Radio Navigation and Instrument Systems
Candidates must understand what each ground and airborne system actually provides, its limitations, and how the receiver interprets it. Cover VOR/DME (radial and slant range), NDB/ADF (relative bearing and the effect of wind), ILS (localiser, glide path, markers, and the sensitive area), GNSS (RAIM, integrity, and the difference between an overlay and an RNP approach), and radar (primary and secondary).
Concepts to master
- VOR: the CDI shows the selected radial; reverse sensing occurs when the OBS is set to the reciprocal.
- ADF: the needle points to the station relative to the aircraft nose; station passage is when the needle swings through 90 degrees relative to the aircraft heading.
- ILS: the localiser is the most sensitive and should be intercepted at a shallow angle; the glide path is a fixed angular surface, so the height at the threshold is constant but the rate of descent varies with ground speed.
- GNSS: RAIM prediction, fault detection, and the requirement to check the approach is in the database and the aircraft is approved.
- Slant range error: DME reads higher than the true ground distance, most noticeably overhead the station.
Common traps
Questions ask what happens to the CDI when you fly through the station (it reverses), or what the glide path needle does as you cross the threshold. Another trap is assuming a GNSS approach can be flown without a RAIM check.
Rule of thumb
On an ILS, the rate of descent on a 3 degree path is roughly 5 x ground speed in knots, in feet per minute.
Worked example
At 120 kt ground speed on a 3 degree glide path: 5 x 120 gives about 600 ft/min. At 90 kt, about 450 ft/min. The path is the same; only the rate changes.