ILS Localizer and Glideslope

5.ils-localizer-glideslope. ILS Localizer and Glideslope

The Instrument Landing System (ILS) is a precision approach navigation aid that provides both lateral and vertical guidance to a specific runway. It enables pilots to descend on a stabilized path to landing in low visibility and low ceiling conditions. A standard ILS has five primary components: the localizer, the glideslope, marker beacons (or DME substitution), approach lighting, and various monitoring/identification features.

Localizer (LOC)

The localizer transmitter is normally located approximately 1,000 feet beyond the departure end of the approach runway and provides lateral guidance along the extended runway centerline. Key facts:

• Operates on 108.10–111.95 MHz on odd-tenth frequencies (108.10, 108.15, 108.30, 108.35, etc.).
• Identified by a three-letter Morse code identifier preceded by the letter "I" (e.g., "I-ORD").
• Course width is tailored to produce a full-scale deflection of 700 feet at the runway threshold, which corresponds to a total beam width of approximately 3° to 6° (typically about 5°). This makes the localizer roughly four times more sensitive than a standard VOR.
• Useful volume: 18 NM within ±10° of the course centerline and 10 NM within ±35°, up to 4,500 feet AGL.
• Two signals are transmitted: 90 Hz modulation predominates on the left side of course (as viewed inbound), 150 Hz on the right. The CDI deflection represents the difference between the two.

A back course transmission may be available off the opposite end of the runway. On a back course, normal sensing is reversed unless the avionics offer a back-course mode; the glideslope is never usable on a back course.

Glideslope (GS)

The glideslope transmitter is located 750–1,250 feet down the runway from the threshold and 400–600 feet to one side of the centerline. It provides vertical guidance along a path that intersects the runway at the touchdown zone.

• Operates in the UHF band, 329.15–335.00 MHz, automatically paired with the localizer frequency when the LOC is tuned.
• The glide path angle is normally 3° above horizontal, producing a touchdown point approximately 1,000 feet down the runway.
• Full-scale deflection is 0.7° above and below the glide path, so the glideslope CDI is significantly more sensitive than the localizer—small pitch corrections produce large needle movement.
• Like the localizer, it uses 90 Hz and 150 Hz modulation; 150 Hz dominates below the path, 90 Hz above the path.

False glideslopes exist at integer multiples of the true path (approximately 6°, 9°, etc.). Always intercept the glideslope from below at the published intercept altitude to avoid capturing a false signal.

Course Sensitivity Comparison

• VOR: full-scale deflection = 10° either side of course (±10°).
• Localizer: full-scale deflection ≈ 2.5° either side of course (±2.5°).
• Glideslope: full-scale deflection = ±0.7° either side of path.

Because the localizer beam narrows as the aircraft approaches the runway, corrections must be progressively smaller and more precise during the approach.

ILS Categories and Minimums

ILS approaches are classified by the lowest authorized minimums:

• CAT I: DH 200 feet, RVR 2,400 (1,800 with touchdown zone and centerline lighting).
• CAT II: DH 100 feet, RVR 1,200.
• CAT III (a/b/c): DH below 100 feet or none, RVR 700 down to 0.

CAT II and III operations require special authorization, qualified aircraft, autopilot/autoland or HUD, and trained crews.

Flying the ILS — Procedure

1. Tune and identify the localizer frequency; verify the Morse code ident with the leading "I."
2. Set the published inbound course in the OBS (modern HSI/CDI auto-slaves, but setting the course supports situational awareness).
3. Intercept the localizer at an angle no greater than 45°, then begin tracking the centerline.
4. Cross the final approach fix (FAF) or glideslope intercept altitude, intercepting the glideslope from below as it becomes alive.
5. Establish the final approach configuration and a power setting that yields the target descent rate. Rule of thumb for a 3° glide path: descent rate (fpm) ≈ groundspeed (knots) × 5. At 90 KGS this is approximately 450 fpm.
6. Apply the "pitch for glideslope, power for airspeed, rudder for localizer" technique (or trim/configuration as appropriate). Make small, prompt corrections—needle movement accelerates as you approach the runway.
7. At the decision altitude (DA), if the required visual references (14 CFR 91.175) are in sight, continue and land; otherwise, execute the published missed approach.

Common Errors

• Chasing the needles instead of flying small, stabilized corrections.
• Capturing the glideslope from above (risk of false glidepath).
• Failing to identify the localizer—an unmonitored or off-the-air signal can present centered needles that are unreliable.
• Allowing airspeed to decay below approach speed while focused on glideslope tracking.