Obstacle climb gradient explained
An obstacle climb gradient describes how much altitude must be gained for each unit of horizontal distance traveled over the ground. In U.S. instrument procedures, climb gradients are commonly stated in feet per nautical mile, such as 200, 300, or 400 ft/NM.
Gradient is not the same as feet per minute
A requirement of 300 ft/NM does not directly tell you what vertical speed to hold. Feet per nautical mile describes the path over the ground. Feet per minute describes altitude change over time. To connect them, you need groundspeed.
Convert ft/NM to FPM
Required FPM = climb gradient × groundspeed ÷ 60. At 120 knots groundspeed, 300 ft/NM requires 600 fpm. At 90 knots, it requires 450 fpm.
Why groundspeed matters
A higher groundspeed means the airplane covers each nautical mile more quickly, so it must gain the required altitude in less time. A lower groundspeed gives more time to climb for each mile traveled. Wind therefore changes the vertical speed associated with a fixed ground-referenced gradient.
Geometric obstacle calculations
You can also estimate a gradient from obstacle height and horizontal distance. That geometric answer tells you the path needed to clear the obstacle from the chosen starting point. It does not prove that the aircraft can achieve the required climb under the actual weight, density altitude, configuration, and engine condition.
Procedure requirements versus aircraft capability
For an IFR departure, first identify the published climb requirement and any speed or routing assumptions. Then determine whether the aircraft can meet it with appropriate margin under expected conditions. Published procedure criteria and approved aircraft performance data are separate pieces of the decision.
Related PilotDesk tools
Climb gradient guide · Gradient vs angle · FPM to gradient
Published departure gradients
The FAA Aeronautical Information Manual says that, unless a procedure specifies otherwise, IFR departure obstacle clearance assumes crossing the runway end at 35 ft, reaching 400 ft above the runway end before the initial turn, then maintaining at least 200 ft/NM until the minimum IFR altitude. A published procedure can require a greater gradient. Pilots must determine whether the airplane can meet it at the expected groundspeed and conditions.
Read the current procedure and its notes, then compare the requirement with aircraft-specific performance. The FAA's AIM, Section 5-2, Departure Procedures explains the obstacle-clearance assumptions.