Definition
A visible line of fine ridges or fins on a molded or forged part where excess material was squeezed out between the two halves of the mold or die at the parting surface. The flash line marks the seam where the mold sections met, and the flash itself is normally trimmed off during finishing, leaving a faint witness line on the finished part.
Plain English
A thin seam-like line on a part showing where the two halves of the mold came together. A small amount of material always squeezes into that joint during manufacture, and even after it's trimmed away, a faint line is usually left behind.
Context Anchor
Seen during aircraft maintenance or preflight inspection, especially when looking closely at tires, seals, and other molded parts.
Derivation
Flash' in manufacturing refers to the thin excess material that escapes (or 'flashes') out of a mold under pressure. The 'line' is the trace this excess leaves along the parting plane of the mold.
Why Pilots Care
Marks a zone of severe turbulence, icing, and wind shear that must be avoided for safe flight.
Analogy
Like the faint seam you can feel running around a plastic toy or a chocolate Easter egg where the two halves of the mold met.
Intuition Check
Flash line does not mean a line of light or an electrical flash. Here, “flash” means extra material left at a mold seam.
Example Sentence 1
During the inspection, the technician noted the mark on the cowling fairing was just a flash line from the mold, not a crack.
Example Sentence 2
Preflight weather showed a flash line approaching from the southwest, so departure was delayed until it passed.