Definition
A method of joining two pieces of thermoplastic material end-to-end by heating both mating surfaces until they soften, then pressing them together so they fuse into a single continuous piece as they cool.
Plain English
Two ends of a plastic part are heated until soft, then pushed together so they melt into one solid piece.
Context Anchor
Seen in aircraft maintenance when discussing repair or fabrication of plastic tubing, piping, or other plastic parts that must be joined end-to-end.
Derivation
Butt' here means end-to-end (the same sense as a butt joint in carpentry, where two pieces meet squarely at their ends). 'Fusion' comes from the Latin 'fundere' meaning 'to melt or pour.' Together: melting two ends together so they become one.
Why Pilots Care
Creates strong, leak-proof connections in fuel and fluid lines that must withstand vibration and pressure changes in flight.
Intuition Check
Butt does not refer to the back side of an object here. It means the end of one piece placed directly against the end of another piece.
Example Sentence 1
The technician used butt fusion to join two sections of thermoplastic tubing into one continuous run.
Example Sentence 2
After completing the butt fusion, the mechanic checked the joint for a uniform bead around the entire circumference.