An appropriate method for a structured design process is defined by VDI guideline 2221 .

One of the most important steps is the precise definition of functions. In order to guarantee a thorough understanding of the product requirements at the very beginning of its development, functional analysis must be organised involving experts from all disciplines (structure, systems, aerodynamics). Functional analysis enable to think in terms of functions and not in terms of solutions. The precise definition of functions also helps understanding and identifying the root cause of unwanted intermediate product features (i.e. in case it is discovered that a design solution for a given part would lead to too much weight or cost) and consequently adapting the design solution, or challenging (and potentially changing) a requirement in order to meet more important requirements from superior levels of the requirement cascade. A good example for a purely functional requirement is: “The floor must be able to carry payload (maximum load 32 t)”. A bad example (as it is already solution oriented) in this context would be: “The floor structure is an aluminium honeycomb design capable to withstand a maximum load of 32 t”. The basic procedure for a functional analysis is described in below Figure . After a description of the product (step 1), the functions are systematically analysed (step 2) by a team of experts from different disciplines. Step 3 is the hierarchical organisation of the different functions, while step 4 characterises all functions. The final step prioritises these functions.