DME arc lead points
Where to start the turn onto a arc from a radial, and how many radials early to turn off the arc onto an inbound course, from the arc's DME and your standard-rate turn.
Planning and education aid. Not for primary navigation. Full disclaimer
Experimental: not yet fully verified. How results are checked
Turn onto the arc 0.64 NM before it, and off it 3.9° before the inbound course.
- Lead onto the arc
- Lead off the arc
- Lead by the rule
Provenance
- Computed by
- aviation.ifr.dme-arc-lead 1.0.0, core 0.1.0
- Model
- A standard-rate turn (3°/s) at groundspeed GS has radius GS ÷ (60π) NM. Turning 90° from a radial onto the arc, start one turn radius r short of the arc. Turning 90° from the arc onto an inbound radial, the turn circle sits inside the arc, tangent to it and to the radial, so the lead is asin(r ÷ (R − r)); the rule of thumb is 60 × r ÷ R radials (Instrument Flying Handbook, ch. 9)
- Accuracy
- Exact for a steady turn in still air; wind moves the lead points
- Notes
- 1 shown with the answer
- Cites
- Federal Aviation Administration, Instrument Flying Handbook, FAA-H-8083-15B
Something look off?
How we got thisFormula, worked example, sources, and proof
Model: A standard-rate turn (3°/s) at groundspeed GS has radius GS ÷ (60π) NM. Turning 90° from a radial onto the arc, start one turn radius r short of the arc. Turning 90° from the arc onto an inbound radial, the turn circle sits inside the arc, tangent to it and to the radial, so the lead is asin(r ÷ (R − r)); the rule of thumb is 60 × r ÷ R radials (Instrument Flying Handbook, ch. 9)
Accuracy: Exact for a steady turn in still air; wind moves the lead points
Worked example: A 10 DME arc at 120 kt. Source: Hand check: radius 120 ÷ (60π) = 0.64 NM; lead asin(0.64 ÷ 9.36) = 3.9°, rule 3.8°. It is golden test vector v001, and every build checks the tool still gives its answer within its tolerance.
You enter
- Arc
- 10 NM
- Groundspeed
- 120 kt
You get
- Turn radius
- 0.64 NM
- Lead onto the arc
- 0.64 NM
- Lead off the arc
- 3.9°
- Lead by the rule
- 3.8°
Review: Not yet independently reviewed by a flight instructor (CFI).
Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.
Status: version 1.0.0, core 0.1.0. See this tool in the verification report.
Checked against: 7 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.
Sources
- Instrument Flying Handbook, FAA-H-8083-15B, Federal Aviation Administration, FAA-H-8083-15B. Chapter 9 (navigation systems: DME slant range, time and distance to a station from a bearing change, DME arcs).
Terms
- DME: distance measuring equipment
- A radio aid that shows the straight-line distance from the aircraft to a ground station, in nautical miles. Up high and close in, that slant distance is longer than the distance over the ground. Source: Instrument Flying Handbook
Experimental means this tool has not yet met the stable bar: at least 20 golden vectors, differential tests, and an independent worked example.
DME — distance measuring equipment
A radio aid that shows the straight-line distance from the aircraft to a ground station, in nautical miles. Up high and close in, that slant distance is longer than the distance over the ground.
Source: Instrument Flying Handbook