aircraft, including the F-4, F-111 and A-10. the gun was still in operation. Use in aircraft certification Chicken guns are routinely used in the process of proving compliance with certification regulations. Given their complexity and the expertise required to operate them, an aircraft manufacturer will typically contract with a facility that operates a gun to perform a test against a given standard. The component to be tested is mounted securely on a frame, the gun fires a bird at it, and the results are examined for compliance with the relevant standards. Most tests are performed with the gun pressurized to around this results in a bird being launched at around , approximately the resultant velocity in a collision between a bird and an aircraft. The FAA do not specify the species of bird that should be used for testing, but do state that the birds should not be frozen, as this would not accurately reflect the reality of a strike. Chickens are used as they are cheap, and readily available. There have been efforts to develop artificial bird analogs for use in impact tests, to replace the use of carcasses. The motivations for this range from ensuring that results are easily reproducible across the industry, cost, and sensitivity to the views of animal rights activists. However, concerns have been expressed by some engineers that tests with artificial birds do not accurately represent the forces involved in real bird strikes as the analogs lack bones. Some go further and state that the farm-raised birds commonly used in tests are also unrepresentative owing to the lower density of their muscle tissue. Notable uses During the development of the Boeing 757 in the 1970s, the cockpit roof was subjected to a bird strike test wherein a chicken was fired at into a stationary cockpit. To the surprise of the Boeing engineers, the chicken penetrated the skin of the aircraft. As a result, the cockpit of the 757, and that of the 767, which shared the same design, had to be reinforced. Several 767s were already in service, and had to be recalled for retrofitting of the reinforcements. Later in the 757's development process a bird strike test was conducted on the aircraft's windows, again by firing a chicken at them. The UK Civil Aviation Authority's certification requirements at the time were more stringent than the FAA's, and required the metal around the windows