Although commissioned in mid-December 1948, "Sydney" did not enter service until 5 February 1949, as work was still being completed. On 25 December 1948, while still fitting out in Devonport, a helicopter from carrying Santa Claus became the first aircraft to land on "Sydney"s flight deck. The ship's sea trials and post-commissioning workup were not without event: in early December 1948, a 771 Naval Air Squadron de Havilland Mosquito crashed while helping "Sydney" calibrate her radar suite, killing both British aircrew, and a RAN pilot undergoing landing qualifications aboard on 17 March 1949 crashed while attempting to land: none were killed, but the pilot's Fairey Firefly and four others in the deck park were destroyed.