Definition
A standard maximum descent rate used in instrument approach design, expressed as 400 feet of altitude lost for every nautical mile flown along the approach course. It is the default obstacle-clearance descent gradient applied to non-precision approach segments unless terrain or obstacles require a steeper one to be published.
Plain English
On this kind of approach, the path is built so the aircraft loses no more than 400 feet of altitude for each nautical mile flown forward. It is the standard slope used when designing the approach.
Context Anchor
Seen in RNAV (GPS) approach discussions when evaluating the vertical path from a fix toward the runway or toward a published minimum altitude.
Derivation
“Descent” comes from a Latin word meaning “to climb down.” “Gradient” comes from a Latin word meaning “step” and is used for slope. Together, the phrase describes how many feet the airplane steps downward for each nautical mile forward.
Why Pilots Care
Allows pilots to meet altitude restrictions while maintaining a stable descent without excessive speed or configuration changes.
Analogy
It is like a road grade measured by how much height the road loses over a set distance. Here, the “road” is the airplane’s flight path, and the measurement is 400 feet down for each nautical mile forward.
Grounding Statement
Picture being 400 feet lower each time you pass another one-nautical-mile point along the approach path.
Intuition Check
Do not read 400-ft/NM as 400 feet per minute. It is based on distance traveled, not time; your actual descent rate per minute changes with groundspeed.
Example Sentence 1
The approach was designed to the standard 400-ft/NM descent gradient, so a normal stabilized descent at approach speed kept us comfortably on profile.
Example Sentence 2
At a groundspeed of 120 knots, a 400-ft/NM descent gradient calls for a descent rate of 800 feet per minute.