Definition
An imaginary sloping evaluation surface used in instrument approach design that rises one foot vertically for every twenty feet horizontally outward from the runway or heliport landing area. It is assessed during procedure design to identify obstacles penetrating the final approach segment near the landing environment. Penetrations of the 20:1 OCS typically result in restrictions such as Visual Descent Point (VDP) removal, increased visibility minimums, or the publication of a 'Visual Segment - Obstacles' note on the approach chart.
Plain English
It's an invisible slanted surface tilted gently upward from the landing area. Procedure designers check whether anything (trees, towers, terrain) sticks up through it. If something does, it triggers a warning or higher minimums on the approach chart so pilots know the visual portion of the approach isn't entirely clear.
Context Anchor
Seen in helicopter GPS approach discussions, especially when the FAA explains how obstacle clearance and visibility minimums are set for copter approaches to an airport or heliport.
Derivation
The '20:1' is a slope ratio: 20 feet horizontal for every 1 foot vertical, a shallow upward tilt. 'OCS' stands for Obstacle Clearance Surface — a defined surface used to evaluate whether obstacles intrude into protected airspace.
Why Pilots Care
It sets the minimum climb performance needed to keep the helicopter clear of terrain and obstacles after departure or during a missed approach.
Analogy
Picture a long, shallow ramp rising away from the approach area. If a tree or tower sticks up through that ramp, it is too high for that protected surface.
Intuition Check
Do not read 20:1 as a descent angle to fly. It is a design surface used to check obstacle clearance, not a cockpit pitch setting or required flight path.
Example Sentence 1
Because a stand of trees penetrated the 20:1 OCS, the approach chart carried a 'Visual Segment - Obstacles' note and no VDP was published.
Example Sentence 2
Before flying the approach, the pilot confirms that no terrain penetrates the 20:1 OCS.