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Mountain effect, occurring for instance with NDBs, is caused by what physical phenomenon?
  • A
    refraction
  • B
    diffraction
  • C
    reflection
  • D
    absorption
Refer to figure.
MOUNTAIN EFFECT

Mountainous areas can cause both reflections and diffraction of the transmitted radio waves to produce errors in ADF systems. Such errors increase at low altitude and can be minimized by flying higher.

Note: The new learning objectives state the following "Describe diffraction of radio waves in mountainous terrain (mountain effect)." => it seems that the correct option for this question differs:
  • EASA2016: reflection
  • EASA2020: diffraction

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