In-Flight Navigation: Visual Navigation, Map Reading, Position Lines and Radio Aids
Visual navigation and map reading
In-flight visual navigation uses the chart, the ground and the clock together. Orient the map to the ground (or mentally rotate it), follow a deliberate sequence of checkpoints, and compare expected with observed features. Good checkpoints are unambiguous, near the track, and spaced to give a useful time interval. Map reading requires reading the chart’s scale, relief, airspace and symbols, then converting map measurement to time using GS.
Position lines and fixes
A position line (PL) is a locus of possible positions. A bearing to a station gives a straight PL; a DME range gives a circle; a transit of two features gives a line through both. Two PLs intersecting at a good angle (ideally near 90°) give a fix; three give a check. A single PL can be used with a DR position, but it only constrains position along that line. Radio aids provide PLs: VOR gives a magnetic radial from the station, ADF gives a relative bearing (converted to true/magnetic), DME gives slant range, and GNSS gives a position.
Worked example
Overhead a VOR, DME reads 40 NM; GS is 120 kt and the aircraft is tracking directly to the station. Time to station = 40/120 h = 1/3 h = 20 min. If instead the aircraft is 30° off the direct track, the along-track component of GS is 120 cos 30° ≈ 104 kt, so time ≈ 40/104 h ≈ 23 min. The DME range is slant range, so at high altitude it slightly exceeds ground distance.
Traps
A VOR radial is from the station; the bearing to the station differs by 180°. Do not fix from two nearly parallel PLs. Station passage is indicated by cone-of-confusion effects, not by a single stable bearing. Do not use TAS for time. Confirm chart variation and the sense of ADF relative bearings, and do not treat a DME arc as a straight line.