Definition
A swirling, circular movement of air or fluid that runs counter to or breaks away from the main flow. In aviation, eddies are small rotating pockets of disturbed air that form when smooth airflow is interrupted by an obstacle, a sharp change in terrain, or the wake of another aircraft.
Plain English
A small whirlpool of air -- a pocket where the air spins or tumbles instead of flowing smoothly along with the rest.
Context Anchor
Pilots encounter this term in discussions of wind near mountains, hangars, tree lines, buildings, and other places where airflow can become rough or uneven.
Derivation
From the Old Norse 'ida,' meaning a whirlpool or backflow. The original sense -- water swirling against the main current -- carries directly into aviation, where the same idea applies to air rather than water.
Why Pilots Care
Eddies can suddenly change an aircraft's attitude, airspeed, or altitude, requiring prompt corrections especially near terrain or behind other aircraft.
Analogy
Think of a stream flowing past a rock. Just behind the rock you see little swirls of water spinning the wrong way. Air does the same thing behind hills, buildings, and aircraft.
Grounding Statement
Smooth airflow breaks into small rotating pockets when it meets resistance or speed changes.
Intuition Check
Do not think of an eddy as only something in water or something you can see. In aviation, an eddy is a small rotating pocket of air, and it may be invisible.
Example Sentence 1
On a gusty day, the pilot expected eddies on the lee side of the hangars and stayed alert for turbulence on short final.
Example Sentence 2
Wake turbulence includes eddies that can affect a following airplane on final approach.