Localizer Receiver, S-TEC VIR-351 (cont’d)




Issue No : 000

The difference data, which is directly proportional to the localizer course The difference data, which is directly proportional to the localizer course deviation, is applied to the deviation amplifier. deviation, is applied to the deviation amplifier.

Equal amounts of each signal result in a localizer left/right deviation bar that is Equal amounts of each signal result in a localizer left/right deviation bar that is centered on the CDI, while an un-proportional relationship results in a deviation centered on the CDI, while an un-proportional relationship results in a deviation of the left/right bar from center. of the left/right bar from center.

On larger aircraft the ILS and VOR control panels are usually combined and On larger aircraft the ILS and VOR control panels are usually combined and located on the instrument panel; located on the instrument panel;

the receivers are typically separate units located in the radio equipment the receivers are typically separate units located in the radio equipment rack. rack.

Light-aircraft ILS receivers are usually combined with the receiver for VHF Light-aircraft ILS receivers are usually combined with the receiver for VHF omnirange (VOR) and are often designated as VOR LOC receivers. omnirange (VOR) and are often designated as VOR LOC receivers.





Issue No : 000

The indicators for The indicators for the system are the system are often combined often combined with other with other indicators that indicators that provide a number provide a number of navigation of navigation indications.indications.

Among these are Among these are the attitude-director the attitude-director indicator (ADI) and indicator (ADI) and the horizontal-the horizontal-situation indicator situation indicator (HSI) (HSI)

Attitude-director indicator





Issue No : 000

Glide SlopeGlide Slope

The typical glideslope transmitter is usually: The typical glideslope transmitter is usually: located 750 feet from the beginning of the runway and located 750 feet from the beginning of the runway and radiates a 5-watt RF signal from a horizontally polarized antenna array at radiates a 5-watt RF signal from a horizontally polarized antenna array at an inclined glidepath angle of 2.50 to 3.00. an inclined glidepath angle of 2.50 to 3.00. operates on one of 40 available channels provided by 150-kHz spacing operates on one of 40 available channels provided by 150-kHz spacing UHF frequency range of 329.15 MHz to 335.00 MHz. UHF frequency range of 329.15 MHz to 335.00 MHz.

Like the localizer station, it also generates two separate directional lobes on the same carrier frequency.

The lobe radiated above the glidepath angle is amplitude-modulated at 90 Hz. The lobe below is 150 Hz amplitude-modulated.

The glideslope receiver compares the amplitude of the 90-Hz and 150-Hz modulation components to determine the aircraft's position in the vertical plane.

Principles of Glideslope (GS)




Issue No : 000

Principles of Glideslope (cont’d)

Flying the glidepath




Issue No : 000

When the two signals are equal, the aircraft is centered on the glidepath. When the two signals are equal, the aircraft is centered on the glidepath.

It is indicated by zero deflection of the up/down needle on the CDI.It is indicated by zero deflection of the up/down needle on the CDI.

The glideslope needle is much more sensitive than the localizer needle.The glideslope needle is much more sensitive than the localizer needle.

Its course width is set at 0.70 off-centers for full-scale deflection. Its course width is set at 0.70 off-centers for full-scale deflection.

At five miles from the end of the runway, a full-scale deflection of the up/down At five miles from the end of the runway, a full-scale deflection of the up/down needle would correspond to a distance of roughly 250 feet above or below the needle would correspond to a distance of roughly 250 feet above or below the centerline of the approach path. centerline of the approach path.





Issue No : 000


Glideslope and Localizer path




Issue No : 000

An illustration of a typical localizer course and glidepath used for an ILS An illustration of a typical localizer course and glidepath used for an ILS approach is shown in Figure above. approach is shown in Figure above.

Aircraft "A” is centered on the ILS approach path where: Aircraft "A” is centered on the ILS approach path where:

The localizer and glideslope 90-Hz and 150-Hz signals are equal in The localizer and glideslope 90-Hz and 150-Hz signals are equal in amplitude. amplitude.

The localizer The localizer left/rightleft/right needle and glideslope needle and glideslope up/downup/down needle to be needle to be centered on the course deviation indicator. centered on the course deviation indicator.

Aircraft "B" is flying above the glidepath and to the right of the localizer course. Aircraft "B" is flying above the glidepath and to the right of the localizer course. In this case, the 90-Hz glideslope signal dominates. In this case, the 90-Hz glideslope signal dominates. The “The “downdown” needle deflection on the CDI.” needle deflection on the CDI.The “The “leftleft” needle deflection is the result of the 150-Hz localizer signal.” needle deflection is the result of the 150-Hz localizer signal.

The steering needles command the pilot to fly the aircraft down and to the left The steering needles command the pilot to fly the aircraft down and to the left to intercept the center of the ILS approach path to the runway. to intercept the center of the ILS approach path to the runway.





Issue No : 000

The localizer course and glidepath signals intersect at a common point along The localizer course and glidepath signals intersect at a common point along the center of the ILSthe center of the ILS

Approach to provide an “X” and “Y” axis, respectively, along the path of Approach to provide an “X” and “Y” axis, respectively, along the path of descent. descent.

Marker Beacon transmitters are located at predetermined locations beneath this Marker Beacon transmitters are located at predetermined locations beneath this path to provide the pilot with an indication of the aircraft's distance from the end path to provide the pilot with an indication of the aircraft's distance from the end of the runway. of the runway.





Issue No : 000

Glideslope receivers operate in the UHF portion of the RF frequency band Glideslope receivers operate in the UHF portion of the RF frequency band between 329.5 MHz and 335.0 MHz. between 329.5 MHz and 335.0 MHz.

The glideslope is designed to work in conjunction with the localizer to provide The glideslope is designed to work in conjunction with the localizer to provide the pilot with both a vertical and horizontal reference along the ILS approach. the pilot with both a vertical and horizontal reference along the ILS approach.

It also has 40 channels which are paired to the 40 localizer channelsIt also has 40 channels which are paired to the 40 localizer channels

To select the appropriate glideslope channel: To select the appropriate glideslope channel:

The pilot merely tunes in the desired localizer frequency on the navigation The pilot merely tunes in the desired localizer frequency on the navigation control head control head The glideslope frequency is automatically channeled to the corresponding The glideslope frequency is automatically channeled to the corresponding localizer frequency. localizer frequency.

Table bellow shows the paired glideslope and localizer frequencies. Table bellow shows the paired glideslope and localizer frequencies.

Glideslope Circuit Theory




Issue No : 000

108.95 108.90110.55110.50108.55108.50110.75110.70108.75108.70

329.15 329.30329.45329.60329.75329.90330.05330.20330.35330.50

334.70 334.55334.10333.95329.90329.75330.50330.35329.30329.15

108.10 108.15108.30108.35108.50108.55108.70108.75108.90108.95

PAIRED LOC

FREQ (MHz)

GS FREQ (MHz)

PAIRED GS FREQ

(MHz)

LOC FREQ (MHz)

110.95110.90111.95111.90109.15109.10111.15111.10109.35109.30

330.65330.80330.95331.10331.25331.40331.55331.70331.85332.00

331.40331.25332.00331.85332.60332.45333.20333.05333.80333.65

109.10109.15109.30109.35109.50109.55109.70109.75109.90109.95

PAIRED LOC

FREQ (MHz)

GS FREQ (MHz)

PAIRED GS FREQ

(MHz)

LOC FREQ (MHz)

Table. Localizer and glideslope frequency pairing





Issue No : 000

111.35111.30109.55109.50111.55111.50109.75109.70111.75111.70

332.15332.30332.45332.60332.75332.90333.05333.20333.35333.50

334.40334.25335.00334.85329.60329.45330.20330.05330.80330.65

110.10110.15110.30110.35110.50110.55110.70110.75110.90110.95

PAIRED LOC

FREQ (MHz)

GS FREQ (MHz)

PAIRED GS

FREQ (MHz)

LOC FREQ (MHz)

109.95109.90108.35108.30110.15110.10108.15108.10110.35110.30

333.65333.80333.95334.10334.25334.40334.55334.70334.85335.00

331.70331.55332.30332.15332.90332.75333.50333.35331.10330.95

111.10111.15111.30111.35111.50111.55111.70111.75111.90111.95

PAIRED LOC

FREQ (MHz)

GS FREQ (MHz)

PAIRED GS

FREQ (MHz)

LOC FREQ (MHz)

Table. Localizer and glideslope frequency pairing (cont’d)





Issue No : 000

Superheterodyne receivers are used to receive the UHF glideslope signals.Superheterodyne receivers are used to receive the UHF glideslope signals.

The instrumentation circuitry for glideslope operation is identical to that used The instrumentation circuitry for glideslope operation is identical to that used for the localizer. for the localizer.

The detector supplies the 90-Hz and 150-Hz modulation components to two The detector supplies the 90-Hz and 150-Hz modulation components to two bandpass filters. bandpass filters.

It separates the signals to provide an output to drive the up/down needle It separates the signals to provide an output to drive the up/down needle on the CDI. on the CDI.

A "GS" flag is also provided to indicate an unusable glideslope signal when A "GS" flag is also provided to indicate an unusable glideslope signal when the flag appears on the indicator. the flag appears on the indicator.

Glideslope Circuit Theory (cont’d)




Issue No : 000

The glide slope transmitter operates on a principle similar to that of the The glide slope transmitter operates on a principle similar to that of the localizer. localizer. The glide slope transmitter is located at a distance from the approach end of The glide slope transmitter is located at a distance from the approach end of the runway.the runway.

Approximately 15 percent of the length of the runway. Approximately 15 percent of the length of the runway.

Glideslope Circuit Transmitter

A schematic of radiation pattern from a glide slope transmitter




Issue No : 000

If an airplane is approaching the runway and is above the glide path:If an airplane is approaching the runway and is above the glide path:

The 90-Hz signal will predominateThe 90-Hz signal will predominate

If the airplane is below the glide path:If the airplane is below the glide path:

The 150-Hz signal will predominate The 150-Hz signal will predominate

The glide slope receiver will provide an output for the cross-pointer indicatorThe glide slope receiver will provide an output for the cross-pointer indicator

The pilot will have a visual indication of the airplane's position with respect The pilot will have a visual indication of the airplane's position with respect to the glide path. to the glide path.

If the horizontal pointer is above the center of the indicatorIf the horizontal pointer is above the center of the indicator

The airplane is below the glide path. The airplane is below the glide path.

Glideslope Circuit Transmitter (cont’d)




Issue No : 000

A beam provided by the A beam provided by the combination of localizer and combination of localizer and glide slope transmitter is glide slope transmitter is electronically exactelectronically exact

Provides a precise path Provides a precise path by which an airplane may by which an airplane may approach a runwayapproach a runway

Reach the point of Reach the point of touchdown. touchdown.

This is a most valuable This is a most valuable aid in conditions of poor aid in conditions of poor visibility in the vicinity of visibility in the vicinity of an airport. an airport.

ILS transmission pattern from localizer and glide slope

Glideslope Circuit Transmitter (cont’d)

Entertainment & Humor

Nav Topic 10 instrument landing systems