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For CAT II and CAT III approaches require that protected area around the ILS antennas is kept clear from obstacles so it does not disrupt the ILS signal.

So when there is an airplane on the final approach, at which moment at latest must the protected area be clear (which includes the preceding aircraft must be out of it)? Before the landing aircraft reaches 1000' above (stabilized criteria)? Or before it reaches CAT I minima? Or even further out?

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  • $\begingroup$ Which country are you asking about? $\endgroup$
    – user22445
    Commented Feb 14, 2023 at 22:52

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ICAO Annex 10 Vol. 1 provides some general guidance on ILS critical and sensitive area protection:

2.1.9.3 Technical and operational logic associated with critical and sensitive areas. Ideally, the critical area is enforced during all ILS operations with protection afforded down to at least the Category I decision height. A critical area disturbance would normally impact all aircraft using the ILS signal at a given time (entire approach). [...] From an operational perspective, the sensitive area would ideally protect aircraft operations at least from the Category I decision height down to the runway, and be activated during low visibility conditions only (e.g. Category II and III). [...]

Note.— Guidance on operational procedures for the protection of critical and sensitive areas is provided in ICAO EUR DOC 013, “European Guidance Material on All Weather Operations at Aerodromes”.

(ICAO Annex 10 Volume 1 "Radio Navigation Aids", bold emphasis mine)

The referenced document contains some more details, first on when LVP apply in general:

7.5.6 LVP Operations Phase

7.5.6.1 The point at which LVPs are in force must be clearly defined in terms of a specific RVR and/or height of cloud base and must be promulgated in the LVP. LVP should be in force at the latest when the MET conditions deteriorate below RVR value of 550 m and/or cloud base of 200 ft for approach and landing operations and below RVR value of 550 m for departure operations.

(ICAO EUR Doc 013, bold emphasis mine)

And then further on ILS operations: the critical areas should always be clear and the sensitive area should be clear at the latest when the landing aircraft is 2 NM before the threshold (recommended is 4 NM when possible or no radar available):

7.5.6.4 ILS operations

7.5.6.4.1 To ensure that the integrity of the guidance signal radiated by the ILS is maintained during aircraft approaches, all vehicles and aircraft on the ground should remain outside the ILS critical and sensitive areas. The ILS critical areas must be clear of all vehicles, persons and aircraft at all times.

7.5.6.4.2 These objectives are normally achieved by providing appropriate spacing between successive landing and/or departing aircraft. [...] To accord with the basic requirements, the spacing specified should provide sufficient separation between successive approaching aircraft, normally to allow the leading aircraft to land, to vacate the runway, and to clear the ILS localizer sensitive area before the following aircraft reaches a point 2 NM from touchdown. [...] At aerodromes where the traffic density is low or where the range of the approaching aircraft cannot be monitored by radar, the separation should be increased to enable the leading aircraft to clear the runway and ILS localizer sensitive area before the following aircraft reaches a point 4 NM from touchdown, i.e. about the position of the outer marker (or equivalent DME position).

(ICAO EUR Doc 013, bold emphasis mine)

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