Instrument Landing System (ILS)

Overview

• ILS (Instrument Landing System): A system enabling exact alignment and descent of an aircraft to a runway using radio signals.
• Scenario: Aircraft descending through clouds, approaching runway using lights and smooth touchdown due to ILS.

Key Components of ILS

• Directional Radio Signals: Provide the approach path for alignment.
• Approach Chart/Plate: Published FAA charts showing the ILS approach path, used for visualization.
• PAPI/VASI Lights: Used by pilots to determine if they are on the correct glide slope.
  • PAPI: Precision Approach Path Indicator.
  • VASI: Visual Approach Slope Indicator.
• Glide Slope Antenna: Provides vertical guidance.
• Localizer Antenna: Provides horizontal guidance.
• Marker Beacons: Provide distance information

Visual Indicators

• PAPI/VASI Operation:
  • Red and white lights signal if too high or too low.
  • Correct Path: Red over white.
  • Too Low: Red over red.
  • Too High: White over white.

Radio Signals & Frequencies

• Replacing Visual with Radio: When visibility is limited, radio frequencies are used.
• Two Frequencies:
  • 90 Hz and 150 Hz.
  • Signals overlap to create a glide slope.
• Glide Slope Antenna: Located near PAPI/VASI lights.
• VOR Receiver: Interprets signals, shows if the aircraft is centered on the glide slope.

Adjustments & Corrections

• Needle Movements: Indicates if too high or low and the degree of deviation.
• False Glide Slopes: Avoided by intercepting the glide slope from beneath.

Approach Plates & Symbols

• Feather Symbols: Represent glide slope and localizer on approach plates.
• Localizer: Provides lateral guidance like the glide slope works for vertical.
• Localizer Coverage:
  • 35 degrees either side of runway center line (up to 10 miles).
  • 10 degrees either side (up to 18 miles).
• Localizer Sensitivity: Increases closer to the runway.

Marker Beacons

• Types:
  • Outer Marker: Usually 4-7 miles from the runway.
  • Middle Marker: Located at decision altitude (typically 200 feet AGL).
• Alternate Technologies: DME or GPS now often provide distance information instead.

Flying the ILS Approach

• Equipment Tuning: Set to correct frequency (e.g., 109.3 for Sioux City, IA approach).
• Navigation Equipment: Automatically picks up paired signals.
• Identification Signal: Broadcasts localizer ID in Morse code.
• 3D Guidance:
  • Lateral from Localizer.
  • Vertical from Glide Slope.

Decision Altitude

• Decision Process:
  • Continue descent if runway visible at decision altitude.
  • Initiate missed approach if not.

Practical Example

• ILS Approach at Night:
  • Maintain glide slope with PAPI lights and needle indications.
  • Decision altitude prompts landing decision based on visibility.

Summary

• Key Elements: Radio signals, visual aids, and precise guidance for landing.
• Importance: Helps pilots land safely in poor visibility conditions.