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As an aeroplane is approaching the runway while taxiing for departure, the flight crew observes two sets of yellow lines drawn across the taxiway. One line further away from the runway has a sign stating "S2 CAT II/III," and the line closer to the runway has a sign stating "SI". Why is there a separate holding position for category II and Ill operations when compared to all other operations, including CAT l?
  • A
    Because of the lower visibility observed during CAT II and III operations, the landing pilot(s) may mistake the lights from the waiting aircraft as runway lighting, causing a distraction during the landing.
  • B
    The landing minima for CAT II and Ill operations are lower and therefore require greater precision, and an aircraft waiting too close to the landing runway may affect the quality of the ILS signals.
  • C
    Since aeroplanes perform an autoland during CAT I operations, the autopilot is able to fly the aeroplane more accurately and therefore the CAT I position may be closer to the runway.
  • D
    Aircraft must be kept further away from the runway during CAT II and Ill operations as arriving aeroplanes may land automatically and therefore require more space.

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.

CAT II and CAT III operations require high accuracy standards. The aircraft flying CAT II or CAT III approaches follow the ILS LOC and GP signals, and based on these signals they are flown down to a low height above the touchdown, or even all the way to the touchdown, depending on the category.

Since on precision approaches CAT II and III the system is capable of bringing the aircraft too low or even on touchdown, measures need to be taken to ensure that their signals will not be affected by interference of any kind.

ILS localizer and glide slope signals are radio waves, and can therefore be affected by interference. The most common type of interference for ILS signals are 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. An aircraft waiting too close to the runway may affect the quality of the ILS signals, making it harder for the landing aircraft to maintain its course and height accurately. Therefore, a separate holding position is required to ensure that the signals are not affected by other aircraft movements..

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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