Localizer
A localizer is the lateral guidance component of the Instrument Landing System (ILS), transmitting on a VHF frequency (108.10–111.95 MHz) and providing left-right guidance aligned with the runway centreline. The localizer signal defines the approach course and is used by the CDI/HSI to display deviation from the runway centreline.
Localizer Characteristics
- Frequency: 108.10–111.95 MHz (on odd tenths, e.g., 108.10, 109.30 — not 108.20)
- Course width: approximately ±2.5° full-scale deflection (5° total)
- Usable range: approximately 18 NM on the front course
- Frequency is tuned manually or selected through the ILS frequency — glide slope is automatically paired
Localizer Back Course
The localizer signal also radiates behind the antenna (the back course). Some airports have published back course approaches. Flying a back course requires reversing CDI sensing (unless the instrument has a back course switch) — the needle deflects opposite to the normal sense. Flying a back course approach with conventional CDI in “normal” mode is a common procedural error.
Localizer-Only Approaches
When the glide slope is inoperative, a localizer (LOC) approach may be published with higher minimums than the full ILS. Localizer-only approaches are non-precision — the pilot descends to MDA and levels off, using only lateral guidance from the localizer.
Related Terms
See also: ILS, CDI, Glide Slope.