he deviation bar and the deviation indicator needle together form the Course Deviation Indicator or CDI. If the needle is over the centre point the aircraft is then located at some position along the selected radial – or its reciprocal. The five division marks or dots either side of the centre point are spaced at two degree intervals, thus if the needle is over the third mark, left or right of centre, the aircraft is positioned at a radial six degrees in azimuth from the selected radial, or its reciprocal. (Actually the aircraft is at the centre mark and the needle indicates the position of the selected radial). Full travel of the needle from the centre to either side represents 10° – or more – of azimuth. The ambiguity of whether the OBS selection is the radial or the reciprocal is determined by the TO / FROM indication; in the diagram at left 030 must be the radial as the aircraft is in the FROM area. When the aircraft passes overhead the beacon the needle will swing from side to side, the alarm flag may temporarily indicate that navigation is 'OFF' and the TO/FROM indication will reverse.
A difficulty for a non IFR trained pilot using the VOR is a lack of perception of which way to turn the aircraft to fly to a selected radial, using the CDI indications. However, for VFR purposes, this is easily ascertained if the pilot follows two simple rules:
1. To track FROM a VOR select the radial required and ensure FROM is indicated.
2. To track TO a VOR rotate the OBS until the CDI is centred and TO is indicated.
In both cases as wind effect drifts the aircraft off track the deviation indicator needle will move to one side and that movement indicates the direction to turn to regain track. i.e. turn towards the needle.