Definition
An imaginary sloped surface used in instrument approach design that rises one foot vertically for every twenty feet horizontally, extending outward from the landing area along the final approach course. The surface is evaluated for obstacles; any object penetrating it must be accounted for in the approach procedure, often by raising minimums, adding a note, or restricting the procedure at night.
Plain English
A gentle invisible ramp drawn out from the runway or helipad that rises one foot for every twenty feet of distance. Approach designers check whether anything (trees, towers, terrain) sticks up through this ramp, because anything that does could be a hazard during the final part of the approach.
Context Anchor
Seen in helicopter GPS approach discussions, especially when checking the visual path from the instrument approach portion to the airport or heliport landing area.
Derivation
The ratio 20:1 simply expresses the slope: 20 units horizontal for every 1 unit vertical. That works out to roughly a 2.86° climb gradient — a shallow slope, chosen because it represents a realistic obstacle-clearance path for an aircraft transitioning from instrument flight to a visual landing.
Why Pilots Care
The surface must be verified clear of obstacles before the approach can be flown without added restrictions or visual requirements.
Analogy
Think of laying a long, shallow board from the landing area outward. If a tree or pole pokes through where that board would be, it is too high for that protected slope.
Grounding Statement
A 20:1 inclined plane is not something you see from the cockpit; it is a design surface used to judge whether the space near the landing path is clear enough.
Intuition Check
Do not read 20:1 as a steep climb or descent angle. It means a shallow slope: 20 units across for every 1 unit up.
Example Sentence 1
Because a stand of trees penetrated the 20:1 inclined plane on the final segment, the approach was published with a note prohibiting its use at night.
Example Sentence 2
During procedure design, the 20:1 inclined plane is checked to ensure safe visual descent to the heliport.