Definition
Imaginary sloped and level surfaces extending outward and upward from a runway, helipad, or instrument approach segment, used by procedure designers to identify obstacles that penetrate protected airspace and that must therefore be considered when establishing minimum altitudes, climb gradients, and approach minimums for an instrument procedure.
Plain English
Invisible angled surfaces drawn out from a runway or helipad on a designer's chart. Anything sticking up through them — a tower, tree, building, or terrain — has to be accounted for when setting the safe altitudes for an approach or departure.
Context Anchor
Seen in helicopter instrument procedure discussions, especially when explaining how approach paths, missed approach paths, and obstacle protection are designed.
Derivation
Straightforward: surfaces used to identify obstacles. The word 'surface' is borrowed from geometry — these are mathematical planes, not physical things you can see or touch.
Why Pilots Care
Helicopter pilots depend on these surfaces to confirm that published approach altitudes provide real clearance from terrain and structures.
Analogy
Think of an OIS like an invisible safety screen laid along the flight path. Anything sticking through that screen gets the designer’s attention.
Intuition Check
Do not picture a physical surface on the ground. An obstacle identification surface is an imaginary design boundary used to find obstacles that matter to the procedure.
Example Sentence 1
Because a ridge penetrated the obstacle identification surface for the final approach segment, the procedure designer raised the MDA by 200 feet.
Example Sentence 2
Because no obstacles broke the OIS, the minimum descent altitude could be published at 800 feet.