Definition
An obstacle clearance surface used in instrument approach design that rises one foot vertically for every 34 feet horizontally, extending from the runway threshold outward along the final approach path. If this surface is clear of obstacles, the approach can be published with standard (lower) visibility minimums; if penetrated by obstacles, visibility minimums must be raised or the approach must include a higher descent angle to clear them.
Plain English
An invisible sloped surface starting at the runway and angling upward into the approach path. The FAA checks whether anything sticks up through it. If nothing does, the approach gets normal visibility minimums. If something does (a tower, a hill, trees), the minimums get raised to keep pilots safely above it.
Context Anchor
Seen in instrument approach procedure discussions and chart notes about obstacles in the visual segment near the runway.
Derivation
The name describes the slope itself: 1 foot of rise for every 34 feet of horizontal distance. That slope corresponds to roughly a 1.7-degree angle, which sits just below a normal 3-degree glidepath, giving a built-in margin between the descent path and any obstacles below.
Why Pilots Care
It confirms the final approach path is free of terrain or obstacles, allowing a safe descent to the runway without last-minute surprises.
Analogy
Picture laying a long, shallow ramp out from the runway. If a tree, tower, or terrain sticks up through that ramp, it is a concern for the visual part of the approach.
Grounding Statement
Picture a long, gentle ramp sloping up from the runway threshold out into the final approach area. The ramp rises 1 foot for every 34 feet you travel away from the runway. Anything that pokes through that ramp is an obstacle the approach designer has to account for.
Intuition Check
Do not read “surface” as pavement or ground here. In this term, “surface” means an imaginary sloping plane used for obstacle evaluation.
Example Sentence 1
Because a ridge penetrated the 34:1 surface, the approach was published with higher visibility minimums than standard.
Example Sentence 2
During the briefing, the crew verified clearance above the 34:1 surface for the transition segment.