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Refer to figure.
If the course pointer in situation 2B is rotated 2.5 degrees in a clockwise direction, what will happen to the LOC course deviation indicator?
  • A
    is at the left outer dot.
  • B
    is centred.
  • C
    is at the right inner dot.
  • D
    does NOT change position.

Refer to figure.

From the course selector knob at the bottom left corner we can infer that the equipment is a HSI (Horizontal Situation Indicator).

If we rotate the course selector knob the whole central CDI (course deviation indicator) display will rotate accordingly. This would also change the selected course for use with a VOR, similar to the OBS (Omni-Bearing Selector) on a fixed CDI display. However, it is important to note on this question that we can see a glideslope indication to the right of the HSI in figure 2B, meaning that this is tuned to an ILS frequency, and the CDI is locked on to localiser indications.

The indications on the central CDI will not change (as it is tuned to ILS LOC) when it is turned. This is because the ILS localiser produces two lobes of differently modulated signal that the receiver interprets into a deviation from the centreline, this does not rely on the selected OBS course in any way. Pilots usually set the correct inbound course anyway as an aide memoir and to show the fly left/fly right indications the correct way round on the HSI.

The course pointer will now be pointing to 232.5 degrees on the compass rose after rotating, 2.5 degrees clockwise from 230 degrees (which is also our heading), but will be indicating the same amount of "dots" off centreline, as this is unaffected by the selected course. This can be seen on the above annex, but you have to look carefully, as 2.5 degrees is a very small change.

Notes for further learning: This feature of being able to swivel the CDI around makes flying back course approaches with HSI convenient. We can set the front course QDM and fly the back course approach as if it were a front course approach (without glideslope)

Also note that, if we have OBS on the bottom left corner then the equipment will be a fixed CDI (Course Deviation Indicator). Rotating the OBS knob while tuned to ILS will only rotate the compass card but have no effect on the deviation display.

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