Hesselyn joined the New Zealand Territorial Force in 1939, transferring to the Royal New Zealand Air Force the following year. After training, he was posted to Britain in September 1941, joining first No. 61 Operational Training Unit, then No. 234 Squadron RAF as a flight sergeant. In February 1942, Hesselyn was posted to Malta, flying a SpitfireMk V from the deck of the aircraft carrier, to the island and joining No. 249 Squadron RAF on 9 March 1942. On April Fools' Day 1942, Hesselyn recorded his first two kills, a Bf 109 of JG 53 and a Ju 87 of StG 3. Later that month, he shot down another Bf 109 and Ju 87, and was awarded the Distinguished Flying Medal. The citation for the award, promulgated in a supplement to the London Gazette on 22 May 1942, read: May proved to be just as productive, with Hesselyn claiming four 109s and a Ju 88 of KGruppe 806 over a four-day period. For this he received a Bar to his DFM, and was commissioned as a pilot officer. Before returning to England, he downed one more 109, for a total of 12 claimed victories. During a spell at No. 61 OTU, Hesselyn and fellow No. 249 Squadron pilot, Flight LieutenantVirgil "Paul" Brennan, collaborated with journalist Henry Bateson on writing Spitfires over Malta, an award-winning book based on Brennan and Hesselyn's experiences on Malta. In early 1943 Hesselyn joined No. 501 Squadron RAF, before going on to No. 222 Squadron RAF as a flight commander later in the year. He shot down two 109s of JG 3 on 17 August 1943, and received the Distinguished Flying Cross in October for his "great skill, courage and keenness" in downing 17 aircraft. His luck finally ran out on 3 October 1943. After downing at least one 109, bringing his total to 18 and 1 shared destroyed, he was forced to bail out of his burning plane and was taken prisoner. He was probably shot down by Fbw. Weigand of I./JG 26. Hesselyn escaped once, but was recaptured and spent the rest of the war making repeated attempts to rejoin the fight. As a result of his conduct as a prisoner of war, he was made a Member of the Order of the British Empire in December 1945.
RAF career
Hesselyn transferred to the Royal Air Force after the Second World War, being appointed to a four-year commission as a pilot officer on 16 March 1947 while retaining his war substantive rank of flight lieutenant. The latter rank was made substantive the very next day, and he was granted a permanent commission in September 1948. Hesselyn's final promotion, to squadron leader, came on 1 January 1951. Hesselyn died at RAF Hospital Uxbridge on 14 November 1963, aged 42.
