Definition 1 of 2
Definition
A thin, sprayed application of liquid asphalt or emulsion applied to an existing pavement surface to create a bond between the old surface and a new layer of asphalt placed on top of it.
Plain English
A sticky liquid sprayed onto an old paved surface so the new layer of pavement laid over it will stick properly and not separate later.
Context Anchor
Seen in aircraft maintenance and repair instructions for painting, fabric covering, sealing, or attaching repair materials.
Derivation
From the everyday sense of 'tack' meaning slightly sticky, as in adhesive that has not fully set. The 'coat' is simply the layer applied. Together it describes a sticky layer whose only job is to make the next layer adhere.
Why Pilots Care
Fresh tack coat on a taxiway or ramp can track onto tires, brakes, and wheel pants. Pilots operating shortly after pavement work should be alert to NOTAMs about resurfaced surfaces and avoid taxiing across freshly coated areas when possible.
Analogy
It is like putting down a light sticky base before placing tape, so the tape has something it can grip evenly.
Intuition Check
“Tack” does not mean a small nail here, and it does not mean changing direction like a boat. In this term, it means slight stickiness used to help one layer hold another.
Example Sentence 1
The airport closed Taxiway B for two hours after the crew sprayed the tack coat, allowing it to set before the new asphalt was laid.
Example Sentence 2
After the tack coat flashed off to the proper feel, the technician applied the topcoat without risk of lifting.