At the outbreak of war, the appearance of RAF Old Sarum had changed little. Its line of hangars still looked out onto the grass flying field, while a Roman road still formed the northern border of the airfield. The squadron continued to be primarily engaged in training and developing ground support techniques, including the spraying of poison gas, although this was never actually used. In February the 16 Squadron left for France via Hawkinge and its place was taken by the first two Canadian flying units to arrive in Britain – 110 and 112 Royal Canadian Air Force (RCAF) Squadrons. The final operational unit to be based here during this period was 225 Squadron (RAF), another Lysander equipped unit. This took the place of 110 RCAF Squadron on 9 June 1940 and with the growing threat of a Nazi invasion of the Britain, it was engaged in patrolling the south coast for enemy landings.