Thursday, March 18, 2021

Marker Beacon

 

II. Technical Subject Areas

A. Aircraft Flight Instruments and Navigation Equipment

    2. Flight instrument systems and their operating characteristics

      d. Marker beacon receiver/indicators (also described under ILS)
        1) Ground elements
          a) Low-powered VHF transmitter directs its signal upward in a small fan-shaped pattern
          b) Outer marker (OM) located 4-7 miles from airport near position where aircraft at appropriate altitude will intercept glidepath
          c) Middle marker (MM) located approximately 3,500 feet from landing threshold at position where glide-slope centerline is about 200 feet above touchdown zone elevation
          d) Inner marker (IM) located at decision height on glidepath for Category II approach
          e) There may be a back-course marker to indicate the back-course FAF
        2) Airborne marker beacon receiver indicates, aurally and visually, when the aircraft is directly over the transmitter facility
          a) OM - low-pitched tone, continuous dashes, two per second and purple or blue light ------
          b) MM - intermediate tone, alternate dots and dashes, 95 .- per minute and amber light .-.-.-
          c) IM - high-pitched coninuous dots, six per second and white light ......
          d) BCM - (back-course marker) - high-pitched pairs of dots, 72-75 pairs per minute and white light .. .. ..
          e) Many units allow selection of 2 sensitivities
            i) Low gives sharpest indication of position and should be used during an approach
            ii) High provides an earlier indication that aircraft is approaching the beacon

      References:
        Instrument Flying Handbook, FAA-H-8083-15, 1999
        AIM 1-1-9

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