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Performing an ILS approach, you will fly overhead the markers in an specific order. This order is:
  • A
    OM, MM, IM (if available).
  • B
    OM, IM (if available), MM.
  • C
    MM, IM (if available), OM.
  • D
    IM (if available), MM, OM.

Refer to figure.

There are three types of markers installed as part of a Instrument Landing System (ILS)

Outer marker: (OM)

This marker normally indicates the Final Approach Fix (FAF). It is located between 4 and 7 nm from the runway threshold on the same course as the localizer. When passing the outer marker the pilot receives an audio tone in continues series of 400 Hz accompanied with a flashing blue light in a continuous series of 2 second dashes.

Middle marker: (MM)

This marker normally indicates the CAT I Missed approach point and it is positioned between 0.5 and 0.8 nm from the runway threshold. When passing the middle marker the pilot receives an audio tone in continues series at 1300 Hz accompanied with a flashing amber light in an alternating dots and dashes sequence.

Inner marker: (IM)

This marker normally indicates the passing of the runway threshold. When passing the inner marker the pilot receives an audio tone in continues series of 3000 Hz accompanied with a flashing white light in a dots sequence.

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