ILS — Instrument Landing System (Awareness)
ILS is the standard precision approach system of civil aviation for IFR approaches in low-visibility conditions. For PPL VFR, awareness matters — most runways at larger fields are ILS-equipped, and ILS approach corridors often overlap VFR traffic.
Source: ICAO Annex 10 Volume I, Chapter 3.1 ILS; ICAO Doc 8168 PANS-OPS.
Components
1. Localiser (LOC)
- Transmitter in line with the runway (at the end of the landing direction).
- Sends two overlapping beam lobes (90 Hz left, 150 Hz right of centreline).
- Pilot derives the lateral position from the difference relative to runway centreline.
- Frequency band: 108.10–111.95 MHz (even 100 kHz channels, paired with glide path).
- Beam width at threshold: about ±2.5° (5° total), narrowing toward the transmitter.
2. Glide Path (GP) / Glide Slope (GS)
- Transmitter beside the runway, at the touchdown point.
- Generates a vertical path of typically 3° (standard ILS glide slope).
- Frequency band: 329.15–335.00 MHz (UHF), paired with LOC frequency.
- Beam width: about ±0.7° (1.4° total).
3. Marker beacons (largely replaced by DME today)
- OM (outer marker): about 4–7 NM before threshold; deep blue light + Morse "−·−".
- MM (middle marker): about 0.5 NM before threshold; yellow + "·−·−".
- IM (inner marker, CAT II/III only): about 250 ft before threshold; white + "····".
- All on 75 MHz.
4. DME / approach DME
- Today routinely co-located with the glide path → distance-precise approaches without markers.
Categories (CAT)
| Category | Decision Height (DH) | Runway Visual Range (RVR) |
|---|---|---|
| CAT I | ≥ 200 ft (60 m) | 550 m |
| CAT II | ≥ 100 ft (30 m) | 300 m |
| CAT IIIa | < 100 ft | 200 m |
| CAT IIIb | < 50 ft | 50 m |
| CAT IIIc | 0 ft | 0 m (zero visibility, theoretical) |
Source: ICAO Annex 6 / Doc 8168.
Higher categories require: special aircraft avionics, crew qualification, runway lighting, RVR measurement.
What does this mean for PPL VFR?
- Localiser corridor: approach corridor along the LOC-transmitter-runway centreline, up to ~10 NM out. VFR traffic inside the LOC corridor can disturb the IFR approach.
- Glide path: a typical 3° profile over several miles — VFR must remain below or above the GP.
- Approach pattern coordination: ATC often instructs VFR traffic to stay outside the ILS approach corridor or to cross the LOC with reported altitude.
Practical note
- For a VFR approach to a field with active ILS traffic: listen carefully to ATC, give IFR traffic priority, position-report precisely.
- If you are ILS-certified yourself (e.g. with IR): detailed procedures in Subject 070 Operational Procedures and Subject 080 Aerodynamics (approach profiles).
Other precision approach systems (awareness)
- GBAS (Ground-Based Augmentation System): GNSS-based, in development as ILS successor.
- MLS (Microwave Landing System): developed since the 1990s, not widely adopted.
- RNP APCH (LNAV/VNAV/LPV): GNSS approach with performance similar to CAT-I ILS (LPV), see lesson "PBN Awareness".