Top of descent

Model: Straight descent path over flat ground at constant groundspeed: distance = Δh / tan θ, vertical speed = GS · tan θ

Show your work

1. Altitude to lose

   Δh = cruise altitude − target altitude

   35,000 ft − 3,000 ft = 32,000 ft

2. Distance to start down

   $$d=\mathrm{\Delta h}/\tan \theta$$

   32,000 ft / tan 3° = 100.5 NM

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

Accuracy: Exact for a constant angle and groundspeed; real descents slow down and meet changing winds. Planning aid, not certified for navigation.

When to use this: Use this in the cruise, when you want to know where to start down: give the altitude to lose and either the descent angle or the vertical speed you intend, and it returns the distance out to begin, the rate to hold, and the time it takes. The 3-to-1 and five-times-groundspeed rules are shown beside the exact answer, so you can see how close the mental arithmetic is.

Limitations: It describes a straight path at a constant angle and groundspeed over flat ground. Real descents change speed, meet changing winds, and are interrupted by level-offs and crossing restrictions, all of which move the top of descent. It does not know terrain, airspace, or an arrival procedure's restrictions, and it is a planning aid rather than a clearance.

Worked example: FL350 to 3,000 ft on a 3° path at 420 kt. Source: add-aviation-suite descent scenario: 100.5 NM (3:1 rule 96 NM), 2,229 fpm (5 × GS rule 2,100 fpm).

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: 20 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).
• Pilot's Handbook of Aeronautical Knowledge, FAA-H-8083-25C, Federal Aviation Administration, FAA-H-8083-25C. Aircraft Performance and Navigation chapters (pressure and density altitude, wind triangle).