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Flight Planning
3° Descent Rate Calculator
Calculate descent rate in feet per minute for a 3-degree path from groundspeed and compare exact geometry with the groundspeed-times-five rule.
kt
Vertical speed—
NM per minute—
Rule-of-thumb (GS × 5)—
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637 fpm versus the groundspeed × 5 shortcut
At 120 kt groundspeed, the exact 3° geometry gives 120 × 6,076.1 / 60 × tan(3°) ≈ 637 fpm. Groundspeed × 5 gives 600 fpm.
Both rates depend on groundspeed. This calculation does not establish obstacle clearance or authorize descent below a charted altitude.
Formula and method
Vertical speed = groundspeed × 6,076.1 ÷ 60 × tan(3°). The familiar groundspeed × 5 rule is a cockpit approximation.