Definition
A small, temporary weld used to hold two metal parts in correct alignment before the final, full-strength weld is made along the joint.
Plain English
A quick spot of weld that pins parts together so they stay lined up while you finish the real welding.
Context Anchor
Seen in aircraft maintenance and repair work when metal parts must be fitted and held in place before final welding.
Derivation
From the old carpentry and sewing sense of 'tack' — a small fastener or quick stitch used to hold something temporarily in place. The welding term carries the same idea: a light, short-term hold before the permanent join is made.
Why Pilots Care
Tack welds keep parts square and aligned during fabrication or repair. Poorly placed tacks can lock in distortion, leaving a finished weldment that doesn't fit or carry loads correctly — a real concern on engine mounts and structural tubing.
Analogy
Similar to using a few staples to hold fabric in place before sewing the final seam.
Intuition Check
A tack weld is not the finished weld by itself. It is mainly a positioning weld used to hold the parts correctly before the final weld is made.
Example Sentence 1
Before welding the engine mount cluster, the technician applied tack welds at each tube intersection to hold the assembly square in the jig.
Example Sentence 2
Once the fit was verified, the tack welds were ground off before performing the continuous seam weld.