Definition
The lengthwise threads in a woven fabric, running parallel to the selvage edge and forming the strongest direction of the cloth. In aircraft fabric covering, the warp threads run the length of the bolt and are oriented along the longest dimension of the surface being covered.
Plain English
The threads that run the long way down a piece of cloth. They are the strongest threads in the fabric, and when covering an aircraft surface, they are laid along the longest part of that surface.
Context Anchor
Seen in aircraft fabric covering, fabric repair, and discussions of how woven covering material is laid out on an airframe.
Derivation
From Old English 'wearp,' meaning that which is thrown or laid down. In weaving, the warp is set up first on the loom -- thrown lengthwise across it -- before the cross threads (the weft or fill) are woven through. Knowing this helps you remember that warp = lengthwise, the foundation threads.
Why Pilots Care
Correct alignment of warp threads determines whether the finished covering meets required strength under flight loads and air pressure.
Analogy
Think of a woven placemat: one set of threads runs lengthwise, and the other set crosses side to side. The warp threads are the lengthwise set.
Intuition Check
Do not read “warp” here as “bend” or “twist.” In fabric, warp means the lengthwise set of threads.
Example Sentence 1
When recovering the wing, the mechanic laid the fabric so the warp threads ran spanwise from root to tip.
Example Sentence 2
Inspectors checked that the warp threads were straight before applying the first coat of dope.