Definition
An imaginary sloped surface used in instrument departure procedure design that rises one foot vertically for every 40 feet horizontally from the end of the runway. Obstacles that penetrate this surface must be either removed, marked, lighted, charted, or accounted for by requiring a higher-than-standard climb gradient on the departure procedure.
Plain English
It is an invisible ramp that climbs gently away from the end of the runway. Procedure designers check whether anything sticks up through that ramp. If something does, the departure has to be adjusted so airplanes climb steeply enough to stay above it.
Context Anchor
Seen in instrument departure and procedure-design discussions, especially when explaining how obstacle departure procedures are created.
Derivation
The 40:1 ratio describes the slope itself: 1 foot up for every 40 feet forward. That works out to about a 152 foot-per-nautical-mile climb, which is the standard climb gradient assumed for instrument departures. OCS stands for Obstacle Clearance Surface, meaning a surface used to evaluate obstacle clearance.
Why Pilots Care
An obstacle that penetrates this surface may force higher minimums, a non-standard climb gradient, or denial of the procedure.
Analogy
Picture a long, shallow ramp rising away from the runway. Obstacles need to stay below that ramp unless the departure procedure gives you a way to safely clear them.
Grounding Statement
From the runway outward, the protected surface climbs gradually, and obstacles that rise into it must be accounted for in the departure procedure.
Intuition Check
40:1 OCS is not the climb path the pilot is being told to fly. It is a design surface used to decide whether the normal climb is safe or whether extra departure requirements are needed.
Example Sentence 1
Because a ridge penetrated the 40:1 OCS off Runway 12, the departure procedure required a climb gradient of 280 feet per nautical mile until 4,000 feet.
Example Sentence 2
Because a tower penetrated the 40:1 OCS, the approach required a higher decision altitude.