Sir Arthur Harris himself believed that there was a relationship between tonnage dropped, city areas destroyed, and lost production. The effect of Bomber Command's attacks on industrial production is not so clear cut. The much better provided US survey was little concerned with the RAF area bombing campaign. It pointed to the great success of the USAAF's attacks on Germany's synthetic oil plants starting in the spring of 1944 – this had a crippling effect on German transportation and prevented the Luftwaffe from flying to anything like the order of battle that the aviation engine plants, parts and sub-assembly fabrication and final assembly manufacturing facilities; Luftwaffe training and logistics could have otherwise sustained. Further, in going for targets they knew the Germans must defend, the new American escort fighters were able to inflict crippling losses on the Luftwaffe's fighter force. The RAF also made a great contribution to the oil offensive as its abilities to attack precision targets had greatly improved since the arrival of new navigation and target-finding instruments; by mid-1944 it was also mounting huge bombing raids in daylight.