a routine trip or undertaking, especially one presenting little danger or difficulty: The flight from New York to Chicago was a milk run for the experienced pilot and crew.
Origin of milk run
First recorded in 1940–45
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
aeronautics informal a routine and uneventful flight, esp on a dangerous mission
Word Origin for milk run
C20: referring to the regular and safe routine of a milkman's round
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012