 As a result, 100 octane fuel was able to be issued to all front-line Fighter Command aircraft starting in the spring of 1940.Although U-boats and surface raiders had begun to take a heavy toll of tankers, in the summer of 1940 there was a surplus of these ships because of the incorporation into the British merchant marine of tanker fleets from countries overrun by Germany.The combination of CS propellers and 100 octane fuel put the British fighters on par with the Luftwaffe.