Instrument Landing System (ILS)

Introduction to ILS

• Instrument Landing System (ILS) is used for precision landing using instruments, especially in adverse weather conditions.
• Provides horizontal and vertical guidance.

Components of ILS

Localizer

• Provides horizontal guidance.
• Uses frequencies in the VHF band (108 - 111.95 MHz).
• Differentiated from VOR by frequency arrangement (odd decimal places for localizer).
• Transmitter aligned with runway, 300 m off the far end.
• Sends 90 Hz (left) and 150 Hz (right) amplitude modulated signals.
• Displays centerline alignment when signals are equal.

Glide Slope

• Provides vertical guidance.
• Uses UHF band frequencies (329 - 335 MHz).
• Transmitter located about 300 m down the runway from the threshold.
• Sends 90 Hz (top) and 150 Hz (bottom) signals.
• Equal signals indicate correct glide path.
• Glide paths typically set at 3°, but can vary.

Marker Beacons

• Used to check height and distance from the runway.
• Outer Marker: Blue light, 4 nautical miles, Morse code "O".
• Middle Marker: Orange light, 0.6 nautical miles, Morse code "M".
• Inner Marker: White light, at threshold, Morse code "I".
• Modern systems may use Distance Measuring Equipment (DME) instead.

ILS Categories

• Different qualities and capabilities based on equipment.
• Categories 1, 2, 3A, 3B, and 3C based on Minimum Descent Altitude (MDA) and Runway Visual Range (RVR).
• Category 1: 200 ft decision height, 550 m RVR.
• Category 3A: Allows lowest RVR landings (e.g., 200 m for Boeing 737-800).

Display and Navigation

Omni Bearing Indicator (OBI)

• Used for both VOR and ILS.
• Vertical deviation for localizer, horizontal for glide slope.
• 2.5° max deflection for localizer, 0.75° for glide slope.

Horizontal Situation Indicator (HSI)

• Shows live heading input and ILS information.
• Localizer and glide slope indicated by arrows and diamonds.

Primary Flight Display (PFD)

• Modern digital display with localizer and glide slope indicators.
• High precision required for approaches close to the ground.

Errors and Considerations

Signal Disruption

• Caused by vehicles/aircraft, terrain.
• Critical and sensitive areas around ILS to ensure reliability.

False Glide Slopes

• Occur as weaker, erroneous paths above the true glide slope.
• Always intercept glide slope from below to avoid.

Back Course Approaches

• Not used in Europe due to reversed fly-left/fly-right indications.
• Can be used for departure procedures.

Summary

• ILS uses localizer and glide slope for precise landing guidance.
• Frequencies range in VHF and UHF bands, coordinated for automatic tuning.
• Display systems allow pilots to track and correct position for safe landings.
• Remain within tested safe areas to avoid errors and disruptions.