ILS FACILITY CLASSIFICATION

1.1 This Aeronautical Information Circular (AIC) outlines the International Civil Aviation Organization (ICAO) system for classifying Instrument Landing Systems (ILS). This classification is typically applied to ILS facilities designed for precision approach operations, particularly those in Category (CAT) II or III.

1.2 To maximize the benefits of modern aircraft automatic flight control systems, it is necessary to describe ground-based ILS facilities in more detail than the basic Facility Performance Categories I/II/III. This is accomplished through the ILS classification system, which uses three designated characters as described below. The classification scheme helps identify the additional capabilities of a specific ILS ground facility, allowing for the determination of its specific operational application.

2.1 An ILS facility classification is defined by a three (3) character string with each character separated by a slash (/) according to the following:

1. The first character - Roman numeral I, II, or III indicates conformance with the Facility Performance standards contained in International Civil Aviation Organization (ICAO) Annex 10, and indicates that the ILS is CAT I, CAT II or CAT III capable.
2. The second character - Letter A, B, C, T, D, or E defines the point along the approach path or runway to which the localizer conforms to the facility performance Category II/III course structure tolerances. The character indicates ILS conformance to a physical location as follows:
   • A: 7.5 KM (4 NM) before the threshold
   • B: 1050 M (3500 FT) before the threshold (CAT I decision point)
   • C: Glidepath altitude of 100 FT height above touchdown (HAT) (CAT II decision point)
   • T: Threshold
   • D: 900 M (3000 FT) beyond the threshold (Touchdown guidance)
   • E: 600 M (2000 FT) before the runway end (Roll out guidance)
3. The third character - Number 1, 2, 3, or 4 indicates the minimum level of integrity and Continuity of Service (CoS) of the ILS. Integrity is needed to ensure that an aircraft on approach will have a low probability of receiving false guidance; CoS is needed to ensure that an aircraft in the final stages of approach will have low probability of being deprived of a guidance signal. The interpretation of each number is as follows:
   • 1: The performance objective of the ILS equipment has not been demonstrated or is less than Level 2.

     Note: Level 1 performance can support low-visibility operations for which positioning guidance below approximately 200 FT height above threshold (HAT) is supplemented by other means, such as visual cues or advanced avionics.

   • 2: The performance objective for ILS equipment used to support low visibility operations when ILS guidance for position information in the landing phase is supplemented by visual cues. This level is a recommended objective for equipment supporting Category I operations.

   • 3: The performance objective for ILS equipment used to support operations which place a high degree of reliance on ILS guidance for positioning through touchdown. This level is a required objective for equipment supporting Category II and IIIA operations.
   • 4: The performance objective for ILS equipment used to support operations which place a high degree of reliance on ILS guidance throughout touchdown and rollout. This level basically relates to the needs of the full range of Category III operations.

3.1 An ILS that conforms to the ICAO Annex 10 Facility Performance CAT III standards, meets the CAT III localizer course structure criteria to ILS point “E” and conforms to the integrity and CoS objectives of Level 4 would be described as Class “III/E/4”.

4.1 The ILS facility classification is published in the AIP, section AD 2.19.