Radio Navigation and Electronic Aids
Scope
Radio Navigation and Electronic Aids tests VOR, DME, NDB/ADF, ILS, GPS/RNAV, and the interpretation of cockpit indications. For PPL VFR, the emphasis is safe use and limitations, not airline automation.
Core rules
- VOR gives a magnetic bearing from the station called a radial; the OBS selects the radial and the CDI shows deviation. TO/FROM indicates whether the selected course leads to or from the station.
- DME gives slant range in nautical miles; it is not ground distance.
- NDB/ADF gives a relative bearing; the ADF needle points to the station.
- ILS has localizer for lateral guidance and glideslope for vertical guidance; it is for precision approaches.
- GPS/RNAV requires a valid database, adequate satellite geometry, and RAIM where required; check NOTAMs for outages.
Worked example
The OBS is set to 090° and the CDI is centred with FROM shown. Reasoning: the aircraft is on the 090 radial, east of the station. If the pilot turns to a heading of 090° magnetic, the aircraft tracks away from the station. If TO were shown with the same OBS, the aircraft would be on the reciprocal 270 radial. Do not simply turn to the OBS number without checking TO/FROM and the heading indicator.
Traps
Confusing a radial with a heading. Reverse sensing when flying away from a VOR with a TO indication. Treating DME as ground distance. Assuming GPS is always available. Using an expired database or ignoring RAIM.