Overview
This lecture explains how the Instrument Landing System (ILS) provides precise vertical and horizontal guidance for aircraft during low-visibility landings using radio signals and cockpit instruments.
Introduction to ILS Approaches
- ILS allows pilots to land safely by providing exact approach paths during low visibility.
- It uses ground-based radio transmitters to guide aircraft to the runway.
Visual Glide Slope Indicators
- PAPI and VASI are arrays of red and white lights that show if an aircraft is above, below, or on the correct glide slope.
- Seeing "red over white" means the aircraft is on the correct approach angle; "white over white" is too high, and "red over red" is too low.
Basic ILS Components
- ILS replaces visual lights with radio signals at different frequencies (e.g., 90 Hz and 150 Hz).
- These signals overlap to create a precise glide slope path to the runway.
Glide Slope (Vertical Guidance)
- The glide slope antenna is placed near the approach end of the runway and transmits overlapping signals to define the descent path.
- Cockpit instruments (needle indicators) show position relative to glide slope; adjustments are made to keep the needle centered.
- Avoid intercepting the glide slope from above due to potential false glide slopes caused by signal reflection.
Localizer (Horizontal Guidance)
- The localizer antenna is placed at the far end of the runway and transmits signals for lateral alignment with the runway center line.
- The localizer course is always 700 feet wide at the runway threshold, making it sensitive near the ground.
- The receiver needle indicates if the aircraft is left or right of center and requires "chasing the needle" to stay aligned.
Additional ILS Features
- Marker beacons (outer and middle markers) indicate distance from the runway and key approach milestones.
- Distance measuring equipment (DME) or GPS can provide similar distance information.
Operating the ILS
- Pilots tune to a published localizer frequency (e.g., 109.3), which also pairs with the glide slope signal.
- The ILS has limited signal coverage (usually 35° to each side up to 10 miles, 10° out to 18 miles); indications are only reliable within this area.
- At the middle marker (typically 200 feet above ground), pilots must decide to land or execute a missed approach if the runway is not visible.
Key Terms & Definitions
- ILS (Instrument Landing System) — Precision landing aid using ground-based radio signals for vertical and horizontal guidance.
- Glide Slope — Vertical guidance path for safe descent, indicated by a needle and/or visual lights.
- Localizer — Horizontal guidance signal aligning the aircraft with the runway center line.
- PAPI/VASI — Visual light arrays at the runway providing glide slope information.
- Marker Beacon — Transmitter providing distance markers along an approach path.
- DME (Distance Measuring Equipment) — Radio navigation aid measuring distance to a ground station.
Action Items / Next Steps
- Review how to tune and interpret ILS signals on navigation equipment.
- Study approach plates to recognize ILS symbols and frequencies.
- Practice identifying ILS indications in a simulator or with sample approach charts.