Definition
A required climb gradient of 400 feet of altitude gain for every nautical mile flown over the ground. It is the standard minimum climb gradient the FAA assumes for instrument departures unless a higher gradient is published, and it ensures the aircraft will clear obstacles along the departure path.
Plain English
For every nautical mile you travel forward, you must climb at least 400 feet. It is a slope, measured by how much altitude you gain over a given distance.
Context Anchor
Seen in helicopter-only GPS instrument approach procedures, especially where the procedure describes the climb needed after the approach is not continued to landing.
Derivation
The slash means “per.” “ft” means feet, and “NM” means nautical mile. So 400 ft/NM reads as “400 feet per nautical mile.”
Why Pilots Care
Ensures the helicopter clears obstacles during the initial climb after takeoff or a missed approach.
Grounding Statement
At 400 ft/NM, flying 3 nautical miles while climbing would require about 1,200 feet of altitude gain.
Intuition Check
Do not read 400 ft/NM as 400 feet per minute. It is based on distance flown, not time; the faster the helicopter moves over the ground, the more feet per minute it must climb to keep the same 400 ft/NM gradient.
Example Sentence 1
The departure procedure requires a minimum climb gradient of 400 ft/NM until reaching 2,000 feet.
Example Sentence 2
If the helicopter cannot sustain 400 ft/NM, the pilot must select an alternate departure route.