VOR radials, phase and TO/FROM

Is a radial the direction the aircraft is flying?

The radial belongs to the station-to-aircraft geometry.

A station-centred direction

A VOR radial is a magnetic direction outward from the station, using the aid’s reference. An aircraft on radial 270 is west of the station whether it is flying north, east, south or west. To fly toward the station along that line in still air, use course 090°. Radial and inbound course are reciprocals.

The VOR receiver derives angular information by comparing reference and variable components. Conventional and Doppler ground-station implementations produce compatible cockpit bearing information by different signal-generation methods. The pilot’s radial interpretation remains the same.

Heading does not determine TO/FROM

With a conventional course-deviation presentation, TO/FROM relates selected course to aircraft position, not the direction of the nose. At radial 270, selecting 090 normally centres with TO; selecting 270 centres with FROM. Rotate aircraft heading in the model while position and selected course remain fixed: the logical TO/FROM result does not change.

Locate position before choosing course

Place an aircraft north-east of a VOR on radial 045°. Selecting course 225° should centre with TO under valid normal geometry, because that course points south-west toward the station. Selecting 045° should centre with FROM. The aircraft can hold heading 180° at the same instant; that heading changes neither its radial nor the selected course’s TO/FROM relationship.

Now move the aircraft around the station while leaving heading fixed. The radial changes because position changes. Rotate heading without moving position and it does not. This paired experiment is more reliable than memorising that a particular compass number means inbound or outbound. The station’s magnetic reference can also differ from a current local variation value, so compare published course information using its intended reference.

What makes a VOR CDI move?

It is an angular comparison with the selected course line.

Angular error becomes lateral distance

A conventional VOR CDI commonly represents about 10° at full-scale deflection, but the installed display determines its scaling. The same angular error corresponds to a wider corridor farther from the station: cross-track distance is approximately range × sin(angular error). A 2° error at 60 NM is about 2.09 NM.

A centred CDI identifies a course line, but the line extends on both sides of the station. TO/FROM resolves the relevant half-plane. Near the perpendicular ambiguity region and station passage, the indication needs careful interpretation rather than aggressive needle chasing.

Validity is a gate

Ground-station identification and receiver status must be suitable before interpreting deviation. A warning flag is not another form of course guidance. This laboratory removes the CDI and TO/FROM interpretation when the signal is marked invalid; the geometric aircraft position remains visible solely to explain the failure.

The same needle at two ranges

At 10 NM, a 5° angular displacement corresponds to 10 sin 5° = 0.872 NM sideways. At 60 NM it corresponds to 5.229 NM. The same number of conventional VOR dots can therefore represent a much larger physical displacement en route than near the station. Conversely, holding a fixed 1 NM lateral gap makes the angular indication grow as the aircraft approaches.

Near overhead passage, small movements can cause rapid bearing changes and a TO/FROM transition. A large correction made solely to freeze the needle can be inappropriate. Confirm station passage by the expected combination of geometry and valid indications according to the installed system. A flag is not a different kind of TO/FROM transition, and rapid needle motion alone does not prove a healthy station passage.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.