9
RADIO NAV — CHAPTER 9 · NOTES
ILS
by Ghost Aviator
LECTURE PARTS
Ghost Aviator
www.ghostaviator.com
Ghost Aviator
www.ghostaviator.com
Ghost Aviator
DGCA CPL / ATPL Study Notes • Radio Navigation
✈ Chapter 9: ILS
Instrument Landing System
📋 Contents
1. Introduction 2. ILS Components 3. ILS Frequencies & Pairing 4. Identification & Monitoring 5. Marker Beacons 6. ILS Coverage 7. Principle of Operation 8. Displays & Interpretation (CDI, HSI) 9. ILS Categories (ICAO) 10. Errors, Accuracy & Protected Areas 11. ILS Calculations 12. ILS Summary 13. Practice Questions (14 Q)1. Introduction
The Instrument Landing System (ILS) is the most accurate approach and landing aid in current use. It has been in existence for over 40 years and is the ICAO standard precision approach system.
📡 Key Characteristics
- Precision approach system — provides guidance in both horizontal and vertical planes
- Enables automatic landings (autoland) in Cat III conditions
- Guides the pilot from the coverage limit down to Decision Height (DH)
- At DH: land (if required visual references exist) or execute missed approach
Fig 9.1: The Instrument Landing System (ILS) — overview of all components
2. ILS Components
📡 Ground Installation: Three Main Components
| Component | Function | Location | Band |
|---|---|---|---|
| Localizer (LLZ) | Lateral (left/right) guidance — runway centreline | ~300 m from upwind end of runway | VHF |
| Glide Path (GP) | Vertical (up/down) guidance — glide slope | 300 m from threshold, ~200 m from runway edge abeam touchdown | UHF |
| Marker Beacons | Range check — cross-check height vs distance | OM, MM, IM at set distances from threshold | VHF 75 MHz |
✓ Optional/Associated Components
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- Locator (L): Low-powered NDB co-located at the OM beacon site
- DME: Frequency-paired with ILS; zero-referenced to runway threshold; supplements or replaces marker range information
- Back Course: Non-precision approach on back beam of localizer (approved in some countries; NOT permitted in UK)
3. ILS Frequencies & Pairing
Localizer (VHF)
108 – 111.975 MHz
40 channels, odd tenths
Glide Path (UHF)
329.15 – 335 MHz
40 channels, freq. paired
Markers (all)
75 MHz
OM + MM + IM
⚡ Localizer Frequency Sharing
In 108–112 MHz: Odd tenths = ILS Localiser (108.1, 108.3, 108.5…); Even tenths = VOR. Each ILS frequency also has a second step: e.g., 108.1 AND 108.15 for ILS.
📡 Frequency Pairing Advantages
- Selecting the localizer frequency automatically activates the GP receiver
- Reduces pilot workload — one selection activates both
- Reduces potential for wrong frequency selection
- Only one identifier needed for both localizer and glide path
DME Paired with ILS
📡 ILS/DME
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- DME ranges are zero-referenced to the ILS runway threshold
- Protected within ILS localizer service area up to 25,000 ft
- May also be used for published SIDs and STARs (with extended coverage when notified)
- Do not use DME outside stated limits — may give errors
4. Identification & Monitoring
📻 ILS Identification
- 2 or 3-letter Morse code; first letter is usually “I”
- Transmitted at 7 groups per minute
- Only one ident needed (shared by localizer and GP)
- Ident automatically suppressed if ILS becomes unserviceable
- During maintenance: ident replaced by continuous tone or removed completely
⚠ Critical Rules
- If ident is a continuous tone → ILS under maintenance → DO NOT USE
- If ident ceases during approach → ILS is unserviceable → discontinue immediately
- Unserviceable glide path only: ATC will warn; do not use GP
Ground Monitoring
🔍 Automatic Monitor Triggers Action When:
Actions: Cease radiation / Remove ident & nav components / Lower category (Cat II/III may revert to Cat I/II).
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| Parameter | Cat I Limit | Cat II Limit | Cat III Limit |
|---|---|---|---|
| Localizer shift from centreline | > 35 ft | > 25 ft | > 20 ft |
| Glide path angle change | > 0.075 × basic GP angle | ||
| Power reduction | > 50% | ||
5. Marker Beacons
Markers transmit at 75 MHz in a narrow fan-shaped vertical beam. The signal is received only when the aircraft flies through the fan (not directional). Reception is indicated by synchronous aural tones and cockpit lights.
Fig 9.2: Marker beacon radiation patterns — fan-shaped vertical beams
Fig 9.3: OM and MM positions, plus marker identification table
📡 Marker Beacon Summary
| Marker | Cockpit Light | Ident Signal | Audio Freq | Range from Threshold |
|---|---|---|---|---|
| OM (Outer) | BLUE | 2 dashes/sec | 400 Hz (Low pitch) | 6.5 – 11.1 km (3.5 – 6 NM) |
| MM (Middle) | AMBER | Alt dots & dashes 3/sec | 1300 Hz (Medium pitch) | 1050 m ± 150 m (3500′ ± 500′) |
| IM (Inner) | WHITE | 6 dots/sec | 3000 Hz (High pitch) | 75 – 450 m (250′ – 1500′) |
⚡ Memory Aid — Markers
Big Blue Beacon = Outer Marker (3 Bs, low dashes) —
Amber = Middle (Alternating) —
White & high = Inner (Whitest, Highest pitch, fastest dots)
✓ Z Markers
Z markers have a cylindrical vertical radiation pattern. Used to mark airway reporting points or co-located with NDBs. They indicate when the aircraft is overhead (compensating for the NDB cone of silence).
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6. ILS Coverage
Fig 9.5: Localizer coverage sectors — 25 NM within ±10°, 17 NM within ±35°
Fig 9.6: Glide path horizontal coverage — 10 NM within ±8° azimuth
Fig 9.7: Glide path vertical coverage — 0.45θ to 1.75θ above horizontal
📐 Localizer Coverage
Note: May be reduced to 18 NM (±10°) when alternative navaids provide satisfactory intermediate approach coverage.
| Sector | Range |
|---|---|
| Within ±10° of centreline | 25 NM (46.3 km) |
| 10° to 35° from centreline | 17 NM (31.5 km) |
| Outside ±35° | 10 NM (18.5 km) if provided |
📐 Glide Path Coverage
Where θ = promulgated glide path angle (usually 3°).
| Parameter | Value |
|---|---|
| Azimuth coverage | ±8° either side of centreline |
| Range | At least 10 NM |
| Vertical — upper limit | 1.75 × θ above horizontal |
| Vertical — lower limit | 0.45 × θ (may reduce to 0.3θ if required) |
⚡ Outside Coverage Sectors
If an aircraft uses the ILS outside published coverage sectors, it may receive false on-course or reverse sense signals. The ILS cannot guarantee correct indications outside its coverage area.
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7. Principle of Operation
7.1 Localizer
Fig 9.8: Localizer radiation pattern — two overlapping lobes with 90 Hz and 150 Hz modulation
📡 Localizer Operation
| Sector | Frequency | Colour Code | CDI Indication |
|---|---|---|---|
| Left-hand lobe (aircraft left of ℅) | 90 Hz | Yellow | Needle deflects RIGHT → Fly Right |
| Right-hand lobe (aircraft right of ℅) | 150 Hz | Blue | Needle deflects LEFT → Fly Left |
| On centreline | DDM = 0 | — | Needle centred |
DDM (Difference in Depth of Modulation) increases with distance from the centreline. The DDM drives the vertical needle of the CDI/HSI.
⚡ Memory Aid — Localizer Lobes
“150 = Blue = Right = Fly Left” (Right of centre → fly left to return to centre)
“90 = Yellow = Left = Fly Right”
“90 = Yellow = Left = Fly Right”
7.2 Back Course ILS
⚠ Back Course Disadvantages
- Glide path indications are incorrect (would guide to wrong end of runway)
- CDI needle sense is REVERSED for back course approach (HSI gives correct sense if front QDM selected)
- No range-check markers available
- Not permitted in the UK — ignore any back course indications in UK
7.3 Glide Path
Fig 9.9: Glide path radiation pattern — two lobes in vertical plane, with false glide slope above
📡 Glide Path Operation
| Lobe | Modulation | Aircraft Position | CDI Indication |
|---|---|---|---|
| Upper lobe (large) | 90 Hz | Aircraft above GP | GP needle deflects DOWN → Fly Down |
| Lower lobe (small) | 150 Hz | Aircraft below GP | GP needle deflects UP → Fly Up |
| On glide path | DDM = 0 | On the GP | GP needle centred |
Standard GP angle: 3° (ICAO requirement: 2° – 4°).
GP transmitter: UHF, located ~300 m upwind of threshold, ~200 m from runway edge.
7.4 False Glide Slopes
⚠ False Glide Slopes
- Caused by metallic structure reflections and antenna height/propagation characteristics
- First false GP at approximately twice the promulgated glide path angle
- For standard 3° GP: first false GP at approximately 6°
- False GPs always occur above the true glide slope
- Normal practice: intercept glide slope from below
- At Heathrow: continuous descent approach may capture GP from above — confirm height with DME/markers
✓ ILS Reference Datum Point
Located vertically above the intersection of the runway centreline and threshold at a specified height (~50 ft). The downward extended portion of the ILS glide path passes through this point. Height is published in the UK AIP, AD section.
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8. Displays & Interpretation
8.1 CDI (Course Deviation Indicator) in ILS Mode
Fig 9.10: ILS CDI — localizer needle (vertical) and glide path needle (horizontal) with annotations
📐 CDI Sensitivity in ILS Mode
| Parameter | Localizer | Glide Path |
|---|---|---|
| Sensitivity | 0.5° per dot | 0.14° per dot |
| Full Scale Deflection | ±2.5° (= 5 dots) | ±0.7° (= 5 dots) |
| Max safe deviation below GP | — | 2.5 dots fly up (= 0.35°) |
Compare with VOR mode: CDI sensitivity = 2°/dot in VOR mode, 0.5°/dot in ILS mode. 5× more sensitive!
⚠ Maximum Safe Deviations
- If left/right localizer deviation exceeds half full scale (>2.5 dots) → initiate immediate go-around
- If glide path fly-up exceeds 2.5 dots fly up → initiate immediate go-around (terrain clearance may be compromised)
8.2 HSI (Horizontal Situation Indicator) in ILS Mode
Fig 9.11: A typical HSI showing ILS components — lateral deviation bar and GP needle
📐 HSI in ILS Mode
- Course selector should be set to the QDM of the runway
- Deviation bar and GP needle give correct sense indications
- HSI gives correct sense even on back course (if front course QDM is set)
- Azimuth failure flag, ILS failure flag, and loss of power flag visible
8.3 Localizer On-Course Indications
Fig 9.12: Localizer on-course line indications — fly left/fly right CDI deflections
Fig 9.13: Glide path indications — fly up/fly down
⚡ Reading ILS CDI Summary
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| What you see | What it means | Action |
|---|---|---|
| Localizer bar right | Aircraft LEFT of centreline | Turn RIGHT |
| Localizer bar left | Aircraft RIGHT of centreline | Turn LEFT |
| GP needle high (fly up) | Aircraft BELOW glide slope | Pitch UP / reduce descent |
| GP needle low (fly down) | Aircraft ABOVE glide slope | Pitch DOWN / increase descent |
9. ILS Categories (ICAO)
📐 Facility Performance Categories (Ground Installation)
| Facility Cat | Glide Path Height at Reference Datum |
|---|---|
| Category I | Guidance to ≤ 200 ft (60 m) above threshold horizontal plane |
| Category II | Guidance to ≤ 50 ft (15 m) above threshold horizontal plane |
| Category III | Guidance from coverage limit to and along the runway surface |
📐 Operational Performance Categories (Aircrew/Aircraft)
| Operational Cat | Decision Height (DH) | RVR |
|---|---|---|
| Cat I | ≥ 200 ft (60 m) | ≥ 550 m |
| Cat II | ≥ 100 ft (30 m), < 200 ft | ≥ 300 m |
| Cat IIIA | < 100 ft (30 m) or no DH | ≥ 200 m |
| Cat IIIB | < 50 ft (15 m) or no DH | ≥ 75 m but < 200 m |
| Cat IIIC | No DH | No RVR limitation |
⚡ Memory Aid — ILS Categories
Cat I = 200ft / 550m | Cat II = 100ft / 300m | IIIA = 0ft / 200m | IIIB = 0ft / 75m | IIIC = Zero Zero
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10. Errors, Accuracy & Protected Areas
⚠ Sources of ILS Error
- Beam bends: Caused by atmospheric conditions — smooth, slow deflections
- Scalloping: Caused by reflections → rapid needle fluctuations impossible to follow
- Beam noise: Generated by transmitter interference
📐 ILS Protected Areas
| Area | Definition | When Enforced |
|---|---|---|
| Critical Area | Defined area around LLZ and GP antennae where vehicles/aircraft excluded | ALL ILS operations (any category) |
| Sensitive Area | Extends beyond critical area; controls parking/movement of vehicles/aircraft | Low visibility operations only |
| Cat II/III Hold | Pre-take-off holding points further from runway than in good weather | Cat II and III ops; marked with signs and red stop bars |
⚡ Other Factors Affecting Accuracy
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- Weather: Snow and heavy rain attenuates ILS signals, reducing range and accuracy
- FM Broadcasts: FM stations below 108 MHz can overspill into radio nav band. FM suppression circuits mandatory in ILS receivers since late 1990s.
- Serviceability check: Ensure failure flags not visible; monitor ident throughout approach
11. ILS Calculations
Fig 9.14: ILS approach chart — showing initial, intermediate and final approach segments
📐 Approach Chart Segments
| Segment | Between | Notes |
|---|---|---|
| Initial approach | From entry to IAF (Initial Approach Fix) | Sector Safety Altitudes (SSA) apply |
| Intermediate | IAF to FAF (Final Approach Fix) | Intercept localizer and configure aircraft |
| Final approach | After FAF to DH/touchdown | ILS guidance; check markers/DME |
Height Check on Glide Path (1:60 Rule)
✓ Formula: Height on GP
Height (ft) = GP angle × Range (NM) × 100
(Simplified from the 1:60 rule: GP angle × range/60 × 6076 ft ≈ GP angle × range × 100)
✓ Formula: Rate of Descent (ROD) for 3° GP
ROD (fpm) = 5 × Groundspeed (kt)
For other GP angles: calculate for 3° then ÷3 × actual angle.
⚡ Worked Example
Aircraft at 4 NM from touchdown, 3° GP, groundspeed 150 kt:
Expected height = 3 × 4 × 100 = 1200 ft (exact trig = 1274 ft)
ROD required = 5 × 150 = 750 fpm (exact trig = 796 fpm)
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Expected height = 3 × 4 × 100 = 1200 ft (exact trig = 1274 ft)
ROD required = 5 × 150 = 750 fpm (exact trig = 796 fpm)
12. ILS Summary
| Component | Details |
|---|---|
| Localizer | VHF 108–111.975 MHz, 40 channels (odd tenths), antenna at upwind end |
| Glide Path | UHF 329.15–335 MHz, frequency paired, antenna abeam touchdown |
| Markers | 75 MHz, fan-shaped vertical radiation, OM/MM/IM |
| DME | Frequency paired, zero-referenced to threshold, replaces/supplements markers |
| Ident | 2–3 Morse letters, 7 groups/min, first letter “I”; suppressed when u/s; continuous tone = maintenance |
| OM | Blue, 2 dashes/sec, 400 Hz, 6.5–11.1 km from threshold |
| MM | Amber, alt. dots & dashes 3/sec, 1300 Hz, 1050 m from threshold |
| IM | White, 6 dots/sec, 3000 Hz, 75–450 m from threshold |
| Monitor triggers | LLZ: >35/25/20 ft shift; GP: >0.075θ change; power >50% reduction |
| LLZ coverage | 25 NM/±10°, 17 NM/±35° |
| GP coverage | 10 NM/±8°, 0.45θ to 1.75θ vertical |
| LLZ principle | LH lobe 90 Hz (yellow), RH lobe 150 Hz (blue); DDM=0 on ℅ |
| GP principle | Upper lobe 90 Hz, lower lobe 150 Hz; DDM=0 on GP |
| False GP | First false at ~2× GP angle (6° for standard 3° GP) |
| CDI localizer | 0.5°/dot, FSD ±2.5° |
| CDI glide path | 0.14°/dot, FSD ±0.7°; max safe dev = 2.5 dots fly up |
| Height formula | GP angle × range (NM) × 100 = height (ft) |
| ROD formula (3°) | 5 × groundspeed (kt) = ROD (fpm) |
13. Practice Questions
⚡ Instructions
Click “Show Answer” to reveal the correct answer and explanation.
Q1. The coverage of an ILS localizer extends to ___ either side of the on-course line out to a range of ___ NM.
(a) 10°, 35
(b) 35°, 10
(c) 35°, 17
(d) 25°, 25
Answer: (c)
Localizer: 35° from centreline out to 17 NM; ±10° out to 25 NM. The coverage narrows to 17 NM within the wider ±35° sector.
Localizer: 35° from centreline out to 17 NM; ±10° out to 25 NM. The coverage narrows to 17 NM within the wider ±35° sector.
Q2. The upper and lower limits of an ILS glide path transmitter having a 3.5° glide slope are:
(a) 6.125° – 1.575°
(b) 7.700° – 1.225°
(c) 5.250° – 1.350°
(d) 3.850° – 3.150°
Answer: (a)
Upper = 1.75 × 3.5° = 6.125°; Lower = 0.45 × 3.5° = 1.575°.
Upper = 1.75 × 3.5° = 6.125°; Lower = 0.45 × 3.5° = 1.575°.
Q3. The minimum angle at which a false glide path is likely to be encountered on a 3° glide path is:
(a) 6 degrees
(b) 5.35 degrees
(c) normal glide slope times 1.75
(d) normal glide slope times 0.70
Answer: (a)
First false GP ≈ twice the true GP angle = 2 × 3° = 6°. False GPs always occur above the true glide slope.
First false GP ≈ twice the true GP angle = 2 × 3° = 6°. False GPs always occur above the true glide slope.
Q4. The visual and aural indications when overflying an ILS middle marker are:
(a) continuous low pitched dashes with blue light
(b) continuous high pitched dots with amber light
(c) alternating medium pitch dots and dashes with amber light
(d) one Morse letter with white light
Answer: (c)
MM = Amber light + alternating dots and dashes at 3 per second + 1300 Hz medium pitch audio.
MM = Amber light + alternating dots and dashes at 3 per second + 1300 Hz medium pitch audio.
Q5. An aircraft on ILS approach is receiving stronger 150 Hz signals than 90 Hz. Correct actions to reach centreline and glide path:
(a) DOWN and LEFT
(b) UP and LEFT
(c) UP and RIGHT
(d) DOWN and RIGHT
Answer: (b)
Stronger 150 Hz on localizer = aircraft in RIGHT (blue) lobe = aircraft is RIGHT of centreline → fly LEFT.
Stronger 150 Hz on GP = aircraft BELOW glide slope (lower lobe = 150 Hz) → fly UP.
Combined: UP and LEFT.
Stronger 150 Hz on localizer = aircraft in RIGHT (blue) lobe = aircraft is RIGHT of centreline → fly LEFT.
Stronger 150 Hz on GP = aircraft BELOW glide slope (lower lobe = 150 Hz) → fly UP.
Combined: UP and LEFT.
Q6. Upper and lower limits of ILS glide path with 3.0° glide slope:
(a) 0.35° / 0.70°
(b) 3.00° / at least 6°
(c) 5.25° / 1.35°
(d) 10.0° / 35.0°
Answer: (c)
Upper = 1.75 × 3° = 5.25°; Lower = 0.45 × 3° = 1.35°.
Upper = 1.75 × 3° = 5.25°; Lower = 0.45 × 3° = 1.35°.
Q7. A category II ILS installation in the UK:
(a) provides accurate guidance down to 50 ft above the horizontal plane containing the runway threshold
(b) has a steep glide path, normally 7.5°
(c) provides guidance to and along the runway surface
(d) has a false glide path exactly twice the true GP angle
Answer: (a)
Cat II ILS ground installation provides guidance to the point where the glide path intersects at 50 ft (15 m) above the threshold horizontal plane.
Cat II ILS ground installation provides guidance to the point where the glide path intersects at 50 ft (15 m) above the threshold horizontal plane.
Q8. Which ILS indicator shows aircraft on final approach left of centreline and at maximum safe deviation below the glide path?
(a) (a) — bar right, GP needle centred
(b) (b) — bar left, GP needle high
(c) (c) — bar centred, GP needle centred
(d) (d) — bar right, GP needle high (fly up) at 2.5 dots
Answer: (d)
Aircraft LEFT of centreline → localizer bar deflects RIGHT. Below glide path at maximum safe deviation (2.5 dots fly up) → GP needle shows fly up, 2.5 dots. This is option (d).
Aircraft LEFT of centreline → localizer bar deflects RIGHT. Below glide path at maximum safe deviation (2.5 dots fly up) → GP needle shows fly up, 2.5 dots. This is option (d).
Q9. An aircraft tracking to intercept the ILS localizer inbound but outside the published coverage angle:
(a) will receive false on-course or reverse sense signals
(b) will not normally receive signals
(c) will receive signals without coding
(d) can expect correct indications
Answer: (a)
Outside the coverage sectors, the ILS may give false on-course or reverse sense signals. The signal exists outside the protected coverage area but cannot be guaranteed to give correct guidance.
Outside the coverage sectors, the ILS may give false on-course or reverse sense signals. The signal exists outside the protected coverage area but cannot be guaranteed to give correct guidance.
Q10. The outer marker of an ILS has visual identification of:
(a) alternating dots and dashes on a blue light
(b) continuous dots at 3 per second, blue light
(c) continuous dashes at 2 per second, amber light
(d) continuous dashes at 2 per second, blue light
Answer: (d)
OM = Blue light + 2 dashes per second (400 Hz, low pitch).
OM = Blue light + 2 dashes per second (400 Hz, low pitch).
Q11. The specified maximum safe fly-up indication on a 5-dot CDI is:
(a) half full scale deflection above centreline
(b) 2.5 dots fly up
(c) just before full scale deflection
(d) 1.3 dots fly up
Answer: (b)
Maximum safe deviation below glide slope = 2.5 dots fly up (half the 5-dot full scale). Beyond this, initiate immediate go-around as terrain clearance may be compromised.
Maximum safe deviation below glide slope = 2.5 dots fly up (half the 5-dot full scale). Beyond this, initiate immediate go-around as terrain clearance may be compromised.
Q12. An aircraft using ILS outside coverage sectors — which is correct?
(a) Glide slope false course, localizer intermittent
(b) No transmissions, OFF flags visible
(c) Localizer false course, glide slope reverse sense
(d) Localizer false course/reverse sense; glide slope intermittent/incorrect
Answer: (d)
Outside coverage: localizer may give false on-course and reverse sense indications; glide slope may give intermittent and incorrect indications. Answer (d) correctly states this combination.
Outside coverage: localizer may give false on-course and reverse sense indications; glide slope may give intermittent and incorrect indications. Answer (d) correctly states this combination.
Q13. Coverage of ILS glide slope in azimuth:
(a) ±8° out to 10 NM
(b) ±10° out to 8 NM
(c) ±12° out to 17 NM
(d) ±35° out to 25 NM
Answer: (a)
GP horizontal coverage: ±8° azimuth out to at least 10 NM.
GP horizontal coverage: ±8° azimuth out to at least 10 NM.
Q14. ILS receivers showing predominant 90 Hz on both localizer and glide slope; runway QDM 264°. Aircraft is:
(a) north of localizer, below glide slope
(b) south of localizer, above glide slope
(c) north of localizer, above glide slope
(d) south of localizer, below glide slope
Answer: (b)
Runway QDM 264° (westward). Standing at the runway facing approach (264°), LEFT is to the south, RIGHT is north.
90 Hz on localizer = aircraft in LEFT (yellow) sector = aircraft is south of centreline (left of runway facing west, looking east).
90 Hz on GP = upper lobe = aircraft is above glide slope.
Answer: south, above glide slope.
Runway QDM 264° (westward). Standing at the runway facing approach (264°), LEFT is to the south, RIGHT is north.
90 Hz on localizer = aircraft in LEFT (yellow) sector = aircraft is south of centreline (left of runway facing west, looking east).
90 Hz on GP = upper lobe = aircraft is above glide slope.
Answer: south, above glide slope.
DGCA CPL/ATPL Radio Navigation Study Notes
Chapter 9 — ILS (Instrument Landing System)
Ghost Aviator | www.ghostaviator.com
For personal study use only.
Chapter 9 — ILS (Instrument Landing System)
Ghost Aviator | www.ghostaviator.com
For personal study use only.
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