What is preflight in printing?

The name is borrowed from aviation, and the logic is the same: a checklist run every time, in the same order, before the point at which problems become costly. A file that fails preflight costs an email. The same file discovered on press costs plates, make-ready and a slot in the schedule.

The checks are mostly unglamorous. Are all fonts embedded or outlined, or is the press about to substitute something? Are images 300 dpi at the size they will actually print, or 72 dpi scaled up 400 per cent? Is the document CMYK, or is there RGB imagery that will shift when it converts? Is there 3 mm of bleed on every edge? Are there hairline rules thin enough to disappear? Are there five spot colours where the job was quoted for one?

Automated preflight catches most of that in seconds. What it cannot catch is intent — whether the black text was meant to be rich black, whether an overprint setting was deliberate, whether the dieline layer is meant to print. Those need a human, which is why preflight is a step in a process rather than a piece of software.

The practical advice for anyone supplying artwork: send the file even if you are not sure it is right. Preflight will tell you what is missing, and finding out at that point costs nothing.