Definition
An imaginary sloped clearance surface evaluated along the missed approach path of an instrument procedure, rising one foot vertically for every twenty feet horizontally measured from the departure end of the runway or landing area. Obstacles that penetrate this surface must be identified and either marked on the chart, accounted for by a published climb gradient steeper than the standard 200 feet per nautical mile, or addressed by a visual segment note alerting pilots that obstacle clearance is not assured during the missed approach.
Plain English
A slanted line in the sky that starts at the end of the runway and climbs gently outward. The FAA checks whether anything sticks up through it. If something does, they either tell you about it on the chart or require a steeper climb on the missed approach.
Context Anchor
Seen in helicopter-only instrument approach design and in missed approach discussions for copter procedures to airports or heliports.
Derivation
The 20:1 ratio is simply the slope of the surface: one unit up for every twenty units out. Expressed as an angle, that is roughly a 2.86 degree climb. The number describes the geometry directly, so there is no deeper word origin to unpack.
Why Pilots Care
Provides the protected climb path that guarantees obstacle clearance if a missed approach is initiated, directly affecting go-around safety margins.
Analogy
Think of it like an invisible ramp climbing away from the missed approach point. A 20:1 ramp is not flat, but it is also not a wall; it rises 1 foot for every 20 feet of forward travel.
Grounding Statement
Picture a long, gentle ramp starting at the end of the runway and stretching out into the missed approach path. The 20:1 surface is that ramp, and procedure designers check whether anything pokes up through it.
Intuition Check
Do not read 20:1 as 20 degrees. It means a ratio: 20 forward for 1 up. Also, “surface” here means an imaginary clearance plane, not pavement or terrain.
Example Sentence 1
The chart had a note warning of a tower that penetrated the 20:1 missed approach surface, so we briefed a steeper climb before starting the approach.
Example Sentence 2
The approach chart confirmed all obstacles lay below the 20:1 missed approach surface during the climbout.