A milk run is a highly routine flight operation that crews consider easy, repetitive, and low-stress. Traditionally, it referred to a multi-leg passenger or cargo route that stopped at several small airports in sequence, much like a delivery truck. Military and commercial crews use the term to describe missions with clear weather, familiar airspace, and no operational complications.
Heard on the job
Tonight's cargo trip to Memphis is just a milk run, so the new first officer will fly the leg.