Definition
The weight of a substance per unit volume, typically expressed in pounds per cubic foot or pounds per gallon. It indicates how heavy a given volume of a fluid or material is, and is used in aviation to compare fuels, oils, and other fluids by their density-related weight characteristics.
Plain English
How much a set amount of a substance weighs. For example, how many pounds one gallon of fuel weighs.
Context Anchor
Seen in aircraft maintenance and powerplant discussions involving fluids, gases, pressure, and material properties.
Derivation
From Latin specificus, meaning 'of a particular kind,' combined with weight. 'Specific' here means 'per specific amount' — in this case, per unit of volume — rather than 'exact' or 'particular' in the everyday sense.
Why Pilots Care
Used to convert fuel or oil volume into accurate weight for performance, range, and weight-and-balance calculations.
Analogy
Imagine two same-size containers: one filled with oil and one filled with water. Specific weight is the number that tells how much each full container weighs for that same amount of space.
Intuition Check
Specific weight does not mean the exact weight of one particular part or object. It means weight compared to volume: how much a material weighs for a set amount of space.
Example Sentence 1
Using the specific weight of avgas at 6 pounds per gallon, the pilot calculated that 40 gallons of fuel added 240 pounds to the aircraft's loaded weight.
Example Sentence 2
Oil with a higher specific weight requires adjustment of the engine oil quantity limits listed in the maintenance manual.