Rate of descent calculation method 1 The resulting number is our approximate descent rate in feet per minute. For example, if our groundspeed is 100 knots indicated airspeed (KIAS), and we multiply it by 5, that would equate to a 500 FPM descent rate to achieve a 3-degree descent angle.

In aviation, a standard 3-degree descent rate is calculated using a high-fidelity "rule of thumb" based on the aircraft's ground speed. To find the required Vertical Speed (VS) in feet per minute (fpm), you multiply your ground speed in knots by 5. For example, if a plane is approaching at 120 knots, the 3-degree descent rate would be 600 fpm (120 x 5). If traveling at a higher ground speed of 180 knots, the rate would be 900 fpm. This high-fidelity calculation ensures the aircraft stays on a stable "glideslope" toward the runway. For even more precision, pilots sometimes use the formula: VS=Ground Speed×(606076​)×tan(3∘), but the "multiply by 5" method is the high-fidelity standard for quick mental math in the cockpit. Understanding this descent rate is a high-fidelity requirement for safe landings, as it prevents the aircraft from becoming too "high and fast" or "low and slow" during the critical final phases of the flight.