Definition
The upward force exerted by a fluid on an object placed in it, equal to the weight of the fluid the object displaces. If that upward force is greater than the object's weight, the object rises or floats; if less, the object sinks. In aviation, buoyancy is the principle that keeps lighter-than-air aircraft (balloons and airships) aloft, and it also acts on aircraft floats and on parcels of air in the atmosphere.
Plain English
Buoyancy is the push upward that a fluid gives to anything sitting in it. The more fluid the object pushes out of the way, the bigger the push. If the push is bigger than the object's weight, it rises.
Context Anchor
Seen in basic physics, aircraft floats, fuel and fluid systems, and lighter-than-air aircraft discussions.
Derivation
From the French 'bouée' (buoy) and ultimately from a Germanic root meaning 'to float.' The word literally describes the tendency to float, which is exactly what the force produces.
Why Pilots Care
Determines whether floats or hulls will support an aircraft on water and affects safe loading limits for seaplane operations.
Analogy
Hold a beach ball underwater and you feel it pushing back up against your hand. That push is buoyancy — the water is trying to take back the space the ball is occupying.
Grounding Statement
If an object is in water or air and that surrounding water or air pushes upward on it, buoyancy is involved.
Intuition Check
Buoyancy does not only mean floating on water. In aviation and maintenance, it can involve any upward support from a liquid or a gas, including air.
Example Sentence 1
The hot-air balloon climbs because heating the air inside the envelope makes it less dense than the surrounding air, giving the balloon positive buoyancy.
Example Sentence 2
Proper buoyancy calculations ensure the seaplane remains stable when loaded for a water landing.