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Function of the ILS localizer

Aviation Mechanic Airframe (AMA) practice question 17 of 26 on Communications, Light Signals, and Runway Lighting Systems. ACS element AM.II.I.K11 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONWhat guidance does the localizer portion of an instrument landing system (ILS) provide?

  1. AHorizontal guidance to the runway centerline
  2. BVertical guidance down the proper descent slope
  3. CDistance remaining to the runway threshold
Show the answer and explanation

Correct answer: A. Horizontal guidance to the runway centerline

  • A — correct The handbook states the localizer is used to provide horizontal guidance to the center line of the runway.
  • B — incorrect Vertical guidance for the correct angle of descent is supplied by the separate glideslope broadcast.
  • C — incorrect Distance-from-the-runway information is provided by the marker beacons, not by the localizer.

BACKGROUND

An ILS uses several separate radio transmissions to guide an aircraft to the touchdown area when visibility is poor. The localizer is a VHF broadcast in the lower range of the VOR frequencies, 108 to 111.95 MHz on odd frequencies only, radiated from a horizontally polarized antenna complex beyond the far end of the approach runway. It creates a field about two and one-half degrees wide five miles out that tapers to runway width near the threshold, with a 90 Hz modulated signal on one side and a 150 Hz modulated signal on the other. The aircraft's VOR receiver interprets the two signals and drives the course deviation indicator so the pilot can center the aircraft on the runway centerline.

STUDY TIP

Split the ILS into its jobs before the test: localizer for left and right, glideslope for up and down, marker beacons for distance along the approach.

KEY POINTS

  • The localizer gives horizontal guidance to the runway centerline.
  • It broadcasts on odd VHF frequencies from 108 to 111.95 MHz.
  • The glideslope, not the localizer, supplies vertical descent guidance.

SOURCES

  • FAA-H-8083-31B ch11 "Instrument Landing Systems (ILS) — Localizer"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.