Definition
In a woven fabric or fabric-reinforced composite, the direction of the threads that run across the width of the cloth, perpendicular to the warp threads which run along the length of the roll.
Plain English
The side-to-side threads in a piece of woven fabric. If the fabric is unrolled like a paper towel, the weft threads run across the short way, while the long threads run down the length.
Context Anchor
Seen in aircraft fabric covering, fabric repair instructions, and material layout notes before cutting or applying a patch.
Derivation
Weft comes from the Old English wefan, meaning 'to weave.' It refers specifically to the threads that are woven across the stationary lengthwise threads (the warp) on a loom. Knowing this helps because weft and warp are easy to mix up, and the origin makes clear which one is which: the weft is what is actively woven across.
Why Pilots Care
When repairing fabric-covered aircraft or working with composite layups, the orientation of the weft and warp affects the strength and stretch behavior of the material. Cutting or applying fabric in the wrong direction can weaken the structure.
Intuition Check
Do not read “direction” here as the direction the fabric happens to be placed on the airplane. Weft direction means the actual crosswise thread direction built into the woven cloth.
Example Sentence 1
When applying the covering, the technician aligned the weft direction across the wing rib spacing as specified in the repair manual.
Example Sentence 2
Verify the weft direction before cutting panels to maintain proper strength alignment.