Radio Navigation Instrument Interpretation and Practical Use
Core concepts
A candidate must be fluent in reading the CDI, HSI, RMI, ADF card and DME display, and in converting between the various presentations. The CDI shows deviation from the selected course, with TO/FROM sensing; the HSI combines heading and course; the RMI shows magnetic bearing directly. Practical use includes station passage, intercepting radials, tracking with wind, and cross-checking position with multiple aids. Key concepts are the difference between course and heading, the effect of wind on tracking, and the correct interpretation of flags (OFF, NAV, GS).
Traps in questions
Expect questions on: reverse sensing when the OBS is set to the reciprocal; the difference between the bearing pointer and the CDI; the meaning of a TO indication when the aircraft is on the selected course; and the effect of station passage on the TO/FROM flag. Another trap is assuming the CDI shows track rather than deviation from the selected course. Also remember that DME ground speed is only valid when tracking directly to or from the station.
Worked example
Tracking inbound on the 270 radial with a 20 kt crosswind from the north, the aircraft must crab into wind. If the heading is 265°M and the CDI shows one dot right, the aircraft is left of course; turn right to intercept, then reduce the correction as the CDI centres.
Rule of thumb
Fly toward the needle. If the CDI shows deviation to the right, turn right to intercept, then apply the wind correction once established.
Radio Navigation Errors, Limitations and Interference
Core concepts
Every aid has characteristic errors. VOR suffers from terrain and multipath, and its accuracy is specified as ±5° (or better); DME has slant range error; ADF suffers from night effect, coastal refraction, mountain effect and static; ILS suffers from reflections causing bends; GNSS suffers from multipath, ionospheric delay, interference and jamming. Key concepts are the difference between system accuracy, integrity and availability, and the effect of human factors on interpretation. Interference includes unintentional sources (electrical equipment, TV transmitters) and intentional jamming or spoofing.
Traps in questions
Expect questions on: the fact that GNSS jamming can affect a wide area and is not always detected by RAIM; the effect of precipitation static on ADF; the effect of terrain on VOR and ILS; and the need to cross-check aids when an error is suspected. Another trap is assuming an error is always the equipment's fault; human misinterpretation of the display is common. Also remember that spoofing can produce a false but consistent position.
Worked example
If the VOR and DME disagree with the GNSS position by more than the specified accuracy, the crew should suspect interference or a system fault, cross-check with another aid, and notify ATC if navigation performance is affected.
Rule of thumb
When indications disagree, trust the cross-check and the published accuracy, not a single aid. Report suspected interference, and never assume RAIM will catch jamming or spoofing.