Visual descent point

Model: A straight path at the descent angle from the MDA to the threshold crossing height; a VASI or PAPI path at the same angle and TCH puts you TCH + d·tan θ above the threshold at distance d

Show your work

1. Height to lose

   h = height above touchdown − threshold crossing height

   400 ft − 0 ft = 400 ft

2. Distance from the threshold

   $$d=h/\tan \theta$$

   400 ft / tan 3° = 1.26 NM

The same steps an agent gets from the MCP server with explain: true.

Accuracy: Exact geometry. A published VDP on the chart always governs. Planning aid, not certified for navigation.

When to use this: Use this on a non-precision approach with no published visual descent point: it gives the distance from the threshold where a normal descent from the MDA begins, so you can decide before you get there rather than diving late. It also shows the HAT-over-300 rule and the descent rate the angle asks for at your groundspeed.

Limitations: A published VDP on the chart governs whenever there is one, and so do the minimums, the missed approach point, and any step-down fix. The geometry assumes a straight path at the angle you give from the MDA to the threshold crossing height, over flat ground; it does not know obstacles, terrain, or the runway's lighting, and it is a planning aid rather than an approach procedure.

Worked example: An MDA 400 ft above the touchdown zone, 3° path, 90 kt. Source: Instrument Procedures Handbook chapter 4: VDP ≈ HAT/300 NM for a 3° path (400/300 = 1.33 NM).

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.

Changes

• 2026-09-19, changed: Top of descent and the visual descent point are now stable. Both match the FAA Instrument Procedures Handbook examples (FL 310 to 6,000 ft is 75 NM by the 3-to-1 rule; 300 ft per NM on a three-degree path) and 18 to 19 vectors each from a separate implementation. Results are unchanged. Changelog

Checked against: 24 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• Instrument Procedures Handbook, FAA-H-8083-16B, Federal Aviation Administration, FAA-H-8083-16B. Chapter 2 (descent planning) and Chapter 4 (visual descent point, vertical descent angle).
• Aeronautical Information Manual, Federal Aviation Administration, Current edition. Chapter 4 (tower and ATIS winds, magnetic) and Chapter 7 (METAR and TAF winds, true).

Terms

HAT: height above touchdown

The height of the minimum descent or decision altitude above the touchdown zone elevation. Source: Instrument Procedures Handbook

MDA: minimum descent altitude

The lowest altitude on a non-precision approach to which a pilot may descend without the runway environment in sight. Source: Instrument Procedures Handbook

PAPI: precision approach path indicator

A row of four lights beside a runway: two white and two red means on the glidepath, more white means high, more red means low. Source: Aeronautical Information Manual

VASI: visual approach slope indicator

Rows of red and white lights beside a runway that show whether you are above, on, or below the visual glidepath, usually 3°. Source: Aeronautical Information Manual

VDP: visual descent point

A point on a non-precision approach from which a normal descent from the MDA to the runway can begin. Source: Instrument Procedures Handbook