ADF tracking, homing and environmental errors

Why does homing create a curved ground track?

Pointing at the station does not cancel crosswind.

Heading is not track

Homing keeps the station close to the nose by repeatedly turning toward the needle. In crosswind, the aircraft drifts off the desired inbound line. The bearing then changes, inviting another turn. The resulting ground track curves downwind toward the beacon. It may reach the beacon but it does not preserve an assigned track.

Tracking uses a wind-corrected heading to maintain the required bearing line. Once established inbound, a crosswind correction generally means the beacon is not exactly on the nose. The useful question is whether the bearing line remains constant, not whether the needle reads zero.

Follow the experiment

The laboratory integrates a constant-speed aircraft and a constant wind on a flat plane. Select homing and advance time. Compare tracking, whose heading cancels the crosswind when the wind is weaker than airspeed. This is a causal illustration; bank dynamics, turn limits and station passage are intentionally omitted.

Compare heading and ground velocity

A synthetic aircraft flies north at 120 kt airspeed while the wind moves the air mass east at 20 kt. Its ground velocity initially has a north component of 120 kt and an east component of 20 kt. Ground track is about 9.5° east of north. Keeping the beacon on the nose will not remove that eastward drift.

To hold a northbound track, the westward airspeed component must equal 20 kt. The ideal heading is 360° − arcsin(20/120) = 350.4°. The northward groundspeed is then √(120² − 20²) = 118.3 kt. A station due north will appear about 9.6° to the right of the nose. A non-zero relative bearing is therefore exactly what correct tracking requires in this case.

Which physical effect could move the bearing?

Use the environment to challenge a believable indication.

Different causes, different patterns

At a coastline, a ground-wave path crossing from one conductivity region to another can be refracted. Terrain can reflect or distort reception. At night, sky wave can mix with ground wave. Thunderstorm electrical activity may pull the indication toward the disturbance. The resulting error is not a single fixed correction that applies to every route.

Aircraft bank and antenna installation can introduce additional errors. Diagnose an implausible bearing by checking the reference, operating mode, identification, attitude and independent position. Do not turn a list of named errors into confidence that one can be precisely corrected in flight.

Independent information matters

If the NDB bearing changes while a valid VOR/DME fix and the expected track remain stable, treat the disagreement as evidence to investigate. Independence matters: two displays driven by the same ADF receiver repeat one measurement. In the tracking experiment the wind remains constant; a wandering bearing without the predicted geometric cause lies outside the model.

Use an error hypothesis as a test

An aircraft follows an independently confirmed coastal track. The ADF bearing changes near a shoreline while heading and other position cues remain consistent. Coastal refraction is a possible mechanism because the radio path crosses surfaces with different conductivity. It is a hypothesis to investigate, not a numerical correction determined from the fact that a coast is nearby.

If the bearing also becomes noisy near a thunderstorm, another mechanism may be present. Reducing bank may remove an aircraft-attitude contribution but cannot remove sky-wave interference. Compare the observed change with the conditions each mechanism requires. The operational objective is to recognise when the information is unsuitable and select a valid alternative; identifying an elegant name for the error is secondary to maintaining a trustworthy navigation basis.

Can you explain and apply the mechanism?

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