by the CH-47C. The CH-47s in Vietnam were generally armed with a single 7.62 mm M60 machine gun on a pintle mount on either side of the aircraft for self-defense, with stops fitted to keep the gunners from firing into the rotor blades. Dust filters were also added to improve engine reliability. Of the nearly 750 Chinook helicopters in the U.S. and South Vietnam fleets, about 200 were lost in combat or wartime operational accidents. The U.S. Army CH-47s supported the 1st Australian Task Force as required. Four CH-47s were converted into ACH-47As by adding armor and improved engines. Its armament included two fixed forward firing M24A1 20mm cannons, one turret with 40 mm automatic grenade launcher on the nose, five .50 in machine guns and two weapon pods on the sides that could carry either XM159B/XM159C 70 mm rocket launchers or 7.62 mm miniguns. They arrived in Vietnam in 1966, and they engaged in six months of operational testing at An Khê Army Airfield. They performed well in combat, but its high maintenance costs and demand for use in troop and cargo transport was stronger. Three ACH-47s were lost. One collided with a CH-47 while taxiing. Another had a retention pin shake loose on a 20 mm cannon and was brought down when its own gun fired through the forward rotor blades. The third was grounded by enemy fire and destroyed by enemy mortar rounds after the crew escaped. Iran During the 1970s, the United States and Iran had a strong relationship, in which the Iranian armed forces began to use many American military aircraft, most notably the F-14 Tomcat, as part of a modernization program. After an agreement signed between Boeing and Agusta, the Imperial Iranian Air Force purchased 20 Agusta-built CH-47Cs in 1971. The Imperial Iranian Army Aviation purchased 70 CH-47Cs from Agusta between 1972 and 1976. In late 1978, Iran placed an order for an additional 50 helicopters with Elicotteri Meridionali, but that order was canceled immediately after the revolution; 11 of them were delivered after multiple requests by Iran. In the 1978 Iranian Chinook shootdown, four Iranian CH-47Cs penetrated into Soviet airspace in the Turkmenistan Military District. They were intercepted by a MiG-23M which shot down one, killing eight crew members, and forced a second one to land. Chinook helicopters were used in efforts by the Imperial Iranian loyalist forces to resist the 1979 Iranian