Definition
An imaginary sloped surface evaluated along the final approach segment of an instrument procedure, rising one foot vertically for every twenty feet horizontally outward from the runway threshold. It is used by procedure designers to identify obstacles that penetrate the protected airspace below the normal descent path, which may then require higher minimums, a note on the chart, or a steeper-than-standard descent angle.
Plain English
It is a gently sloped invisible surface that stretches out from the runway end along the approach path. If anything sticks up through it -- a tree, tower, or terrain -- the people who design the approach have to flag it, because it could be a hazard for an aircraft flying the final approach.
Context Anchor
Seen in discussions of standard instrument approach procedures, especially when explaining how obstacles are evaluated near the final approach and runway environment.
Derivation
The name comes from the slope ratio itself: 1 foot of rise for every 20 feet of horizontal distance, written as 20:1. 'Obstacle identification surface' simply means a reference plane used to spot (identify) obstacles, not to clear them.
Why Pilots Care
These surfaces determine whether obstacles require higher approach minimums or special procedures to maintain safe clearance during landing approaches in instrument conditions.
Analogy
Picture a long invisible ramp rising away from the runway. Anything tall enough to poke through that ramp gets flagged for the procedure designer to deal with.
Grounding Statement
A 20:1 surface is shallow: over 2,000 feet of horizontal distance, it rises only 100 feet.
Intuition Check
“Surface” here does not mean pavement or the ground. It means an imaginary measuring plane used to find obstacles that could matter during the final part of an approach.
Example Sentence 1
The approach plate carried an obstacle note because a transmission tower penetrated the 20:1 final approach obstacle identification surface near the threshold.
Example Sentence 2
Reviewing the approach revealed several trees penetrating the 20:1 final approach obstacle identification surfaces.