The effect of Allied numerical superiority in the air over Normandy in 1944
Without doubt, air superiority was a key to the Allied victory at Normandy in June - August 1944. This air superiority was based on both a qualitative superiority and a numerical superiority.
The qualitative superiority manifested itself both regarding the technical field and pilot training. The Allied fighters generally were superior to the German Bf 109 G and Fw 190 A in service in 1944. Moreover, at this stage, the quality of the Luftwaffe pilot standard was being worn down to a mere shadow of what it had once been, and this was the result of a terrible attrition in a long fight against numerically superior US formations over Germany.
In March 1944, the German Luftflotte Reich had performed 3,672 combat sorties and lost 349 fighters. That equals a loss ratio of 9.4 %. (Prien, "JG 1/11", p. 821)
These 3,672 combat sorties were flown against approximately 18,000 sorties by US 8th Air Force (including 8,773 heavy bomber missions; I don't have totals for fighter escort missions, but usually by this time there were more escort fighters than heavy bombers on each mission), plus several thousand sorties over Germany and the Netherlands by the 15th AF and the RAF.
Even if the Americans lost more aircraft than the number of Luftwaffe aircraft they were able to shoot down, the sheer numbers made the US losses have a less serious impact. Because of the large numbers of aircraft deployed on each mission by the Americans, the loss ratio in 8th AF heavy bombers was 3.3 % in March 1944, and in the US fighter units it was even lower. Such a loss ratio can be sustained by any air force without having any negative effect on the quality of the crews.
Luftwaffe fighter pilot losses in March 1944 alone reached nearly 22 % of pilots present on 29 February 1944. In February 1944, nearly 18 % of the pilots present on 31 January 1944 had been lost. The losses surpassed the replacements, so rookies had to leave their pilot training schools before their training was completed and were sent into action - against numerically superior Allied air forces.
Thus, pilot training quality inevitably suffered. It started with the calling of many instructors to first-line service. This first step lowered the quality of the trainers themselves. Next, the pilot training schemes were shortened. Already in early 1944, the Luftwaffe fighter pilot training was shortened to an average of 160 flight hours. A few weeks later, it was further shortened to only 112 hours. Finally, in the spring of 1944, the B flight schools were disbanded, and the pilots were sent into first-line service directly after A schools. The condition for the A2 flight certificate included a basic training of sixty training flights with a total of 15 flight hours. Meanwhile, the average USAAF or RAF fighter pilot's training consisted of 225 flight hours.
In his 380-page study on the air war over Normandy 1944, David Clark writes: "We noted how many German super-aces appeared in the air battle. The skill level of the pool of German pilots was not homonogenous but rather, presented a dramatic contrast. The killing of so many good pilots in the first six months of 1944 left most Gruppen with a smattering of super-aces, a small number of experienced but not yet expert pilots, and the vast majority with but a few hours flying experience. These latter had been desperately pressed into service without sufficient training." ("Angels Eight: Normandy Air War", p. 59)
However, a study of the air war shows that the factor which was most decisive to the Allied air superiority over Normandy in the summer of 1944, was their huge numerical superiority.
On 6 June 1944, the Allies had a total of 13,000 aircraft ready to support the Normandy invasion. The Luftwaffe had 1,300 aircraft at its peak (reached on 10 June) in France.
Challenging the numerical superiority
The effect of this is described by one of the German veterans who flew over Normandy, Major Hans-Ekkehard Bob, commanding JG 3:
"I often found myself alone pursued by eight or ten Mustangs, and was able to survive only by mobilising all my flight skills, twisting and turning around small woods and church towers in low-level flight. I was aided by the lacking skills on behalf of the American pilots, since each one of them wanted to shoot me down, and thus they blocked each other."
III./JG 3's War Diary, 13 June 1944:
"Die zahlenmässige Überlegenheit des Gegners, abgesehen von der technischen, ist derart gross, dass Starts in Schwarm- oder Staffelstärke zu untragbaren Verlusten führen. . . . Eigene Verbände werden in kürzester Zeit in Luftkämpfe mit überlegenen Feindkräfte, die laufend Verstärkung erhalten." (Prien, "III./JG 3", German edition, p. 364.)
As a consequence, the Luftwaffe was instructed to operate in "Gefechtsverbände" - intended to consist 40 or more fighters, but in reality, due to circumstances often composed of rather only half that number. These became subject to repeated Allied fighter attacks from several units.