Thiele was posted to an operational training unit before being transferred to the Canadian No. 405 Squadron at RAF Pocklington a few miles east of York, equipped with Wellington IIs. Thiele's first mission was almost his last. Returning from Cherbourg in France in January 1942, Thiele got the green light to land, but as he touched down was confronted by another aircraft. The two planes collided engine to engine, ripping the wings off. The court of inquiry absolved him. On 30–31 May 1941, flying a Halifax, Thiele took part in the thousand-bomber raid on Cologne. By late October 1941, Thiele had completed 32 operations flying both Halifaxes and Wellingtons. There followed a number of missions over Essen amidst very heavy flak and German searchlight exposure. Thiele was promoted straight from pilot officer to flight lieutenant, skipping the intermediate rank of flying officer, and then to squadron leader in the space of a few months. Later, 22-year-old Thiele would revert to the rank of flight lieutenant in order to return immediately to operations rather than becoming a flight instructor.