INSTRUMENT LANDING SYSTEM (ILS)

For a normal Visual Flight Rules (VFR) Approach, a pilot must have atleast 3 miles horizontal visibility with a ceiling not less than 1000 ft.

This is not always possible and might lead to plenty of diversions and flight cancellations.

Instrument Landing System (ILS) basically is used under Instrument Flight Rules (IFR) approach.

Purpose : Assist in landing

Principle: Difference in Depth of Modulation (DDM)

It has 3 components :

• Localizer – Horizontal Guidance

• Glideslope – Vertical Guidance

• Marker beacons – Distance from Touchdown

CATEGORIES OF ILS

Category IIIA is also called “See to Land”

Category IIIB is also called “See to Taxi”

LOCALIZER (LOC)

• Antenna present at the far end of the Runway

• Frequency: VHF

• Frequency Range: 108- 112 MHz (specifically from 108.10 – 111.95 MHz)

Note: LOC uses the odd tenth decimal whereas VOR uses the even tenth decimal

• Identification frequency: 1020 Hz morse code ( Consists of 4 letters, transmitted atleast 6 times/min)

• Channel spacing: 50 KHz (also for VOR)

• Total number of channel : 200

• Range: 25 NM (10° variation) & 17 NM (35° variation)

OPERATION

The localizer antenna emits two set of signals of the same frequency used by that particular airport for its landing purpose.

But the signal towards the right side is modulated at 150 Hz.

And

The signal to the left is modulated at 90 Hz.

The direction left and right is with respect to the view of the pilot at the time of landing.

The 150 Hz modulation will tell the pilot that the aircraft is towards the right side from the centre of the runway

The 90 Hz modulation will tell the pilot that the aircraft is towards the left side from the centre of the runway.

The modulation at the centre of the runway is 20% from each beam.

With the help of the percentage of modulation the pilot will know to what extent/degree of deviation the aircraft is in from the centre of runway.

Percentage can be in terms of 20%, 30%…..

Since both the 90 Hz and 150 Hz modulation will deliver a 20% modulation exactly at the centre,

Therefore,The Difference in Depth of Modulation (DDM) will be,20% – 20% = 0

When the aircraft receives a 0% modulation, its exactly at the centre of the runway.

In addition to the 90 and 150 Hz tones, the localizer carrier is modulated with an identification tone of 1020 Hz and possibly (exceptionally category III) voice modulation for ground-to-air communication.

The identification of a beacon consists of two or three letters transmitted by keying the 1020 Hz tone so as to give a Morse code representation.

The identification is transmitted not less than six times per minute when the localizer is operational.

COCKPIT INDICATION

The measured DDM will be converted to voltage which will be generated to drive the lateral deviation bar from the set of command bars.

If the bar is moving left, the aircraft needs to turn left to align to the runway

If the bar is moving right, the aircraft needs to turn right to align to the runway.

The pilot needs to maintain the bar at the centre of the LOC deviation scale to make an approach.

The LOC deviation scale comprises of 5 dots where each dot represents 1.5°

Localizer backcourse – Its the feature of the antenna which allow aircrafts to have a non precision approach or take-off by reversing the signals. Gradually the indications in the cockpit will also be reversed.

GLIDESLOPE (G/S)

• Antenna present at the Threshold of the runway

• Frequency: UHF

• Frequency Range: 328.6 – 335.4 MHz

• Channel spacing: 150 KHz

• Total number of channel : 40 (paired with Localizer)

OPERATION

Glideslope works on the similar principle as the Localizer.

i.e. Two beams are radiated from the antenna. The upper beam and the lower beam with the same frequency used by that particular airport for their landing purpose.

The upper beam is modulated at 90 Hz and the lower beam is modulated at 150 Hz.

The percentage of modulation will determine to what extent/degree of deviation the aircraft is from the ideal glidepath.

Percentage can be in terms of 40%, 50%…..

The ideal glidepath is 3° which might increase to a maximum of 3.5° for some airports.

A greater angle than that would lead to structural damage.

When the aircraft is exactly at the correct glidepath, where the beams intersect, the Difference in Depth of Modulation will be,40% – 40% = 0

At the exact glidepath the DDM of each modulation will be 40%

Therefore, when the DDM is 0, the aircraft is following an exact designated glidepath

Note: Glideslope is is never a straight line but rather a parabolic curve.

COCKPIT INDICATION

The measured DDM will be converted to voltage which will be generated to drive the vertical deviation bar from the set of command bars.

If the bar is moving up, the aircraft needs to climb up to align to the runway

If the bar is going down, the aircraft needs to sink down to align to the runway.

The pilot needs to maintain the bar of he G/S deviation scale at the centre to make an approach.

The G/S deviation scale comprises of 5 dots where each dot represents 0.5°