Prototype sets became available in late 1940, with the first production examples arriving in January 1941. During a test flight in February, the aircraft was flying at when Hanbury Brown's oxygen supply failed and he passed out. The test pilot, Peter Chamberlain, realized what had happened and quickly landed the plane. Brown awoke in an ambulance. This accident, along with the many previous flights at high altitude, aggravated an ear injury he had received at RAF Martlesham Heath in 1939, and during the spring he was hospitalized for a mastoidectomy operation in Brighton. The operation was successful, but a post-operation infection caused him to go deaf in both ears and several follow-up visits were required.