Navigation Systems
VOR, DME, and the CDI
VOR is line-of-sight; reception degrades with range and terrain. The CDI is full-scale at 10 degrees of deviation, so a full needle means you are 10 degrees off the selected radial. TO/FROM flips as you cross the station, inside the cone of confusion. Reverse sensing appears when the OBS is set to the reciprocal of your course. DME reads slant range, noticeably longer than ground distance when you are high and close to the station.
ILS and RNAV
Localizer full-scale deflection is 2.5 degrees; glideslope full-scale is 0.7 degrees. Both indications grow more sensitive as you close on the antenna. GPS/RNAV brings RAIM, WAAS, and the approach types LPV, LNAV/VNAV, and LNAV. Know which equipment (TSO-C129, C145, C146) supports which minima and which procedures require WAAS.
Traps
Database currency, GPS interference NOTAMs, and the difference between using GPS in lieu of VOR/ADF for routing and actually flying a GPS approach are favorite subjects. Remember the VOR check must be logged with date, place, frequency, bearing error, and signature.
Worked example: on a VOR approach your CDI shows half-scale deflection. Half of 10 degrees is 5 degrees off course, which justifies a solid intercept angle rather than a small correction.