7.1 Precision Approach Systems: ILS Categories & Tolerances

7.1 Precision Approach Systems: ILS Categories & Tolerances

Quick Summary: The Instrument Landing System (ILS) provides precision lateral and vertical guidance to approaching transport aircraft. Operating categories span Category I (200 ft DH / 2,400 ft RVR) down through Category IIIb (DH < 50 ft / 150 ft RVR). Aircraft dispatchers must master the technical interdependence of ground transmitters, airborne avionics redundancy (fail-passive vs. fail-operational), ILS critical area protection criteria, and the dispatch impact of Minimum Equipment List (MEL) deferrals and NOTAM facility outages.

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1. Instrument Landing System (ILS) Architecture & Classification

A precision approach is defined by the International Civil Aviation Organization (ICAO) and the Federal Aviation Administration (FAA) as an instrument approach procedure based on navigation systems that provide simultaneous lateral course guidance and vertical glidepath guidance with angular convergence toward the runway touchdown point. The terrestrial benchmark for precision approaches remains the Instrument Landing System (ILS).

An ILS installation integrates four primary functional subsystems:

1. Lateral Guidance (Localizer - LOC): Operates on 40 assigned VHF channels between 108.10 MHz and 111.95 MHz (odd-tenth decimals and odd tenths plus 50 kHz). Sited approximately 1,000 to 2,000 feet past the departure end of the runway, it radiates two directional amplitude-modulated audio lobes (90 Hz on the left, 150 Hz on the right) providing a standard angular course width calibrated to produce 700 feet linear width at the landing threshold.
2. Vertical Guidance (Glide Slope - GS): Operates on paired Ultra High Frequency (UHF) channels between 329.15 MHz and 335.00 MHz. Sited 250 to 650 feet to the side of the runway centerline and set back 750 to 1,250 feet past the threshold, it radiates an upper 90 Hz lobe and a lower 150 Hz lobe creating a standard 3.0° descent glidepath.
3. Range Guidance: Marker beacons broadcasting on a fixed frequency of 75 MHz (Outer Marker for FAF/GS intercept, Middle Marker for standard Cat I DH, and Inner Marker for Cat II DH) or colocated precision Distance Measuring Equipment (DME).
4. Visual Aids: High-intensity approach lighting systems (such as ALSF-1, ALSF-2, or MALSR) and transmissometer sensors measuring Runway Visual Range (RVR) at touchdown, midpoint, and rollout locations.

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2. Categories and Actual Authorization

Category I ordinarily involves DA/H not below 200 feet. Category II concerns lower decision heights, conventionally not below 100 feet, with its required low-visibility authorization. Category III covers still lower or no-decision-height operations under specified RVR and equipment approvals; conventional IIIa/IIIb/IIIc labels describe different definitions, not permission for every aircraft to use the same minima.

Representative CAT I values such as 200 feet and RVR 2,400, or an authorized reduction to RVR 1,800, are not universal minima for every runway. Likewise, a commonly taught CAT II 100-foot/RVR 1,200 example must be checked against the actual approved operation. Current C059 is not the old generic CAT II authorization; relevant current low-visibility authority includes C060 as applicable.

Do not impose one universal dual-autopilot count or a universal “all RVR sensors at 150 feet” rule on CAT III. Sensor requirements, fail-operational/passive capability, rollout guidance, and lowest approved RVR depend on the actual system and authorization. Conventional CAT IIIc zero-zero terminology does not itself grant an operation, and ground movement/rescue considerations remain part of the approval.

3. Airborne Avionics & Flight Control Architectures: Fail-Passive vs. Fail-Operational

For low-visibility Category II and Category III approaches, the airworthiness regulations of 14 CFR Part 25 (Airworthiness Standards: Transport Category Airplanes) and FAA Advisory Circular AC 120-28D classify automatic landing systems into two distinct architectural philosophies:

Fail-Passive Automatic Landing Systems

A flight control system is fail-passive if, in the event of an autoflight failure below Alert Height or Decision Height, no significant out-of-trim condition, abrupt control displacement, or flight path deviation occurs. However, the landing is not completed automatically.

• Upon failure of a fail-passive channel, the autopilot automatically disconnects.
• The pilot must immediately take manual control and either complete the landing visually (if visual references were acquired at DH) or immediately initiate a missed approach.
• Because human pilot intervention is required to complete the go-around, fail-passive systems are restricted to Category IIIa operations where an operational Decision Height (typically 50 feet HAT) is mandated, allowing the pilot sufficient height to transition to a go-around.

Fail-Operational Automatic Landing Systems

A flight control system is fail-operational if, in the event of a single failure of any flight control channel below the Alert Height (AH), the approach, flare, touchdown, and rollout can be completed automatically by the remaining operational systems without pilot intervention.

• Fail-operational systems require multiple independent channels—either a triplex architecture (three independent channels voting against each other) or a dual-dual architecture (two pairs of self-monitoring channels).
• If one channel experiences an internal fault, the self-monitoring circuit or majority voting logic disconnects the faulty channel within milliseconds. The remaining channels maintain control of the aircraft through flare and rollout without degrading performance.
• Fail-operational capability is legally required for Category IIIb operations where Decision Height is less than 50 feet or omitted entirely.

The Role of Alert Height (AH) and Radio Altimeters

• Radio Altimeter (RA): In Category II and III operations, barometric altimeters are legally prohibited as the primary reference for decision minimums due to inherent sensor lag and pressure variation errors. The Decision Height is measured exclusively by dual or triple radio altimeters, reading precise Height Above Terrain (HAT) directly beneath the landing gear.
• Alert Height (AH): A radio altimeter altitude (typically 100 to 200 feet AGL) specified for fail-operational Cat III operations. If an equipment failure occurs above the Alert Height, the approach must be aborted and a missed approach executed unless the approach can be continued under higher minimums (e.g., reverting to Cat II). If a failure occurs below the Alert Height in a fail-operational aircraft, the fault is annunciated, but the crew ignores the master caution and allows the aircraft to continue the automatic landing and rollout.

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4. ILS Critical Areas & Airport Surface Movement Control

Because the ILS localizer and glide slope operate via radio waves propagated across the airfield, physical objects—such as taxiing aircraft, fuel tankers, and ground maintenance equipment—act as secondary reflectors. These reflections cause electromagnetic multipath interference, resulting in severe signal distortions known as beam bends and scalloping (rapid oscillations of the cockpit CDI needles).

If an aircraft on an autoland approach follows a bent localizer or distorted glide slope beam, the autopilot may execute aggressive, oscillatory control inputs near the ground, potentially resulting in runway excursions or hard impacts.

ATC Protection Rules (FAA Order 7110.65 & AIM 1-1-9)

To protect precision signal integrity, the FAA establishes ILS Localizer Critical Areas and ILS Glideslope Critical Areas:

• Weather Trigger for Mandatory Protection: Air Traffic Control is mandated to protect ILS Critical Areas whenever:

$$\text{Reported Ceiling} < 800\text{ feet} \quad \text{and/or} \quad \text{Reported Visibility} < 2\text{ statute miles}$$

• Hold Short Mandate: When these conditions exist, ATC will not permit aircraft or ground vehicles to enter or hold within the ILS critical areas while an arriving aircraft is between the Final Approach Fix (FAF) and touchdown.
• Surface Markings: Pilots and ground vehicle operators must hold short at the ILS Critical Area Holding Position Sign (a red sign with white lettering depicting an ILS localizer icon or the letters "ILS") and the associated yellow ladder-style surface painted markings located further back from the runway than standard runway holding position lines.
• Surface Movement Guidance and Control System (SMGCS): When visibility drops below 1,200 feet RVR, airports activate SMGCS (pronounced "SMIG-see") procedures. Taxi operations are restricted to designated low-visibility routes equipped with controllable yellow taxiway centerline lights and red illuminated stop bars at all runway crossings.

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5. False Glideslope Anomalies & Intercept Physics

The glide slope antenna radiates energy toward the approaching aircraft while simultaneously bouncing energy off the ground plane in front of the antenna mast to form the lower portion of the beam. This ground reflection creates interference lobes at harmonic multiples above the published glidepath angle:

• Primary True Glidepath: Nominally calibrated to 3.0° above horizontal.
• First Harmonic False Glidepath: Occurs at approximately 6.0° (double the nominal angle). Crucially, this false glide slope frequently exhibits reversed sensing (the needle commands fly-up when the aircraft is above the false path, or fly-down when below).
• Second Harmonic False Glidepath: Occurs at approximately 9.0° (triple the nominal angle) with normal sensing.

The Severe Hazard of Intercepting from Above

If an aircraft is vectored to intercept the glidepath from above, or if the flight crew initiates glide slope capture while descending from an excessively high altitude, the airborne receiver can capture the 6.0° false glide slope:

• Following a 6.0° descent angle at a standard final approach groundspeed of 140 knots requires a vertical descent rate of approximately 1,500 feet per minute (fpm)—double the normal stabilized rate of ~750 fpm.
• The aircraft rapidly enters an unstabilized condition, triggering Enhanced Ground Proximity Warning System (EGPWS) alerts ("SINK RATE! SINK RATE!") and risking catastrophic high-rate impact.

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6. Dispatch Planning & MEL / Ground Outage Contingencies

Under 14 CFR 121.605 and joint operational control, the aircraft dispatcher and the pilot in command must verify that both ground navigation facilities and airborne equipment are fully certified and operational for the planned approach category.

Airborne Equipment Deferrals Under the MEL

When an aircraft departs with inoperative avionics under the Minimum Equipment List (MEL), the dispatcher must review the Non-Essential Equipment and Furnishings (NEF) and MEL Category relief provisions to identify low-visibility landing disqualifications:

• Autopilot Deferral: Inoperative dual autopilot channels prohibit Category II and Category III operations. The aircraft must be dispatched under Category I landing minimums only.
• Radio Altimeter Deferral: Determine the affected aircraft capability and actual MEL/authorization restrictions. A deferred unit can remove CAT II/III capability, but the result depends on required remaining equipment and approved relief; do not impose a universal rule for every redundant installation.

Ground Facility Outages via NOTAMs

Ground equipment outages reported in D-NOTAMs and FDC NOTAMs dictate operational downgrade penalties:

• Glide Slope Inoperative: The ILS approach is immediately downgraded to a Localizer (LOC) non-precision approach. The Decision Altitude (DA) is replaced by a Minimum Descent Altitude (MDA), increasing the minimum ceiling requirement by typically 300 to 500 feet.
• Approach Lighting System (ALS) Inoperative: If the ALSF-2 or MALSR is out of service, Category I visibility minimums increase by 1/4 to 1/2 statute mile (or 1,000 to 2,000 feet RVR). Cat II/III operations are entirely suspended unless an approved portable lighting authorization exists under specific OpSpecs.
• Transmissometer Outages: If the touchdown zone (TDZ) RVR sensor fails, Category II operations cannot be initiated unless the midpoint sensor is authorized as a substitute under OpSpec C056. Category III operations require all three sensors (TDZ, Mid, Rollout) to be operational, though Midpoint or Rollout may be temporarily unserviceable if RVR is at or above 700 feet.

Official References

Source check: 2026-10-12. FAA instrument-procedure resources; General operating and instrument rules; FAA AIM clearances and holding. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.