By 1960, the Royal Afghan Air Force consisted of approximately 100 combat aircraft including MiG-15 fighters, Il-28 light bombers, transports, and a few helicopters. Also by that time, a small number of Afghan pilots were undergoing undergraduate pilot training in the United States, while others attended training in the Soviet Union, India, and several European countries. In the "bloodless" 1973 Saur Revolution, King Zahir Shah was deposed and Mohammed Daoud Khan became the country's president. During his five years in power, until the Communist coup of 1978, Daoud gained Soviet assistance to upgrade the capabilities and increase the size of the Afghan Air Force, introducing newer models of Soviet MiG-21 fighters and An-24 and An-26 transports. In 1979 the Air Force lost four Mi-8s. Improvements in the early-to-mid-1970s notwithstanding, the Afghan Air Force remained relatively small until after the 1979–80 Soviet invasion of Afghanistan. While the Afghan Air Force was equipped with a large inventory – probably some 400 aircraft in the mid-1980s – many of them were manned and maintained by advisors from Czechoslovakia and Cuba. In many cases, the Soviets were reluctant to entrust Afghan pilots with either the latest aircraft models or high priority missions and, indeed, a number of Afghan pilots were equally reluctant to conduct air strikes against their countrymen. The Afghan Air Force was at its strongest in the 1980s and early 1990s, producing some concern on the part of neighboring countries. The Air Force had at least 7,000 personnel plus 5,000 foreign advisors. At its peak, the Air Force had at least 240 fixed-wing combat aircraft (fighters, fighter-bombers, light bombers), 150 helicopters, and perhaps 40 or more Antonov transports of various models. Midway through the Soviet-Afghan war, one estimate of Afghan air power listed the following inventory: