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

Capt Pankaj Pahil | www.ghostaviator.com

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

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

Example: LLZ 108.1 MHz is paired with GP 334.7 MHz; LLZ 111.95 MHz paired with GP 330.95 MHz.

DME Paired with ILS

📡 ILS/DME

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

Capt Pankaj Pahil | www.ghostaviator.com

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:

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%

Actions: Cease radiation / Remove ident & nav components / Lower category (Cat II/III may revert to Cat I/II).

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).

Capt Pankaj Pahil | www.ghostaviator.com

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

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

Note: May be reduced to 18 NM (±10°) when alternative navaids provide satisfactory intermediate approach coverage.

📐 Glide Path Coverage

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)

Where θ = promulgated glide path angle (usually 3°).

⚡ 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.

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”

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.

Capt Pankaj Pahil | www.ghostaviator.com

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

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

Capt Pankaj Pahil | www.ghostaviator.com

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

Cross-check with other navaids (DME, altimeter, markers) and monitor ident continuously.

📐 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

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)

Capt Pankaj Pahil | www.ghostaviator.com

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.

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°.

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.

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.

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.

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°.

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.

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).

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.

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).

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.

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.

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.

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.

DGCA CPL/ATPL Radio Navigation Study Notes
Chapter 9 — ILS (Instrument Landing System)
Capt Pankaj Pahil | www.ghostaviator.com
For personal study use only.

✈ Chapter 9: ILSInstrument Landing System

📋 Contents

1. Introduction

2. ILS Components

3. ILS Frequencies & Pairing

DME Paired with ILS

4. Identification & Monitoring

Ground Monitoring

5. Marker Beacons

6. ILS Coverage

7. Principle of Operation

7.1 Localizer

7.2 Back Course ILS

7.3 Glide Path

7.4 False Glide Slopes

8. Displays & Interpretation

8.1 CDI (Course Deviation Indicator) in ILS Mode

8.2 HSI (Horizontal Situation Indicator) in ILS Mode

8.3 Localizer On-Course Indications

9. ILS Categories (ICAO)

10. Errors, Accuracy & Protected Areas

11. ILS Calculations

Height Check on Glide Path (1:60 Rule)

12. ILS Summary

13. Practice Questions

✈ Chapter 9: ILS
Instrument Landing System