Radio navigation aids and their use
This domain tests practical use of VOR/DME, NDB/ADF and ILS, including the geometry that produces the indications you see, and the errors that make those indications misleading.
VOR and DME
- The CDI shows course deviation, not heading. With OBS 090 and FROM displayed, the aircraft is on the 090 radial; selecting 270 gives TO and reverse sensing.
- DME measures slant range, so it over-reads ground distance. The error is largest close in and at high altitude.
NDB/ADF
- The ADF needle points to the station relative to aircraft heading; tracking requires applying drift. Night effect, coastal refraction, thunderstorms and quadrantal error all distort the bearing.
ILS
- Localiser and glidepath use 90/150 Hz modulation. A 3° path rises about 318 ft per nautical mile, commonly rounded to 300 ft/nm. Markers, DME or GNSS provide range; the missed approach point is defined on the chart.
Worked example
An aircraft is 6 nm ground distance from a DME station at 6,000 ft (about 1 nm). Slant range = √(6² + 1²) = √37 ≈ 6.08 nm, an error of about 0.08 nm. At 1 nm ground distance and 6,000 ft, slant range is √2 ≈ 1.41 nm - a 0.41 nm error that matters near the facility.
Traps
- Reverse sensing from a wrongly set OBS.
- Cone of confusion overhead a VOR or NDB, where the needle swings.
- Treating DME as ground distance at high altitude close to the station.
- False glidepath indications and localiser interference from aircraft or vehicles.