He led the ROC through the final two years of the war and the difficult period of the V-1 flying bomb raids on southern England.
On 15 June 1937 Crerar was granted an Auxiliary Air Force commission in the immediate rank of squadron leader and was appointed as the first officer commanding of No.
Crerar immediately generated controversy within the ROC when his first decision was to make retirement at age fifty mandatory for all centre personnel, including the duty officers.
During Crerar's command the ROC played a major part in combating the German hit-and-run raids on the south and south-east coasts of Great Britain.
He ordered a large number of additional satellite spotter posts to be quickly constructed along the coast giving a more complete low-level coverage.
Some doubt had been expressed as to the ability of the Corps to deal with this threat, Air Commodore Crerar assured the committee that the ROC could again rise to the occasion and prove its alertness and flexibility.