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Aircraft Requirements and Safety

The design process of an aircraft starts with specification of the requirements. An aircraft design is always a compromise. The first and most important requirement of an aircraft part is that it fulfils its function in all circumstances, particularly in critical situations.
The strength of a structure is a measure of the risks taken – the acceptance that the structure will fail in extreme conditions. Society sets standards for such risks. We accept that all structures fail in certain conditions. When calculating the loads, we name the force which will just make the structure fail, the ultimate load. Structural failures often occur due to a very large series of normal repetitive loads that cause fracturing of the material: metal-fatigue. It is very important to know the rate of crack-growth and the residual strength (the strength in the presence of cracks) of a structure. A number of European countries have formulated a set of Joint Airworthiness Requirements, the J.A.R, which are based on the American Federal Airworthiness Requirements, or F.A.R. The
airworthiness standards define primary structures, those that would endanger the aircraft upon failure, secondary structures, those that do not cause immediate danger upon failure, and non load-bearing structures, which do not carry loads. There are multiple ways of considering part safety. The fail-safe principle accepts that there is a chance that part of the structure fails. However, there should be no chance of the whole structure failing. In the safe-life philosophy, the chance of the structure failing within its prescribed lifetime should be zero. If this were to happen, then the chance of the whole structure failing is substantial. The stiffness of a structure is a measure of its resistance to a change in shape when subjected to forces. The stiffness of a complete structure is always a combination f its material properties and its geometry. Aircraft wings and tail-sections can be subjected to three types of forces, namely aerodynamic forces, elastic forces and mass forces. These forces can work together in such an unfortunate way that they induce a type of vibration known as flutter. Flutter only occurs above a certain speed, which we call the critical speed. Flutter is caused by two coordinated types of vibration that amplify each other’s effect. Air-transport safety is the responsibility of the manufacturer, the user and the government. As part of this responsibility, the government exercises control over the airfield through the State Air-Transport Service (In the Netherlands this is the Luchtvaart Autoriteit). The Luchtvaart Autoriteit is responsible
for monitoring design, manufacturing, use and maintenance of aircraft, education, training and testing of personnel, and operational guidelines, accident investigation, traffic management and traffic regulations.

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