Compounding the problem, both sides were using jet engine-powered fighter aircraft on a large scale for the first time, and the high speeds of combat made visual identification of damaged and destroyed aircraft difficult. USAF pilots were credited with a kill if the gun camera showed their guns striking the enemy aircraft even if no one actually saw it go down. After the war the USAF reviewed its figures in an investigation code-named "Sabre Measure Charlie" and downgraded the kill ratio of the North American F-86 Sabre against the Mikoyan-Gurevich MiG-15 by half. The Soviets also had lax standards for confirming kills at the beginning of the war, leading to widespread over-claiming. Both sides made extensive use of gun cameras to better track effectiveness, but Soviet cameras were less effective, further contributing to over-claiming. Records from the United Nations show that 40 American pilots reached ace status. However, records from China, the Soviet Union, and North Korea conflict widely and accounts vary on how many aircraft on each side were lost and who is credited with the victories. Air victory claims, which are often controversial between two sides of a war, were particularly difficult to measure in Korea thanks to the difficulty recovering crashed aircraft and confirming losses, as well as poor records for the two sides which fought the bulk of their engagements in an area known as MiG Alley. The number of aircraft lost during the war is in dispute among both the UN and the Soviet bloc nations. UN pilots claim 840 aircraft shot down during the war, while Chinese, Soviet, and North Korean sources indicate only 600 were lost among the three nations, including non-combat losses. Conversely, the Soviet Union sources claim to have shot down 800 UN aircraft, while the US claims to have lost only 100 aircraft in combat. Overestimation of victories on both sides has been attributed to the stress and confusion of air combat situations during the war, as well as the tendency for pilots to deliberately exaggerate claims for career advancement. Historians suggest that numbers in these nations were deliberately exaggerated for propaganda purposes and to appease their superiors. For example, Soviet pilots faced penalties for perceived failure or ineffectiveness, making inaccurate or false claims of victories more common. Conversely, data-matching with Soviet records shows that US pilots claimed up to 400 per cent more kills in some combats than they actually achieved, and that they routinely attributed their own combat losses to landing accidents and "other causes". The Soviet bloc nations claim to have destroyed a combined total of between 1,000 and 1,600 UN aircraft in air-to-air combat, the most common number in sources being 1,106 UN aircraft total, including 651 F-86 Sabres.