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An aeroplane is on approach with Nav 1 tuned to the ILS frequency and NAV 2 tuned to the VOR frequency which also provides an approach to the same runway. When the CDI of NAV 1 is at exactly full scale deflection, the NAV 2 CDI will show approximately...

  • A
    quarter scale deflection.
  • B
    half scale deflection.
  • C
    full scale deflection.
  • D
    no deflection.

Refer to figure.
VORs and ILSs often use the same displays in the cockpit, with the lateral display of the CDI (course deviation indicator) useful for displaying VOR or Localiser deviation, and the vertical display showing glide path deviation. They do not, however, have the same level of accuracy, as a localiser has to be much more accurate than a VOR.

Therefore, on a standard 5 dot CDI, VORs have a deviation indicated by 2 degrees per dot, so full scale deflection is 10º deviation from the selected bearing.

ILS Localisers (LOC) on the other hand have just 0.5 degrees per dot deviation, so full scale deflection is 2.5º deviation from the localiser centreline.

Therefore, in this scenario, where we are on exactly full scale LOC deflection, we are 2.5º off the correct inbound course, which is a quarter of the scale deflection on the VOR display.

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