Definition
A biplane in which one wing — usually the lower — is significantly smaller than the other in span, area, or both. The aircraft has two wings stacked vertically, but the smaller wing carries less of the lift and is often used mainly for structural bracing or improved downward visibility.
Plain English
An aircraft with two wings, one above the other, where one of the wings is much smaller than the other.
Context Anchor
Seen in aircraft design descriptions, aircraft identification, and discussions of older or vintage airplanes.
Derivation
From the Latin 'sesqui-' meaning 'one and a half,' combined with 'plane' (wing). The name reflects the design: not quite two full wings like a biplane, but more than one — roughly 'one-and-a-half wings.'
Why Pilots Care
For most pilots, the term matters mainly for recognizing and understanding aircraft design. On vintage aircraft, the unequal wing layout can affect inspection points, structural loads, and how the aircraft is described in manuals or records.
Intuition Check
Do not read sesquiplane as a special kind of monoplane. It still has two wings; one of them is just much smaller than the other.
Example Sentence 1
The Nieuport 17 was a famous World War I sesquiplane, with a narrow lower wing that improved the pilot's downward view.
Example Sentence 2
Ground crew checked the wing bracing on the sesquiplane before the next flight.