Definition
In composite manufacturing and repair, the process of placing layers (plies) of reinforcing fabric — such as fiberglass, carbon fiber, or Kevlar — into a mold or onto a repair area, with each ply oriented and stacked according to a specified schedule before being saturated with resin and cured.
Plain English
Stacking sheets of cloth-like material in a planned order, then soaking them with resin so they harden into a strong part or patch.
Context Anchor
Seen in aircraft composite structure manufacturing and repair, especially when working with fiberglass, carbon fiber, or similar layered materials.
Derivation
From the everyday phrase 'to lay up,' meaning to set something in place in layers. The aviation usage keeps that plain meaning — you are literally laying material up, ply by ply, onto the work surface.
Why Pilots Care
The strength of a composite part depends almost entirely on how the lay-up was done — ply count, fiber direction, and resin saturation. A poor lay-up can fail under flight loads even though the part looks fine from outside.
Analogy
It is like making a layered patch: each layer has to be placed in the right spot, in the right order, and bonded properly, or the finished patch will not be as strong as intended.
Intuition Check
Lay-up does not mean parking or storing an aircraft here. In this maintenance context, it means building or repairing a composite part by placing material layers and resin in position.
Example Sentence 1
The technician completed the four-ply lay-up on the damaged wingtip before vacuum bagging it for cure.
Example Sentence 2
Incorrect fiber angles during lay-up can reduce the strength of the finished composite skin.