Instrument Landing System (ILS) – Ibex Aviation Academy

Instrument Landing System

Instrument landing system (ILS) provides horizontal and vertical guidance by its Localiser and Glide path to assist in bad weather landings
Localiser of ILS operates on frequencies from 108 to 111.975 MHz in the VHF band
Glide slope of ILS operates on frequencies from 329.15 to 335 MHz in UHF band
Since localiser and glide path are paired with each other, selection of localiser automatically selects its glide path
Distance from touch down are provided by marker beacons which are low powered NDB on the final approach
Alternately, DME paired with ILS provides distance information on final approach of ILS

Localizer transmits two overlapping lobes on the same frequency with different modulation in the approach path to runaway
Right hand lobe, called the blue sector is modulated at 150 Hz while the left hand lobe, called the yellow sector is modulated at 90 Hz
Equal signal strength of 150 Hz and 90 Hz modulation indicate that the aircraft is correctly placed along the extended centre line of runway
Difference in depth of modulation (DDM) is zero at the centre line and increases with deviation from the extended centre line
ILS receiver installed in the aircraft measures the DDM and indicates Go Left or Go Right to establish on the extended centre line of runway

In normal approach of less than 3 degrees, lateral coverage is 10 deg up to 25 NM and 35 deg up to 17 NM
In steep approach greater than 3 degrees, lateral coverage is 10 deg up to 18 NM and 35 deg up to 10 NM
Localiser is undependable outside the specified coverage area due to false beams from the localiser antenna

Localiser antenna emits back beams in the reverse direction that show reverse indications during approach on the opposite runway
Although Back beam approaches are permitted in some countries, the indications in aircraft would tell you ‘What not to do’
However, during missed approach procedure, back beams would show correct indications

Localiser indications on OBI are command indications to return to centre line which means Needle Left Fly left and Needle Right Fly Right
In a five dot OBI full scale deflection represents 2.5 degrees and hence each dot indicates 0.5 degrees
Established on localiser means aircraft is within half the full-scale deflection
Localiser indications on HSI are shown by an arrow or a needle
Warning flag on OBI will appear when carrier wave or any one of the modulating signal is absent

Glide-path transmits two overlapping lobes on the same frequency with different modulation in the approach path to runaway
Lower lobe, called the blue sector is modulated at 150 Hz while the higher lobe, called the yellow sector is modulated at 90 Hz
Equal signal strength of 150 Hz and 90 Hz modulation indicate that the aircraft is at the correct glide angle
Difference in depth of modulation (DDM) is zero at the correct glide path and increases with deviation from the correct glide path
DDM is measured in the receiver indicating Fly Up or Fly Down to reach the reference datum on the centre line at 50 ft over the threshold
ILS Transmits false glide path above twice the correct glide path angle due to ariel characteristics and reflected waves
Therefore, glide slope is to be intercepted from a lower height by establishing on the localiser before intercepting glide-path

In a normal approach, horizontal coverage of glide path is up to 8 degrees to a distance of 10 NM reducing to 8 NM in steep approach
Vertical coverage for glide-path extends from 0.45 x glide angle to 1.75 x glide angle
For a standard 3° glide path the vertical coverage would be from 1.65 deg to 2.25 deg deviation from the correct glide path or from 1.35 deg to 5.25 deg deviation from the horizontal

Glide-path indications on OBI are command indications to intercept the glide path which means Needle Up Fly Up and Needle Down Fly Down
Full scale deflection of CDI or OBI in ILS mode is 0.7 degrees above and below the glide path which means 0.14 degrees deflection per dot
Maximum safe deviation permitted is half of the full scale deflection which would mean 0.35 degrees above or below the glide-path

Marker beacons are low powered NDBs emit carrier waves with a frequency of 75 MHz providing distance from threshold on final approach
Outer marker signals blue light in aircraft with a 400 Hz modulated sound of 2 dashes per second at 3.5 to 6 NM from touchdown
Middle marker signals amber light in cockpit with a 1300 Hz modulated sound of alternate dots and dashes around 3500 feet from touchdown
Inner marker signals white light in flight deck with a 3000 Hz modulated sound of 6 dots per second at 250 to 1500 feet from touchdown
Polar diagram of marker beacons could be elliptical, bone, fan or circular in shape
ILS categories refer only to the ground equipment and quality of transmission
The usage of ILS is further restricted by landing aerodrome procedures, capability of airborne receiver and flight crew qualifications

Ground Monitor of ILS monitors emissions from an area of guaranteed coverage and removes the identification code in case of unacceptable accuracy levels
At times, Cat II or III ILS may be downgraded to lower categories depending on test results
Acceptable level for localiser are a shift from runway centre line by more than 35 ft for ILS Cat I, 25 ft for ILS Cat II and 20 ft for ILS Cat III
Acceptable level for Glide Path is a change by more than 0.075 × Basic glide path angle.
Reduction in power by more than 50% from the normal transmission also triggers ILS monitoring system

ILS installations are constrained by certain factors like limitation of 40 channels
Terrain effects restrict installation in hilly areas due to requirement of large clear flat land to
Fixed Glide Path limits the number of aircrafts on approach and does not cater to the needs of STOL aircraft
Protection of ILS site and avoiding interference from civilian FM broadcast is essential for safe ILS Operation