06 Instrument Landing System (ILS)

Question 1

What are the components of an ILS?

Answer 1

• Localiser
• Glide Slope
• Markers (usually)
• DME (co-located)

Question 2

What frequency range does an ILS Localiser work at?

Answer 2

108 to 112 MHz

Question 3

On an ILS, what does a Localiser provide?

Answer 3

Localiser provides Lateral Guidance (Asometh Guidance)

Question 4

On an ILS, what does a Glide Path provide?

Answer 4

a Glide Path provides vertical guidance

Question 5

For an ILS system, where are the Localiser and the Glide Path antenna’s located?

Answer 5

• Localiser antenna is located at the (far) end of the runway
• Glide Path antenna is located 300 meters from the start of the runway (i.e. the touchdown point)

Question 6

For an ILS, what is the principle of operation of the Localiser?

Answer 6

• Antenna placed at the end of the runway emitting a radiation pattern which is amplitude modulated by a 90Hz and 150Hz signal
• Left ‘lobe’ at 90 MHz
• Right ‘lobe’ at 150 MHz
• AC can determine its alignment to the centre of the RW by measuring the depth of modulation from each signal
• The difference will be 0 directly down the centre of the RW

Question 7

For an ILS, what course deviation do the following indicate for the ‘Localiser’?

• Full scale deflection
• Half scale deflection

Answer 7

• Full scale deflection is at 2.5 degrees
• Half scale deflection is at 1.5 degrees
• Assuming a 5 dot indicator, each dot represents 0.5 degrees

Question 8

For an ILS Localiser, what is the purpose of having a ‘protected zone’?

Answer 8

By locking on to the localiser within the protected zone, it guarantees that the AC will not be effected by lateral lobe interference (fake lobes)

Question 9

What are the dimensions of the protected zone for an ILS Localiser?

Answer 9

• A cone of 10 degrees from the centre line out to a distance of 25nm
• A cone either side of the centre cone of 35 degrees from the centre line with a distance of 17nm

Question 10

For an ILS, what is the principle of operation of the Glide Path?

Answer 10

• Antenna is placed 300m from the start of the RW (touchdown point)
• Produces a radiation pattern which is amplitude modulated by a 90Hz and 150Hz signal
• Top one is at 90Hz
• Bottom one is at 150Hz
• AC measures the depth of modulation for each lobe and calculates the difference in depth of modulation between them
• Where this is 0 (i.e. directly between the two lobes) is the ideal Glide Path

Question 11

For an ILS, what does each ‘dot’ on the course deviation indicator for the ‘Glide Path’?

Answer 11

• One ‘dot’ is a 0.14 degree deflection

- Full scale deflection is at 0.7 degrees

Question 12

For an ILS, what is the horizontal coverage of the Glide Path?

Answer 12

Up to 8 degrees from the lateral central line and up to a distance of 10nm

Question 13

For an ILS, name the following for the Marker Beacons:

• Name/location
• Colour
• Range to touchdown
• Modulation frequency
• Pitch
• Ident

Answer 13

Inner Marker:

• White
• 75 to 150m
• 3000Hz
• High
• 6 dots/sec

Middle Marker:

• Amber
• 1050m
• 1300Hz
• Medium
• Alternating dots/dashes, 2 dots and 6 dashes per sec

Outer Marker:

• Blue
• 6.5 to 11.1 km
• 400Hz
• Low
• 2 dashes/sec

Question 14

What is the calculation for the Rate of Descent?

Answer 14

ROD (fpm) = Theta x Ground Speed (nm/min) x 100

Where Theta is the glide path angle

Question 15

What is the equation for the Height Passing?

Answer 15

Height Passing (ft) = Theta x Range from touchdown (nm) x 100

Question 16

At what thresholds will an ILS take action in the event of an error?

Answer 16

Action will be taken if:

• Localiser mean course shifts by:
  ~ CAT 1: +-35ft
  ~ CAT 2: +-25ft
  ~ CAT 3: +- 10ft
• Glide slope changes by 0.075 Theta (glide slope angle)
• Reduction in transmission power exceeds 50%

Question 17

For an ILS, what is the vertical coverage of the Glide Path?

Answer 17

• Between 0.45 x Theta and 1.75 x Theta

- Where Theta is the glide slope

Question 18

What sort of approach is an ILS approach?

Answer 18

Precision approach

Question 19

What frequency does an ILS Glide Slope work at?

Answer 19

• 328 to 335 MHz

- UHF band

Question 20

What frequency do ILS markers work at?

Answer 20

75 MHz

Question 21

How can you determine the difference between a VOR signal and an ILS signal?

Answer 21

• ILS signals and VOR signal share some of the same frequencies (ILS: 108 to 112MHz, VOR: 108 to 118MHz)
• An ILS will have an odd number after the decimal (xxx.yx where y will be odd)

Question 22

To what level above the runway do the following Category ILS glide slopes provide?

• CAT 1
• CAT 2
• CAT 3

Answer 22

• CAT 1: 200ft above the RW
• CAT 2: 50ft above the RW
• CAT 3: surface of the RW

Question 23

An ILS glide slope is usually intercepted at what distance?

Answer 23

3 to 10 nm