In August 1939 Cunningham rejoined his squadron which had been equipped with a version of the Bristol Blenheim. He was not enthusiastic about the aircraft. The turret had been eliminated to reduce weight and increase speed, but the flying suits and cockpit were not heated, which caused discomfort to the crews over the winter period. The radios were too short in range which caused communication problems. The Blenheim's cockpit windshield was made up of several panes and was difficult to clean. It was also liable to reflect light, which made cooperation with searchlights hazardous. Lastly, problems were encountered with blind-flying instruments. The squadron incurred some losses owing to the inexperience of crews flying patrols over the North Sea from RAF North Weald in Essex, a number of them a result of the difficulties with the aircraft. In January 1940 604 moved to RAF Northolt and flew Blenheims adorned in the Finnish Air Force "Swastika" to Finland via Sweden to assist the country after the Soviet invasion.