Definition 1 of 2
Definition
An imaginary inclined surface used in instrument approach procedure design that begins at the runway threshold and slopes upward along the approach path. Any obstacle that penetrates this surface must be accounted for in the procedure, typically by raising minimum descent altitudes, adjusting the missed approach point, or publishing a steeper approach angle to ensure aircraft remain safely above terrain and obstructions.
Plain English
An invisible sloped ceiling near the runway that nothing on the ground is allowed to stick up through. If something does stick through it, the approach has to be designed to keep the airplane higher so it stays clear.
Context Anchor
Seen in runway, airport, approach, departure, and instrument procedure discussions when obstacles near a flight path are being evaluated.
Derivation
Obstacle comes from Latin words meaning “to stand in the way.” Clearance means having enough free space. Surface means an outer face or boundary. Together, the term means a boundary used to check whether anything standing in the way leaves enough safe space for flight.
Why Pilots Care
It sets the protected airspace that keeps flight paths safe from obstacles during the most critical phases of flight.
Analogy
Think of it like an invisible sheet stretched out near a runway. Objects below the sheet are acceptable for that check; objects poking through the sheet need attention.
Grounding Statement
Picture a sloping invisible plane extending away from a runway, with trees, towers, and terrain checked against it.
Intuition Check
Do not read “surface” as a real physical surface an aircraft lands on. Here it means an imaginary measuring boundary used to judge obstacle clearance.
Example Sentence 1
The approach minimums for that runway are higher than usual because a ridge on final penetrates the obstacle clearance surface.
Example Sentence 2
New tower construction was rejected because it would penetrate the Obstacle Clearance Surface for the runway.