Definition
A unit expressing climb or descent gradient as the number of feet of altitude gained or lost for each nautical mile of horizontal distance flown along the ground track. Used on instrument departure and arrival procedures to specify minimum required climb performance or planned descent paths.
Plain English
How many feet you need to climb (or descend) for every nautical mile you travel forward over the ground.
Context Anchor
Seen in instrument departure procedures when a required climb gradient is listed for obstacle clearance.
Derivation
A foot is the standard aviation unit for vertical height, and a nautical mile is the standard aviation unit for distance along the route. Putting them together shows that this is height change compared with distance traveled, not height change compared with time.
Why Pilots Care
Meeting a published FPNM gradient guarantees obstacle clearance during an instrument departure even when the airplane is heavy or the temperature is high.
Grounding Statement
Picture the airplane moving one nautical mile away from the airport; the FPNM value tells how many feet higher it must be by then.
Intuition Check
Do not read FPNM as feet per minute. FPNM is based on distance traveled, not on time.
Example Sentence 1
The departure procedure required a minimum climb gradient of 250 FPNM until reaching 3,000 feet.
Example Sentence 2
With a tailwind the required FPNM gradient becomes harder to meet because groundspeed increases.