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IFH · IFH Chapter 8

Instrument Landing System (ILS) Approach

Master the ILS precision approach: localizer, glide slope, categories, decision height, and FAA procedures from Instrument Flying Handbook Chapter 8.

CFI's Whiteboard Explanation

An ILS gives you two needles: the localizer (left/right to runway centerline) and the glide slope (up/down on a 3° path). Fly the needles centered down to decision altitude—usually 200 ft above the runway. At DA, if you see the runway environment, land. If not, go missed.

Quick tips:

  • Descent rate (fpm) ≈ groundspeed × 5 for a 3° GS
  • The needles get touchier as you get closer—make small corrections
  • Always intercept the glide slope from below to avoid a false glidepath
  • Identify the Morse code before trusting the signal
Handbook Reference
IFH Ch 8

8.precision-approach-ils. Instrument Landing System (ILS) Approach

The Instrument Landing System (ILS) is a precision approach navigation aid that provides pilots with both lateral (course) and vertical (glidepath) guidance to a specific runway, allowing safe descent to landing minimums in low-visibility conditions. As a precision approach, the ILS is the most widely used Category I approach system in the National Airspace System and remains the benchmark against which other approaches (LPV, GLS) are measured.

Ground Components

A complete ILS installation consists of the following ground-based elements:

  • Localizer (LOC): Provides lateral guidance aligned with the runway centerline. Transmits on VHF frequencies between 108.10 and 111.95 MHz on odd-tenth frequencies (e.g., 108.10, 108.15, 108.30). Course width is tailored so that full-scale deflection is approximately 700 feet at the runway threshold, typically resulting in a course width of 3° to 6° (nominal 5°). Localizer sensitivity is about four times that of a VOR.
  • Glide Slope (GS): Provides vertical guidance, normally a 3.00° descent path to the touchdown zone. Transmits on UHF frequencies (329.15 to 335.00 MHz), automatically paired with the localizer frequency. Full-scale vertical deflection is approximately 0.7° above and below the glidepath.
  • Marker Beacons or DME: The Outer Marker (OM) is located 4-7 NM from the runway and identifies the final approach fix (FAF) for glide slope intercept; it transmits a 400 Hz tone (continuous dashes) and flashes blue. The Middle Marker (MM) is approximately 3,500 feet from the threshold and indicates Category I decision height (DH); it transmits 1,300 Hz (alternating dot-dashes) and flashes amber. Many ILS facilities now use DME in lieu of marker beacons.
  • Approach Lighting System (ALS): Visual aid required to descend below DH unless other specific visual references are in sight (14 CFR 91.175).

Airborne Equipment

The aircraft must have an ILS receiver capable of decoding both the localizer and glide slope signals. Standard cockpit indications include a course deviation indicator (CDI) for lateral guidance and a glide slope indicator (typically a horizontal needle) for vertical guidance. Each dot of CDI deflection on a localizer represents approximately 0.5°, and each dot on the glide slope represents approximately 0.14°.

ILS Categories and Minimums

ILS approaches are classified by the lowest authorized minimums:

  • Category I: DH not lower than 200 feet HAT, RVR not less than 1,800 feet (2,400 feet without touchdown zone and centerline lighting).
  • Category II: DH as low as 100 feet HAT, RVR 1,200 feet. Requires special aircraft, crew, and operator authorization.
  • Category III A/B/C: DH below 100 feet (or no DH); RVR as low as 700 feet (IIIA), 150 feet (IIIB), or zero/zero (IIIC). Requires autoland or HUD systems and specific authorization.

Flying the Approach

  1. Preparation: Brief the approach plate—identify the localizer frequency and identifier (Morse code beginning with "I"), final approach course, glide slope intercept altitude, DH, missed approach procedure, and required visibility.
  2. Tune and Identify: Set the localizer frequency in the NAV radio, identify the Morse code, and set the course pointer to the published front-course inbound heading.
  3. Intercept Localizer: Established on the localizer prior to glide slope intercept. Apply wind correction; small heading changes (2°-5°) are usually sufficient because of localizer sensitivity.
  4. Glide Slope Intercept: At the published intercept altitude (often the FAF), reduce power, extend appropriate flaps and gear, and establish a stabilized descent. A useful rule of thumb for a 3° glidepath: Required descent rate (fpm) ≈ groundspeed (kt) × 5. At 90 KIAS groundspeed, target approximately 450 fpm.
  5. Tracking Inbound: Make small, prompt corrections. Cross-check airspeed, altitude, attitude, heading, and the LOC/GS needles. Keep the needles centered—any deflection grows more sensitive as you approach the runway.
  6. Decision Altitude/Height: At DA/DH, decide to land or execute the missed approach. To continue, you must have the required flight visibility AND at least one of the visual references listed in 14 CFR 91.175(c)(3) in sight (e.g., approach lights, threshold, runway markings, REIL, VASI, touchdown zone).

Errors and Limitations

  • False glide slopes exist at higher angles (typically 6°, 9°, 12°). Always intercept the GS from below at the published altitude.
  • Reverse sensing occurs on the back course unless an HSI is properly set.
  • Signal interference from taxiing aircraft (especially heavies) inside the ILS critical area can cause needle fluctuations; ATC protects these areas when ceiling is below 800 feet or visibility is less than 2 SM.
  • Localizer course is highly sensitive close in—overcontrolling near the threshold is a common student error.
Oral Exam Questions a DPE Might Ask
Q1What are the required ground components of an ILS, and what guidance does each provide?
The localizer provides lateral guidance to the runway centerline on a VHF frequency (108.10-111.95 MHz), and the glide slope provides vertical guidance, normally on a 3° path, on a paired UHF frequency. Marker beacons (or DME) identify position along the approach, and the approach lighting system provides visual transition to landing.
Q2What is the decision altitude on a Category I ILS, and what must you see to descend below it?
Cat I DH is no lower than 200 feet HAT with at least 1,800 RVR (with TDZ/centerline lighting). Per 14 CFR 91.175, you can descend below DA only if you have the required flight visibility and at least one of the listed visual references—such as the approach lights, threshold, runway markings, REIL, VASI, or touchdown zone—distinctly visible.
Q3Why must you intercept the glide slope from below at the published altitude?
False glide slopes exist at higher angles (around 6°, 9°, and 12°) above the true 3° path. Intercepting from below at the published intercept altitude ensures you capture the correct glidepath and don't follow a false signal into terrain.
Related FAR References
FAR § 91.175
Plain-English + oral questions
FAR § 91.129
Plain-English + oral questions
More from IFH Chapter 8
Aircraft Approach Categories
Circling Approach
Contact and Visual Approaches
Missed Approach Procedure
Nonprecision Approach
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ILS Approach: IFH Chapter 8 | GroundScholar