By 1943, a second stage of operational training was required to follow the initial stage of operational training that was provided at a Bomber Command Operational Training Unit (OTU). The OTU training was generally done on twin-engined Wellington aircraft but following 1942, they were being removed from front-line service.

Over a five week period, the 'Heavy Conversion Unit' (HCU) familiarized the crews with the four-engined Halifaxes or Lancasters that they would be flying on operations when they were posted to a squadron. At the HCU, the crews also acquired two additional members -a mid-upper gunner and a flight engineer, positions not required on the Wellingtons. Like the OTU's, the Heavy Conversion Units suffered from the dangerous mix of 'clapped-out' aircraft that had been replaced by newer ones on the squadrons, and inexperienced crews.

The RCAF operated four Heavy Conversion Units -1659 based at Leeming and Topcliffe, 1664 based at Dishforth, 1666 based at Dalton and Wombleton, and 1679 based at Eastmoor and Wombleton.

The links to the right lead to the Daily Diaries for the four RCAF Heavy Conversion Units.