Definition
Any fixed or mobile object — natural or man-made — that rises into navigable airspace and could pose a hazard to aircraft in flight or on the ground. In the U.S. system, an object is formally classified as an obstruction to air navigation when it exceeds the height criteria specified in 14 CFR Part 77, which include any object more than 499 feet above ground level, and lower thresholds near airports, runways, and established flight paths.
Plain English
Anything tall enough to get in the way of an aircraft — like a tower, building, tree, crane, or terrain feature — that pilots and chart-makers need to know about so it can be avoided.
Context Anchor
Seen in airport information, approach and departure planning, chart notes, and notices about tall objects near airports.
Derivation
From the Latin obstruere, meaning 'to build against' or 'block up.' The aviation use carries the same idea: something standing in the way of safe passage.
Why Pilots Care
Identifying obstructions is required to maintain required obstacle clearance altitudes and avoid mid-air collision with fixed objects during takeoff, approach, and low-level operations.
Intuition Check
Do not read obstruction as just “something in the way” in a casual sense. In FAA use, it means something that meets specific height or location standards that make it a concern for aircraft clearance.
Example Sentence 1
Before departing the unfamiliar field, she reviewed the sectional chart for any obstruction along her planned climb path.
Example Sentence 2
Sectional charts mark obstructions taller than 200 feet above ground level with distinct symbols.