Under Civil Aviation Regulation 42U (CAR42U), any modification or repair that is carried out on an Australian aircraft must be done in accordance with 'approved data'. This approved data is usually contained within the aircraft's manuals (aircraft maintenance manual, structural repair manual, component maintenance manuals etc.) issued by the aircraft's manufacturer. However, sometimes there is damage to the aircraft that is outside the scope of these manuals or the operator wishes to modify the aircraft to help it perform a specific function (aerial firefighting, search and rescue) or alter the aircraft in some other way. To ensure that the aircraft continues to be airworthy (i.e. safe to fly), the repair or modification must be examined by an aeronautical engineer so that the many complex factors that affect aircraft flight, structures and systems can be assessed and the design change approved as airworthy. To do this the engineer assesses the design of the modification or repair against the airworthiness requirements for the aircraft and then must justify that the design meets these requirements through the use of analysis, testing and experience. If the design has been shown to meet the airworthiness requirements it may be approved under Civil Aviation Regulation 35 (CAR35) and is hence 'approved data', satisfying CAR42U.