Radio Navigation and Instrument Systems
Radio Navigation and Instrument Systems
This domain tests whether you understand what each display is actually telling you: VOR/HSI indications, DME ranging, ILS deviations, GNSS integrity and the failure flags that tell you a sensor has dropped out.
Core concepts
- A VOR radial is a line from the station; the TO/FROM flag tells you which side of the station you are on. With a centred CDI and a FROM indication, the selected OBS equals your radial.
- CDI sensitivity is angular: full-scale deflection equals about 10° on a VOR, so at 30 NM full scale means roughly 5 NM off track. ILS localiser is far more sensitive (about 2.5° full scale) and glideslope about 0.7°.
- DME measures slant range, so it always over-reads ground distance, badly when close in and high.
- GNSS approaches need sufficient satellites and RAIM (or SBAS) integrity; the unit will flag when the required performance cannot be assured.
- Pitot-static instruments fail predictably: a blocked pitot gives a frozen airspeed indicator that behaves like an altimeter in climb or descent; a blocked static source corrupts airspeed, altitude and VSI together.
Worked example
You are 3 NM ground distance from a DME station at 12,000 ft (2 NM) height. Slant range = √(3² + 2²) = √13 ≈ 3.6 NM. The DME reads 3.6, not 3.0 - a 0.6 NM over-read. Near the overhead the error is worst: at 1 NM ground distance and 1 NM height the DME shows 1.41 NM.
Traps
- Reading a VOR radial as a bearing to the station.
- Using DME distance as ground distance for a descent calculation at high altitude.
- Assuming a GPS approach is available when RAIM predicts an outage.