Climb Gradient: Feet per Nautical Mile (ft/NM) to FPM
Need to convert feet per nautical mile to feet per minute? Departure and obstacle requirements are often expressed in ft/NM, while the cockpit vertical-speed indicator shows feet per minute. Groundspeed connects the two.
Formula
At 90 kt groundspeed, a 200 ft/NM requirement works out to 300 fpm. If groundspeed increases, the required vertical speed also increases because the airplane covers each nautical mile faster.
What pilots can miss
The conversion uses groundspeed, not indicated airspeed. Wind therefore changes the vertical speed needed to maintain a given climb gradient over the ground. The calculation does not prove the airplane can achieve that performance.
200 ft/NM at 90 and 120 knots
A 200 ft/NM requirement at 90 knots groundspeed needs about 300 feet per minute because 90 knots is 1.5 nautical miles per minute and 200 × 1.5 = 300. If groundspeed increases to 120 knots, the same 200 ft/NM gradient needs about 400 feet per minute. This is why a stronger tailwind can increase the required vertical speed even though the published gradient itself has not changed.
Gradient is not the same as climb angle
Feet per nautical mile is a distance-based gradient. Percent grade and flight-path angle describe the same geometry in different units, but they are not interchangeable numbers. Use the units printed on the procedure or performance source and convert only when needed.
Before using it operationally
Compare required performance with current approved aircraft performance data, conditions, weight, procedures and any applicable obstacle-departure or operator requirements.
References
See FAA instrument/departure guidance and PilotDesk Sources & Methods.