Definition
Water carried in dedicated tanks aboard an aircraft for the purpose of adding controllable, temporary weight. It is used in some sailplanes to increase wing loading for better high-speed cruise performance, and in certain test or training aircraft to adjust weight and balance. The water can be jettisoned in flight when the added weight is no longer wanted.
Plain English
Water carried on board purely as added weight. It can be dumped overboard when no longer needed.
Context Anchor
Seen in glider and sailplane operations, especially during cross-country flights or contests when pilots adjust aircraft weight for the day’s conditions.
Derivation
‘Ballast’ comes from an Old Norse and Low German word meaning ‘heavy load carried for stability,’ originally referring to stones or sand placed in the bottom of a ship to keep it steady. ‘Water ballast’ simply means water used for that same purpose — added weight that can be taken on or released as needed.
Why Pilots Care
Allows higher cruise speeds in strong lift while preserving the ability to reduce weight for better climb when needed.
Intuition Check
Water ballast is not drinking water or cooling water. It is planned, removable weight carried to change how the glider performs.
Example Sentence 1
Before the contest start, the pilot filled the wing tanks with water ballast to improve cruise speed between thermals.
Example Sentence 2
As the lift weakened she opened the dump valve to release the water ballast and reduce weight.