HSI geometry and VOR interception

Which side of the course are you on?

Draw the desired inbound line before choosing an intercept.

A spatial instrument

An HSI places the selected course and deviation on a heading-referenced compass presentation. This helps relate the desired path to the aircraft. It does not turn the CDI into a direct turn command in every orientation. The selected course, TO/FROM, heading reference and signal validity must all be understood.

For an inbound course of 000° on the south side of a station, an aircraft east of the line needs a westward ground-track component. A 330° intercept track supplies that component. After reaching the line, the required ground track becomes 000°; retaining 330° would cross through and create a new error.

Distance and closure

Intercept angle controls closure rate, together with groundspeed. A 30° intercept at 120 kt produces 60 kt sideways closure in still air: 120 sin 30°. From 3 NM off track the ideal intercept takes 3 minutes. Near the station, angular deviation changes quickly, so the same manoeuvre can look much more sensitive.

Choose an intercept that actually closes

The desired inbound course is northbound and the aircraft is 3 NM east of it. With no wind at 120 kt, a 330° ground track moves 60 kt west and 103.9 kt north. It reaches the line in 3 minutes and travels about 5.20 NM north during that interval. The station must be far enough ahead for this intended intercept to occur on the relevant segment.

A 030° track has the same north component but an eastward component, so it increases the original error. This exposes the weakness of choosing an intercept using only a heading difference. Position, course direction and the side of the line are necessary facts. After finding a valid ground-track solution, include wind and the aircraft’s turn behaviour before treating it as a manoeuvre.

Why does a good intercept still overshoot?

A useful intercept must end before it becomes a crossing.

Lead the capture

A real aircraft needs time and radius to turn. Waiting until the needle centres before starting the capture turn can carry the aircraft through the line. Higher groundspeed, stronger closure and slower roll response all increase this problem. The ideal line intersection in the laboratory has no bank dynamics and should not be mistaken for a capture procedure.

Wind changes ground track. Choose the intercept on the ground-geometry picture, then solve for a feasible heading. Once established, monitor the trend and use a correction that holds the course rather than repeatedly making the original intercept turn.

Reversal and presentation

Selecting a reciprocal course changes TO/FROM and conventional CDI interpretation. A heading-referenced HSI makes the geometry easier to see but still requires appropriate course selection and correct use. If the signal flags, discard the apparent intercept information; a memorised left/right rule cannot repair invalid data.

A deliberately simplified overshoot

Suppose sideways closure is 1 NM per minute. If a pilot waits until crossing the course line and then needs ten seconds to remove that closure, the aircraft can travel roughly one-sixth of a nautical mile across the line during the delay. This rectangular approximation is deliberately crude because the lateral velocity actually changes during the turn, but it shows why a late response creates an overshoot.

At double the sideways closure, the same delay creates roughly double that displacement. Leading capture addresses aircraft motion; changing a CDI scale merely changes its presentation. Avoid deriving a universal lead angle from this example. Real capture depends on intercept geometry, speed, bank and roll limits, wind, distance to the aid and the aircraft’s approved flight-director or manual technique.

Can you explain and apply the mechanism?

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