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ILS — Instrument Landing System

The ILS is the most widely used precision approach system worldwide, providing both lateral and vertical guidance to the runway threshold. Understanding ILS components, categories, and procedures is essential for IFR operations.

ILS Components

Localiser (LOC)

Provides lateral guidance. Operates on 108.10–111.95 MHz (odd decimal tenths). Course width 3°–6° providing 700 ft width at threshold. Antenna sited ~300 m beyond far end of runway. Intercept within 35° of final course.

Glideslope (G/S)

Provides vertical guidance. UHF 329.15–335.00 MHz (auto-paired with LOC). Standard glidepath: 3° (range 2.5°–3.5°). Full-scale deflection = 0.7°; one dot = 0.175°. Usable range: 10 nm.

Marker Beacons

All transmit on 75 MHz. Outer Marker (OM) — blue light, 400 Hz dashes, ~4 nm from threshold. Middle Marker (MM) — amber, 1300 Hz dots/dashes, ~3500 ft from threshold. Inner Marker (IM) — white, Cat II/III only.

DME

Often collocated as ILS/DME to provide slant-range distance. Automatically paired with LOC frequency. Eliminates need for timing from FAF.

Approach Lighting (ALS)

High-intensity runway lighting providing visual cues for the final visual segment of the approach. Extends 2400–3000 ft from threshold. Essential for low-visibility Cat II/III operations.

ILS Categories

Category Decision Height (DH) RVR / Visibility Requirements
Cat I 200 ft above threshold 550 m RVR (800 m if no RVR equipment) Standard airline ILS; basic avionics
Cat II 100–200 ft AGL 350 m RVR HUD or autoland capability; enhanced training
Cat IIIA Below 100 ft or no DH 200 m RVR Autoland required; fail-operational system
Cat IIIB Below 50 ft or no DH 75–200 m RVR Autoland + rollout guidance required
Cat IIIC No DH No RVR requirement Full zero-zero; very rare operationally
Cat I in Practice The vast majority of ILS approaches worldwide are Cat I. Cat II/III require additional aircraft certification, crew training, airfield infrastructure, and specific airline operating approvals.

Flying the ILS

  1. Intercept the localiser — typically at 45° on a base turn or via radar vectors to final. Confirm correct frequency and morse ident.
  2. Establish on LOC at or above the glideslope intercept altitude. Do not descend prematurely.
  3. When the G/S needle comes alive and begins to drop from the top: select gear down, configure aircraft for approach speed and flap setting.
  4. Fly on the glideslope with small corrections. Needle deflections are sensitive — half a dot equals only 0.35°. Use power to control descent rate on the glidepath.
  5. Monitor both LOC and G/S continuously. Use any approach lighting or visual cues as supplementary reference in poor visibility.
  6. At DH: if required visual references are clearly in sight, continue to land. If not — execute immediate go-around. No delay is acceptable.
Tip

Brief your decision height before commencing the approach. At DH, your eyes should be outside looking for visual references — not scanning instruments. If you're not visual at DH, go around immediately without hesitation.

Required Visual References at DH Runway lights, threshold markings, touchdown zone, approach lighting system, or runway end identifier lights must be identified and positioned for a normal landing before continuing below DH.

Errors and Failures

  • LOC off-scale / failure: CDI deflects to full scale. Go around immediately — do not attempt to recover.
  • G/S failure: Revert to LOC-only approach (non-precision). Use published LNAV or NDB step-down minimums; increase MDA accordingly.
  • False glideslope: An upper false G/S course exists at approximately 9° above the true glidepath. Always remain below the false course by staying at or below the published intercept altitude until established on the correct G/S.
  • Back course: The localiser signal radiates behind the antenna. On a back-course approach the CDI sensing is reversed unless using an HSI with the correct course set, or a receiver with back-course mode.
  • CDI sensitivity: Full-scale LOC deflection is approximately 2.5° from centreline. One dot deflection requires immediate, small correction — do not over-control.
False Glideslope Warning Never intercept the glideslope from above unless operationally required and briefed. The false G/S at ~9° is a known hazard that has contributed to CFIT accidents.

Reading an ILS Approach Plate

Plan View

Shows the airport overhead, approach path, IAF, IF, FAF, and associated fixes. Contains the MSA (Minimum Safe Altitude) circle and procedure turn or TORA/TODA boxes.

Profile View

Side view showing the glideslope intercept altitude, step-down fixes, FAF crossing altitude, and missed approach initial climb altitude. Glideslope angle is annotated.

Minimums Section

Lists DH, RVR, and visibility for Cat I/II/III and alternate minimums. Separate rows for straight-in and circling if applicable. Aircraft category columns (A/B/C/D).

Missed Approach

Published track, initial climb altitude, holding fix or routing. Must be briefed before starting the approach. Follow exactly if a go-around is executed.

FAF & G/S Intercept

The Final Approach Fix (FAF) is depicted by a Maltese cross in the profile view. The glideslope intercept altitude (GS-coded altitude) is the altitude to cross the FAF when established on G/S.

Frequencies & Idents

LOC frequency and morse ident, paired G/S frequency (auto-paired), and associated DME/NDB frequencies listed in the header or briefing strip.

Continue Learning — Approach & Landing

Approach & Landing Overview RNAV (GPS) Approaches Approach Minimums (DA/MDA)