ILS Approach System Overview

Overview

This lecture explains how the Instrument Landing System (ILS) guides aircraft to runways in low-visibility conditions using radio signals for vertical and lateral navigation.

Instrument Landing System (ILS) Basics

• ILS uses ground-based directional radio transmitters to provide precise approach paths for landing.
• ILS enables pilots to land safely when visual references (like runway lights) are not visible due to weather.

Glide Slope (Vertical Guidance)

• Glide slope antennas transmit two overlapping signals (90 Hz and 150 Hz) to indicate correct descent angle (usually 3 degrees).
• If the aircraft receives more 90 Hz, it's too high; more 150 Hz means too low.
• Glide slope indicator is shown on approach charts as a "feather" symbol.
• Pilots follow the glide slope by centering a horizontal needle on their navigation display.

Visual Descent Aids (VASI/PAPI)

• VASI/PAPI lights use red and white indications to show if the aircraft is above, below, or on the glide path.
• Red over white means on the right glide slope; red over red is too low; white over white is too high.

Localizer (Lateral Guidance)

• The localizer transmits two signals (90 Hz left, 150 Hz right) to guide pilots to the runway centerline.
• Pilots use a vertical needle; deviations indicate how far left or right they are.
• The localizer's course is always 700 feet wide at the runway threshold.

Marker Beacons and Distance Measuring

• Marker beacons, shown as football symbols, indicate distance and key approach points (outer and middle markers).
• Marker beacons are being replaced by DME (Distance Measuring Equipment) or GPS.

Operating the ILS

• Pilots tune to the published localizer frequency (e.g., 109.3 MHz for Sioux City Runway 31).
• Both localizer and glide slope signals are highly directional, only reliable within specific approach areas.
• Morse code identifier confirms the correct ILS frequency is set.

Approach Procedures and Limitations

• ILS coverage is within 35° of runway centerline (up to 10 miles) and within 10° (up to 18 miles).
• Aircraft must intercept glide slope from below to avoid false signals caused by reflections.
• At decision altitude (typically 200 ft AGL), pilots either continue to land if visual or execute a missed approach.

Key Terms & Definitions

• ILS (Instrument Landing System) — Radio system guiding aircraft on approach in low-visibility.
• Glide Slope — Vertical guidance signal for descent angle.
• Localizer — Lateral guidance signal for runway alignment.
• VASI/PAPI — Visual lighting systems indicating vertical path.
• Marker Beacon — Radio beacons denoting approach points.
• Decision Altitude (DA) — Minimum height for approach decision.

Action Items / Next Steps

• Review FAA approach plates and practice interpreting ILS approach symbols.
• Practice using navigation equipment to track localizer and glide slope needles.
• Complete reading on DME and GPS-based approaches.