Convoys SL 138/MKS 28 were two Allied convoys which ran during the Battle of the Atlantic in World War II. SL 138 was one of the SL convoys from the South Atlantic to Britain, and MKS 28 one of the MKS convoys between Britain and the Mediterranean.They were sailing together on the Gibraltar homeward route, having made a rendezvous off Gibraltar in order to cross the Bay of Biscay with the maximum possible escort.They were the subject of a major U-boat attack in October 1943, the first battle in the Kriegsmarine's renewed Autumn offensive.