Instrument Landing System (ILS): Localizer and Glide Slope

The ILS (Instrument Landing System) is the standard precision approach system, providing lateral and vertical guidance for landing.

System Components

1. Localizer (LOC): Provides lateral guidance. Operates in VHF (108.10 - 111.95 MHz, odd decimals only). Uses two lobes modulated at 90 Hz (left) and 150 Hz (right).
2. Glide Path (GP): Provides vertical guidance. Operates in UHF (329.15 - 335.0 MHz), automatically paired with the LOC frequency. Uses lobes of 90 Hz (above) and 150 Hz (below). The standard angle is 3°.
3. Marker Beacons: All operate at 75 MHz.
   • Outer Marker (OM): Blue, dashes. Indicates the FAF.
   • Middle Marker (MM): Amber, dot-dash. Near CAT I minimums (approx. 1 NM from threshold).
   • Inner Marker (IM): White, dots. Near the threshold (CAT II/III).

Coverage and Accuracy

• Localizer: 25 NM within ±10° and 17 NM within ±35° of the centerline.
• Glide Path: 10 NM within ±8° of the centerline.
• Sensitivity: The ILS is much more sensitive than the VOR. A full-scale deflection on the LOC is 2.5° (compared to 10° for VOR).

Usage and Procedures

• Established: Considered established when deviation is less than half scale (1.25° on LOC, 0.35° on GP).
• Interception: Always intercept the glide path from below to avoid false glide paths (side lobes that appear at higher angles, such as 9°).
• Back Course (BC): Use of the localizer's back beam. On a normal CDI, indications are reversed, but on an HSI, correct indications can be obtained by setting the front course.

Operation Categories (Installation)

• Category I: Guidance down to 200 feet (DH).
• Category II: Guidance down to 100 feet (DH).
• Category III: Guidance down to the runway surface.

Critical and Sensitive Areas

• Critical Area: Zone where the presence of vehicles or aircraft causes unacceptable interference. Protected whenever the ILS is in use.
• Sensitive Area: Wider zone protected during low visibility operations (CAT II/III) to prevent beam fluctuations that the autopilot might erroneously follow.

Approach Charts