The Art of the Helicopter John Watkinson - Karatunov.net

Thus if a VOR receiver detected a 90 ◦ lag in the signals, this would indicate that
the receiver lay on a bearing of 90 ◦ from the beacon. The beacon would then be at
270 ◦ with respect to the receiver. A cockpit indicator makes this reciprocal indication
automatically. In order to fly along a selected radial, the pilot turns the omni-bearing
selector knob on the display until the desired radial heading is at the top. If the autopilot
is now coupled to the VOR receiver, should the track deviate from the required radial,
a phase change will be picked up which will modify the roll reference of the autopilot
until the correct radial is being flown. This will be indicated when the deviation pointer
on the display points straight up to the selected radial heading. The display will also
indicate whether flight is to or from the beacon.
The signals become erratic as the beacon gets very close. Consequently the VOR
coupling is disabled automatically when this condition is sensed and the flight director
uses magnetic heading to continue flight until the beacon is behind the machine when
VOR coupling will resume with the ‘from’ indicator showing.
Figure 7.53 shows how an ILS (instrument landing system) works. There are four
transmitters associated with each landing site. The localizer is sited at the far end of
Fig. 7.53 An ILS installation. The localizer guides the aircraft horizontally onto a path which aligns with the
runway. The glide slope beams make the altitude a suitable function of distance so the aircraft descends at the
correct angle to the threshold. The markers tell the aircraft how far it is to touchdown.
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