Understanding the ILS Approach

The Instrument Landing System (ILS) is the classic precision approach: it gives an aircraft both lateral and vertical guidance down to a low decision altitude, typically 200 ft above the runway.

Two beams

• The localizer provides *lateral* guidance, a narrow course aligned with the runway centreline. On the course deviation indicator the needle is far more sensitive than a VOR — full deflection is only about 2.5° either side.
• The glideslope provides *vertical* guidance, usually a 3° path to the touchdown zone. The needle shows whether you are above or below the path.

When both needles are centred, you are on course and on path. Small, smooth corrections keep them there; chasing the needles leads to oscillation.

Flying the approach

You intercept the localizer first, level, then capture the glideslope from below as it comes down to meet you. From glideslope intercept you fly a stabilised, descending approach — a near-constant power setting and rate of descent (roughly 5× your groundspeed in knots gives feet-per-minute, e.g. ~500 fpm at 100 kt).

At the decision altitude (DA) you must have the required visual references — approach lights, runway environment — to continue. If you don’t, you fly the missed approach immediately: climb, configure, and follow the published track to the holding fix.

Categories and limits

Standard (Cat I) ILS minima are about 200 ft and ½ statute mile (or RVR 2400). Cat II and III approaches go lower but require special aircraft, crew and airport equipment. Always check the specific approach plate — minima vary by runway and equipment, and the chart is the authority.

Related reading: decision altitude, glidepath, and the crosswind calculator for the landing.

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*Reference and training only — not for navigation. Always consult the current AIM, your aircraft’s POH/AFM and a certificated flight instructor.*