Definition
An obstacle identification surface that slopes upward from the runway threshold at a ratio of 20 feet horizontal to 1 foot vertical, used during instrument approach procedure design to identify obstacles in the visual segment between the final approach fix area and the runway. Obstacles penetrating this surface are evaluated and may result in a note on the approach chart, higher visibility minimums, or restrictions on night operations.
Plain English
An invisible sloping surface that rises gently from the end of the runway. Designers use it to spot anything tall — like trees, towers, or terrain — that sticks up into the area a pilot flies through after leaving instruments and looking outside to land.
Context Anchor
Seen in instrument approach discussions when the pilot is transitioning from instrument references to a visual approach near the runway.
Derivation
The name describes the slope ratio itself: for every 20 feet you travel forward, the surface rises 1 foot. It is not an abbreviation — the numbers are the definition.
Why Pilots Care
An obstacle penetrating the surface may require a steeper descent angle, a circling approach, or denial of visual approach clearance, directly affecting safety margins and workload.
Analogy
Picture a long, flat board tilted gently upward from the runway area. If a tree, tower, or hill would poke through that board, it is a concern for the visual descent path.
Grounding Statement
Picture a very shallow ramp rising from the runway threshold out toward the final approach course — anything poking up through that ramp is a concern.
Intuition Check
Do not read “surface” as the runway pavement or the ground. Here it means an imaginary sloping plane used to evaluate obstacles.
Example Sentence 1
The approach chart had a note about obstacles penetrating the 20:1 surface, so we briefed extra caution during the visual segment and confirmed the procedure was authorized for the conditions.
Example Sentence 2
The approach briefing included a quick check of the 20:1 surface to confirm a clear visual path to the runway.