a year later, he gained fame in 1940 during the battle of britain as a part of, and later squadron leader of, no. 605 squadron raf, equipped with the hawker hurricane fighter. the auxiliary unit was moved to southern england and participated in the large air battles. mckellar's combat career proved to be very brief, lasting just over a year. he claimed all but two of his victories within the last two and a half months of his life; 15 august – 1 november 1940. on 7 october 1940 he shot down five messerschmitt bf 109s, thus becoming an ace in a day; one of only 24 allied aces to achieve the feat. at the time of his last mission he had claimed 21 aerial victories and another two shared destroyed against enemy aircraft. included in this total of 21 air victories are 11 bf 109s. mckellar, along with ronald hamlyn and brian carbury, were the only british pilots to achieve the feat of "ace in a day" during the battle of britain.