Definition
An imaginary sloped evaluation surface used in instrument procedure design that rises 1 foot vertically for every 40 feet of horizontal distance measured along the missed approach track, beginning at the end of the required runway or approach reference point. Obstacles that penetrate this surface are identified and must be accounted for by either adjusting the missed approach procedure, raising minimums, or publishing a required climb gradient steeper than the standard 200 feet per nautical mile.
Plain English
A gently sloping invisible ceiling that procedure designers use to check for obstacles along the missed approach path. If anything sticks up through that slope, the procedure has to be changed to keep aircraft safely above it.
Context Anchor
Seen in missed approach discussions, especially when explaining how obstacles are evaluated after a pilot stops an instrument approach and follows the published missed approach instructions.
Derivation
The 40:1 ratio comes from the standard 200-feet-per-nautical-mile climb gradient assumed for a missed approach. One nautical mile is roughly 6,076 feet, so a 200-foot climb over 6,076 feet works out to about 1 foot up for every 40 feet forward — hence 40:1. Knowing this helps the term feel less arbitrary: it is simply the geometric expression of the standard missed approach climb performance.
Why Pilots Care
It determines which obstacles must be accounted for so the published missed approach procedure provides safe vertical clearance.
Analogy
Think of placing a clear, tilted sheet over the missed approach area. Anything that pokes up into that sheet gets flagged for attention.
Grounding Statement
Picture an invisible ramp starting low near the missed approach area and rising gradually as it extends outward.
Intuition Check
Do not read “surface” as the ground or runway surface here. It means an imaginary measuring plane used to identify obstacles. Do not read “40:1” as an instruction to fly that exact slope. It is a design-check surface, not a commanded flight path.
Example Sentence 1
Because a tower penetrated the 40:1 obstacle identification surface, the approach plate published a required climb gradient of 280 feet per nautical mile on the missed approach.
Example Sentence 2
Because no obstacles broke the 40:1 surface, the missed approach could use a standard climb gradient without extra restrictions.