 After training , in August 1940 , he was briefly posted to No. 19 Squadron as a probationary pilot officer . Due to equipment difficulties , 19 Squadron were unable to complete Johnson 's training and he left the unit . On 6 September 1940 Johnson was posted to No. 616 Squadron at RAF Coltishall . Squadron Leader <unk> " Billy " Burton took Johnson on a 50 @-@ minute training flight in <unk> . After the flight Burton impressed upon Johnson the difficulties of deflection shooting and the technique of a killing shot from line @-@ astern or near line @-@ astern positions ; the duty of the number two whose job was not to shoot down enemy aircraft but to ensure the leader 's tail was safe . Burton also directed Johnson to some critical tactical essentials ; the importance of keeping good battle formation and the tactical use of sun , cloud and height . Five days later , Johnson flew an X @-@ Raid patrol in Spitfire <unk> , qualifying for the Battle of Britain clasp . 
