Definition
A standard climb gradient expressed as 318 feet of altitude gained per nautical mile of horizontal distance traveled. This is the default minimum climb gradient assumed by the TERPS (Terminal Instrument Procedures) criteria for departure procedure design, ensuring obstacle clearance when no higher gradient is published.
Plain English
It is a climb rate measured by distance instead of time. For every nautical mile you fly forward, you must climb at least 318 feet upward. This is the standard minimum climb performance that instrument departure procedures are built around.
Context Anchor
Seen on instrument arrival procedures, including STAR descent planning, where a pilot needs to understand the required vertical path.
Derivation
The figure comes from how procedure designers protect departing aircraft from obstacles. 'ft/NM' simply means 'feet per nautical mile' -- a slope expressed as vertical feet gained for each nautical mile of ground covered. 318 is the specific number TERPS uses as the standard minimum slope.
Why Pilots Care
Meeting this gradient keeps the aircraft on the published vertical path, satisfies altitude restrictions, and maintains obstacle clearance.
Analogy
Think of it like a long ramp: for each nautical mile along the ramp, the height changes by 318 feet. The ramp angle stays the same even if you move along it faster or slower.
Grounding Statement
On an arrival, 318 ft/NM is a practical way to describe a smooth 3-degree descent toward the airport area.
Intuition Check
318 ft/NM does not mean 318 feet per minute. It means 318 feet for each nautical mile traveled; the feet-per-minute descent rate changes with speed over the ground.
Example Sentence 1
The departure procedure assumed the standard 318 ft/NM climb gradient, so I checked the chart's climb rate table to confirm I could meet it at my expected groundspeed.
Example Sentence 2
If the aircraft cannot maintain 318 ft/NM, the pilot requests a different routing to avoid arriving high at the next waypoint.