01 MAP
Drag the aircraft to move. Drag its round handle to turn.
02 HSI
03 INSTRUCTION
Final course ·
Turn model
No wind: GS equals TAS. Standard-rate turns (3°/s); bank is set automatically to at the selected TAS. Turn entry and exit are simplified. All directions share the same magnetic reference. The display uses whole-degree radial values for the intercept rule; the drawn flight path uses exact positions.
Three different rules for calculating the intercept heading
Enter the desired VOR radial, then choose inbound or outbound. R160 inbound gives a final course of 340°; R160 outbound gives 160°.
D is the smallest angle between your current radial and the desired radial. The intercept angle α is measured from the desired tracking course.
D + 30°: α = D + 30°, inbound or outbound, as taught at the Lufthansa flight school and in Michael Fröhling’s Aufsteigen zum Instrumentenflug.
ISI / OIS: inbound α = D + 30°; outbound α = 45°.
Twice difference: α = 2D.
Every method is clamped to 20–90°. These are explicit teaching variants; school mnemonics and limits differ. ISI / OIS (inbound Soll zu Ist, outbound Ist zu Soll) only tells you which way to turn; the angle is D + 30° inbound and a fixed 45° outbound.
The green dotted path includes the capture turn. A plus marks where that turn begins. The curve ends tangent to the selected radial. A circle marks that rollout point, not the intersection of two unrounded straight lines. D and the proposed heading are fixed at the start of the intercept. If the capped rule or the turns cannot fit from the present position, the grey straight leg runs parallel to the selected radial in the outbound direction to gain distance. Entry curves preserve the aircraft’s heading, with an S-turn if lateral clearance is needed. D and the rule are evaluated at the end of the grey leg. This is a geometric teaching aid, not a published repositioning procedure. Already established aircraft continue tracking. Play automatically fits the route on the map.
FAA Instrument Flying Handbook, chapter 9
Michael Fröhling, Aufsteigen zum Instrumentenflug, sample pp. 42–43
Angular lead calculation
Start with 90° lead ≈ GS ÷ 3 ÷ distance, using knots and NM. For 45°, take roughly one-third; for 30°, one-sixth. A 60° turn uses one-half. These are fractions of the 90° lead, not fixed numbers of degrees.
The displayed result uses those factors, interpolating between them for other intercept angles, and rounds only the final answer to the nearest whole degree. A displayed 0° means the estimate is less than half a degree; the drawn turn still starts before the radial.
For a standard-rate turn, radius = GS ÷ (60π) NM. Combining that with the 1-in-60 approximation gives GS ÷ π ÷ distance, simplified here to GS ÷ 3 ÷ distance. The Navy’s alternative approximation is 0.3 × GS ÷ distance. The smaller-turn fractions approximate the geometric factor 1 − cos α.
Distance here means horizontal VOR range at capture-turn start. The path uses exact circular geometry, so its lead may differ from the rounded mental estimate. “Intercept Dist” is the VOR distance at rollout. Playback is accelerated.