Definition
A small structural fairing or rounded transition piece installed at the junction of two aircraft surfaces — typically where the wing meets the fuselage, or where the vertical stabilizer meets the fuselage — to smooth the airflow across the joint and reduce interference drag.
Plain English
A curved filler piece fitted into the corner where two parts of the aircraft meet, so the air flows smoothly across the join instead of getting disturbed.
Context Anchor
Seen in airframe structure and inspection discussions, especially around places like the wing-to-body joint, tail surfaces, and other surface intersections.
Derivation
From the French 'filet,' meaning a small thread or band. In engineering it came to mean a small rounded piece that softens a sharp corner — exactly what an aircraft fillet does where two surfaces meet at an angle.
Why Pilots Care
Fillets reduce drag at structural junctions, improving efficiency and handling. Damage to a fillet may seem cosmetic but can affect airflow and must be assessed during preflight or maintenance inspection.
Analogy
A fillet is like a smooth bead of caulk in the corner of a bathtub: it fills the sharp corner and makes the change from one surface to the other smooth.
Intuition Check
Do not read fillet here as a cut of fish or meat. In aircraft maintenance, a fillet is a smooth shaped area or piece at a joint between aircraft surfaces.
Example Sentence 1
During the walk-around, the technician noticed a cracked wing-root fillet and flagged it for repair before the next flight.
Example Sentence 2
During the inspection the technician checked the composite fillet for cracks or separation from the skin.