Definition
A loose, fibrous material — typically finely chopped cotton, wool, or synthetic fiber — added to a resin, sealant, or filler to thicken it, increase its strength, and reduce shrinkage during cure. In aircraft maintenance, flock is most often encountered as cotton flock mixed into epoxy or resin to create a structural paste used for filling, bonding, or reinforcing composite and fiberglass repairs.
Plain English
Tiny chopped fibers added to a glue or resin to make it thicker and stronger, so it holds its shape and bonds better when used to fill or join parts.
Context Anchor
Seen in maintenance discussions of engine air, fuel, or oil filters and the materials used inside them.
Derivation
From the Latin floccus, meaning 'a tuft of wool.' The same root gives us 'flocculent' (woolly or fluffy). It helps here because flock really is just short tufts of fiber — picturing a small pile of wool fluff captures exactly what gets stirred into the resin.
Why Pilots Care
Mechanics and builders working on composite or fiberglass structures need to recognize flock in repair instructions. Using the wrong filler — or skipping flock where it's specified — can produce a repair that lacks the required strength or shrinks as it cures, weakening the bond.
Analogy
Flock is like the fuzzy lint in a dryer screen: many small fibers create a surface that can catch and hold small bits of unwanted material.
Intuition Check
Do not read flock here as a group of birds. In powerplant maintenance, flock means a fuzzy fiber material used in or with a filter.
Example Sentence 1
The repair manual called for cotton flock to be mixed into the epoxy before applying it to the damaged section of the cowling.
Example Sentence 2
After sanding, flock was added to the step tread for better grip during preflight checks.