In order to certify an aircraft to fly it must hold a Certificate of Airworthiness. In order to achieve this authorisation, the aircraft must comply with the regulations prescribed by the relevant authority. This is aimed at ensuring the safety of the aircraft, its occupants and the public. In Britain, this is issued by the Civil Aviation Authority. There is also agreement within Europe where type certificates are issued according to Joint Aviation Regulations (JAR). In the USA the Federal Aviation Authority requires aircraft registered in the USA to comply with its rules. Large aircraft are designed to JAR 25.

When introducing new materials, the attitude of certificating authorities is that the use of new materials should not subject aircraft operators to higher levels of risk than they accept with existing materials. The use of sandwich structures would fall within this category.

A) Design Life (reliability). The design life of a component is subject to JAR/FAR section 25.571 damage tolerance regulations. The main sections cover evaluation and growth of damage initiated by fatigue loading, accidental damage (such as bird strike, tyre debris, and uncontained engine failure). It is also governed by the philosophies such as: "Safe Life Approach -1960", "Fail-safe life approach (redundancy) -1970", "Damage tolerance approach (inspectability) - 1980", and "Improved damage tolerance approach - 1995".

B) Weight reduction. As yet I have found no codes or standards which we use relating to the weight reduction of a sandwich structure. Two relevant papers which I have found in the open literature which cover cost and weight design of fuselage frames using differing manufacturing methods are:

"Minimum cost and weight design of fuselage frames Part A: design constraints and manufacturing process characteristics", C Kassapogolou, Composites: Part A 30 (1999) 887-894.

"Minimum cost and weight design of fuselage frames Part B: cost considerations, optimisation, and results", C Kassapogolou, Composites: Part A 30 (1999) 895-904.

C) Stiffness and deflection. Strengths are designed according to FAR section 25.305, which uses requirements in terms of the "limit load", "proof and ultimate factors". Stiffness and deflections are measured and modelled using many internal BAe and Airbus procedures, some of which use FE analysis, "Engineers Bending Theory", ESDU methods. Modulus, strength, delamination and deflection of honeycomb core sandwich materials are measured according to standard test procedures such as given in ASTM C365, ASTM C273, and other BAe and Airbus internal procedures (e.g. the flexural 4-point bending test).

D) Fire performance. Under FAR Section 25.867 "Fire protection: other components", states that surfaces to the rear of the nacelles, within one nacelle diameter of the nacelle centreline, must be at least fire-resistant. Fire resistant with respect to sheet or structural members means the capacity to withstand the heat associated with fire at least as well as aluminium alloy in dimensions appropriate for the purpose for which they are used. Various Airbus internal standards refer to appropriate methods for determining flammability, smoke density, smoke gas components and heat release of non-metallic materials, but most of these relate to aircraft interiors.

E) Humidity and moisture: There are internal Airbus and BAe procedures which relate to fluid tightness test for sandwich structures, and for environmental exposure of composite panels. These describe the ability of a material to resist the penetration of water into honeycomb cells, and also the performance in terms of strength and modulus changes and any other physical changes when exposed to hot/wet (and other fluids, e.g. anti icing fluid) environments.

F) Manufacturing methods and quality assurance: This section probably falls under FAR sections 25.601 to 25.631, (design and construction). There are numerous internal BAe and Airbus standards which cover all aspects of manufacture and quality assurance. These range from, e.g. climbing drum peel tests, ultrasonic testing, determination of fibre and resin mass per unit area, void content of facings, technical specifications of film adhesives used to bond facings to core, etc.