Plain English Meaning
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.
Full 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.
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.