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The reason why pre take-off holding areas are sometimes further from the active runway when ILS Category 2 and 3 landing procedures are in progress than during good weather operations is..
  • A
    to increase aircraft separation in very reduced visibility conditions.
  • B
    heavy precipitation may disturb guidance signals.
  • C
    to increase distance from the runway during offset approach operations.
  • D
    aircraft manoeuvring near the runway may disturb guidance signals.

Refer to figure.
Learning Objective 062.02.05.05.02: Define the ‘ILS-sensitive area’: an area extending beyond the ILS-critical area where the parking or movement of vehicles, including aircraft, is controlled to prevent the possibility of unacceptable interference to the ILS signal during ILS operations.


ILS signals have to be extremely accurate during CAT II and CAT III operations, as aircraft are being automatically flown down all the way to touchdown, by reference to only the ILS LOC and GP signals. This means that they have to be exact, and cannot be affected by interference of any kind.

ILS localisers and glide paths are radio waves like any other, and can therefore be affected by interference. The most common type of interference for ILSs is that of multi-path interference and beam-bends. These can happen for a multitude of reasons, but primarily, due to the reflections of the radio waves off buildings, terrain, vehicles, etc. This is particularly true for aircraft; they are huge, shiny, round metal objects that can cause a lot of interference to ILS signals.

This is the reason that, during LVOs (Low Visibility Operations) - CAT II/III approaches, at many airports, aircraft are required to use different holding points, further from the runway, in order to remain outside the ILS-sensitive area (also known as the LSA - Localiser Sensitive Area).

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