Definition
A wing spar built as a hollow rectangular structure, typically with two vertical webs joined by upper and lower caps to form a closed box cross-section. The closed shape gives the spar high strength and stiffness in both bending and torsion while keeping weight low.
Plain English
A wing's main supporting beam built in the shape of a long hollow box. The box shape resists both up-and-down loads and twisting forces better than a single solid beam of the same weight.
Context Anchor
Seen in aircraft structure, maintenance, and construction discussions, especially when describing how a wing is built.
Derivation
Spar comes from Middle English, originally meaning a wooden beam or pole. Box describes the cross-sectional shape. So a box spar is literally a load-carrying beam built with a box-shaped cross-section, as opposed to an I-shape or solid bar.
Why Pilots Care
Its closed shape gives superior resistance to twisting forces that other spar shapes handle less effectively, directly affecting wing integrity and safety.
Intuition Check
Do not read “box” as a container used for storage. In “box spar,” it means the spar is shaped like a closed box, which makes it strong and stiff.
Example Sentence 1
The wing was built around a single box spar running from root to tip, with the fuel tank formed inside the spar itself.
Example Sentence 2
The builder chose a box spar design because it simplified rib attachment while providing good torsional rigidity.