Aerospace Composites in airline service

Large civil aircraft have to be operated intensively by airlines to maximise profits and remain competitive in today's business environment. Similarly the major manufacturers are competing to supply the operators with products which meet their demands of minimum purchase price and operating costs. Composites play an important role in this market.

Composites have contributed to the evolution in the efficiency of airframe structures over the last 15 years. There are very few truly revolutionary technologies, like the jet engine, and continuing improvement is the sum of many small steps.

On today's generation of commercial aircraft, the composites provide a weight saving of the order of 1 % of the Operating Weight Empty. The competitive environment in commercial aviation drives us to seek further ways of reducing Direct Operating Costs (DOC). The largest contribution is first cost followed by fuel. The implication for structures is that we have to reduce weight while reducing the overall manufacturing cost of the aircraft.

One option is to increase the use of composites. Current generation (A320 and A340) typically have around 15% by weight of composites in the structure. If the main wingbox was to be composite, this proportion would increase to 40%.

However, to realise the potential weight saving presents a number of formidable technical and economic challenges. The solutions to these challenges are beyond the current state of the art.

At BAe Airbus, the Composite Wing Programme has carried out research into the most effective way of making carbonfibre composites wingboxes. The importance of reliability and maintainability of the structure in service is recognised and a team has been specifically tasked to investigate technical issues relating to the in service environment. The two parts to this investigation are understanding the customers (airlines) needs and investigating in service damage threats.

As the development costs of large technically advanced aircraft continue to escalate, the need to have a product which meets the requirements of the customer and hence will sell successfully in the market becomes ever more critical. The substantial investment which is required to introduce a composite wingbox onto the next generation of large civil airliner demands that the product must achieve customer approval and if possible exceed expectations as soon as it enters into service.

The Original Equipment Manufacturers (OEMs), need to understand the in-service environment with respect to any issues relating specifically to the product. Most of the composite experience would be based on thin laminates typically on removable structures as this is where the majority of the non metallic parts exist at present. Customer participation reduces the risk of new technologies to the OEMs.

Advanced composites and removable aircraft structures 

The industry has introduced advanced composites cautiously to ensure the capabilities of these new materials, hence the majority of carbon components used on current civil aircraft is of removable structures.

Moving aerodynamic surfaces, composite radomes, fairings, and engine cowl doors are all typical examples of composite components and in service experience is showing that there are advantages and disadvantages for both materials. For example, composites eliminate the corrosion problems which metallics suffer (good design and manufacturing practices must be exercised at carbon to aluminium interfaces), but are more prone to erosion on exposed edges, while honeycomb structures, of either material, is generally more complex to repair than monolithics.

For structure specific to wings, trailing and leading edges contain many lightweight composite parts which, due to their location, are vulnerable to impact from service vehicles and other hazards while on the ground. (This is equally applicable to metallics but less so for a wingbox which is protected by 'bumpers' ). If damaged, it is generally accepted that such components are more costly to repair than their equivalent metallic parts.

Damage assessment, preparation, cleaning and curing of bonded patches are time consuming operations and it is of paramount importance to an operator that this type of work can be completed within the scheduled maintenance downtime. In addition, availability, standardisation and interchangeability of repair materials needs to be improved and this again risks leading to longer downtimes; storage of the materials if required is expensive and often have to be purchased in large quantities.

These are some of the concerns which have been raised by the aircraft operators to the OEMs and as a positive response, an organisation called the Commercial Aircraft Composite Repair Committee (CACRC) consisting of OEMs, airlines, composite repair facilities, material manufacturers, and regulatory agencies was established to address the problems.