Definition
The steepest rate of altitude loss permitted on a non-precision instrument approach segment, expressed as 400 feet of descent per nautical mile travelled along the approach course. On approaches such as those served by a Simplified Directional Facility (SDF), the published descent profile must not exceed this gradient; if the geometry would require a steeper descent, the procedure cannot be designed or flown as published.
Plain English
On this kind of instrument approach, you are not allowed to lose altitude faster than 400 feet for every nautical mile you fly forward. It is the maximum 'steepness' the approach is allowed to have.
Context Anchor
Seen in instrument approach discussions, especially for Simplified Directional Facility approaches, when checking whether the final descent is reasonably manageable.
Derivation
Gradient comes from a Latin word meaning “to step” or “to walk.” In aviation, it means the slope of a climb or descent measured over distance. Here, the “steps” are 400 feet down for each nautical mile forward.
Why Pilots Care
Exceeding this gradient risks obstacle contact or an unstabilized approach on an SDF procedure that lacks vertical guidance.
Grounding Statement
If 3 nautical miles remain, a 400 ft/NM descent gradient allows up to 1,200 feet of altitude loss over that distance.
Intuition Check
Do not read “maximum” as the descent you are supposed to aim for. It is the upper limit of how steep the required descent should be, not a target descent rate.
Example Sentence 1
Because the SDF approach was built within the 400 ft/NM maximum descent gradient, the crew was able to fly a stable descent from the final approach fix to the missed approach point.
Example Sentence 2
Reviewing the approach plate, the crew noted the 400 ft/NM maximum descent gradient and began reducing altitude earlier than planned.