ILS (Instrument Landing System)

The Instrument Landing System is defined under ICAO Annex 10, Volume I, Section 3.1, and in the United States is regulated under 14 CFR Part 171 (Non-Federal Navigation Facilities) and FAA Advisory Circular AC 90-105B (Approval Guidance for RNP Operations, which cross-references ILS CAT requirements). EASA addresses ILS airborne equipment under CS-ACNS-D-ILS and operational approval under Commission Regulation (EU) 2018/1048. The ILS comprises three primary radio components that together guide an aircraft to the runway threshold with precise lateral and vertical accuracy.

The localizer transmits a narrow VHF signal on frequencies between 108.10 MHz and 111.95 MHz, projecting a course that is nominally 5° wide at the antenna (varying from 3° to 6° depending on the distance at which full-scale deflection is set, typically so that the full-scale width equals 700 ft at the runway threshold). The glideslope transmitter operates on paired UHF frequencies between 329.15 MHz and 335.00 MHz, projecting a beam at a typically 3° angle (ranging 2.5° to 3.5° depending on terrain and obstacles), which the aircraft intercepts and tracks to the runway. Legacy outer marker (75 MHz), middle marker, and inner marker beacons are progressively being replaced by co-located DME or DME/N, which provide distance-to-threshold readouts more precisely and more reliably.

ILS performance is categorized by the minimum decision height (DH) and runway visual range (RVR) at which the approach can be conducted. CAT I operations require a DH of not less than 200 ft above touchdown zone elevation and an RVR of not less than 550 m (1,800 ft); aircraft and pilots must meet requirements under 14 CFR §91.189 or applicable EASA OPS rules. CAT II operations require a DH of not less than 100 ft and RVR not less than 300 m (1,000 ft), and demand specific aircraft equipment certification, simulator-based crew training, and airline/operator approval. CAT IIIa operations permit DH below 100 ft with RVR of at least 200 m. CAT IIIb allows DH below 50 ft with RVR between 75 m and 200 m. The theoretical CAT IIIc, permitting zero-zero (no DH, no RVR), is defined in ICAO Annex 10 but not operationally approved anywhere as of 2026, due to the absence of runway surface guidance systems that would allow safe taxi after touchdown in zero visibility.

Aircraft equipment required for an ILS approach includes a VHF NAV receiver capable of receiving the localizer frequency, a glideslope receiver, and a course deviation indicator (CDI) or horizontal situation indicator (HSI). For CAT II and III operations, redundant receivers, a flight director or autopilot coupled to the ILS, a radar altimeter for DH callouts, and fail-passive or fail-operational autopilot systems are mandatory. Ground-based critical areas must be protected from vehicle and aircraft intrusion during low-visibility operations to prevent signal distortion, which is why airports implement Low Visibility Procedures (LVP) restricting taxiway and runway usage.

Although ILS remains the global standard for precision approaches at major airports, it is progressively being supplemented — and at many GA airports replaced — by GPS-based LPV approaches providing equivalent CAT I minima (200 ft / 550 m) without ground infrastructure costs. The FAA's Minimum Operational Network plan retains ILS at airports where satellite approaches cannot provide equivalent protection, particularly those requiring CAT II or III minima. Despite its age (first deployed commercially in the 1940s), the ILS standard is expected to remain operationally dominant at high-traffic and low-visibility airports well into the 2030s.

For flight schools offering instrument rating training, ILS approaches are a mandatory syllabus element. FAA Airman Certification Standards (ACS) for the instrument rating require applicants to demonstrate at least one precision approach during the practical test; an ILS is the canonical precision approach for this purpose. Training schools must have access to airports with serviceable ILS installations, or use FAA-approved aviation training devices (ATDs) that simulate ILS deflection, intercept, and CAT I decision height procedures. Ground failure of an ILS facility — NOTAMed as unserviceable — can disrupt training schedules, and schools must have contingency airports identified for instrument approach practice.

For schools operating under Part 135 or Part 141 with multi-engine or advanced aircraft, ILS CAT I currency requirements apply to pilots in command: 14 CFR §61.57(c) requires six instrument approaches within the preceding six calendar months. Schools that conduct instrument currency flights must log approach types precisely; an ILS approach counts as a precision approach for currency, while an RNAV (GPS) approach with LPV minimums does not automatically substitute in all operator-specific CAT I approval frameworks.

Aviatize tracks ILS approach completions within the training management module, allowing instructors to log each approach by type — ILS CAT I, ILS CAT II, LOC-only, backcourse — directly against the student's lesson record. The system tallies precision approaches separately from non-precision approaches, reflecting the ACS requirement to demonstrate at least one precision approach at the checkride, and flags when a student has not yet logged an ILS in actual IMC versus simulated conditions.

For Part 135 operators and schools tracking pilot currency, Aviatize monitors the six-approach-in-six-months requirement under §61.57(c), breaking down the approach log by type so that dispatchers and chief pilots can instantly see whether a pilot's recent ILS experience is current before assigning them to a low-visibility dispatch. When an ILS NOTAM closes a training airport's precision approach capability, the smart planning and booking module can surface alternate airports with serviceable ILS installations within routing range, minimizing disruption to the instrument training schedule.