The instrument landing system
Navigation · Chapter 40 · DGCA CPL and ATPL
The localizer and glide path in real 3D, a free ILS simulator you can fly from the cockpit, the markers, the dots and the sums, categories, and what spoils an ILS.
Remember
- Localizer 108–111.975 (odd tenths) VHF; glide path UHF 329–335 paired; markers 75 MHz.
- 90 Hz left and above; 150 Hz right and below. More 150: fly left and up.
- False glide paths are above; capture from below. 3° ≈ 300 ft/NM; descent ≈ 5 × GS.
- Localizer dot 0.5° (full 2.5°); glide path dot 0.14° (full 0.7°); beyond half scale, go around.
- OM blue 400 Hz; MM amber 1 300 Hz; IM white 3 000 Hz. Cat I 200 ft/550 m, Cat II 100 ft/300 m.
What does an ILS give you that a VOR or NDB cannot?
A precision approach
- Left-right and up-down • An ILS (instrument landing system) guides you down to the runway. A VOR or NDB tells you only left or right; an ILS also tells you up or down • Guidance in both directions is what makes it a precision approach • Watch the panel: as the aircraft wanders, both bars move
- Down to decision height • You follow the bars down to the dh (decision height) • There you decide: runway lights in sight, land; if not, go around
- Three parts • Localizer: left-right, lined up with the runway • Glide path (also called the glideslope): up-down, a slope of about 3° • Markers or a DME: how far you have to go
Where are the aerials, and on what frequencies?
Where it stands, what it uses
- The whole airfield • Localizer about 300 m past the far end, on the centre line • Glide path about 300 m in from the threshold, to one side • Markers under the approach
- The localizer aerials • A wide row of aerials on 108–111.975 MHz, odd tenths (VHF, 40 channels) • A wide row of aerials makes narrow beams, just as a wide dish makes a narrow radar beam
- The glide-path mast • A short mast on 329.15–335 MHz (UHF), beside touchdown • It sits off to the side, so it is not something you can hit
- Pairing • Every localizer frequency has its own glide-path frequency: pairing • You dial one number and both tune: less work, fewer mistakes, one ident
- Ident and DME • Ident on the localizer only: 2 or 3 letters, usually starting with I • A paired DME reads zero at the threshold, protected up to 25 000 ft
What do the three markers look and sound like?
The markers
This lesson is part of the Navigation plan. See plans
How far out does it work, and what if it goes wrong?
Monitoring and coverage
This lesson is part of the Navigation plan. See plans
How does the localizer tell you left or right?
Localizer: 90 and 150 Hz
This lesson is part of the Navigation plan. See plans
What happens behind the localizer?
The back course
This lesson is part of the Navigation plan. See plans
How does the glide path tell you up or down?
Glide path, false paths, datum
This lesson is part of the Navigation plan. See plans
How many degrees is a dot, and when do you go around?
Dots and the safe limit
This lesson is part of the Navigation plan. See plans
How high should you be, and how fast should you descend?
Height and descent rate
This lesson is part of the Navigation plan. See plans
Solve it: height and rate of descent on a 3° path
Solve it: height and descent rate
This lesson is part of the Navigation plan. See plans
Free ILS simulator: sit in the cockpit and fly the diamonds down to the runway
ILS simulator
- You are in the left seat • A real runway through the windscreen, with its approach lights, PAPI and markings • A full flight display: the localizer and glide-path diamonds sit on it, as in an airliner
- Fly it • ↑ pushes the nose down, ↓ pulls it up, ← → bank, W S thrust: an on-screen stick and lever on a phone • Keep both diamonds in the middle: about 2.5° nose up holds the 3° path at 140 kt
- Real weather, real decisions • A clear day, low cloud, Cat I and Cat II limits, night, and a day below limits: will you go around at decision height? • Hoops to learn with, a crosswind, an outside view with the beams, and a score out of three stars
How low can you go with an ILS?
Categories
This lesson is part of the Navigation plan. See plans
What can bend the beams?
What spoils an ILS
This lesson is part of the Navigation plan. See plans
What are the parts of an ILS approach?
The approach chart
This lesson is part of the Navigation plan. See plans
Test yourself on chapter 40
Test yourself
This lesson is part of the Navigation plan. See plans