For Condensed Formula What You Do To The 3 Bonding Aircraft Composites Repair Decides the Future

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Aircraft Composites Repair Decides the Future

Commercial Aircraft Composite Repair Committee CACRC

This working group covers:

Composite Repair Materials, Composite Repair Techniques, Composite Inspection, Composite Design, Facility Training, Airline Inspection and Composite Repair Terms

This work is progressing.

It should be noted that the CACRC is currently focused on service problems and is therefore primarily concerned with thin components, often sandwich constructions (e.g. SAE AE-27, ‘Guide for the Design of Durable, Repairable and Repairable Aircraft Composites.’ whose design case studies are often sandwich are parts). As service reliance on composites increases, particularly in lightweight sandwich panels or adhesively bonded structures used in aircraft cabin interiors, more composite primary structures are introduced. Today, for example, we find Airbus’ vertical wings and composite horizontal tailplanes as well as the outer wingboxes of Aerospatiale’s ATR72s. However, this construction is monolithic and usually of greater thickness than secondary structures. Experience has shown that these structures are highly tolerant to service risks resulting in more complex maintenance procedures (compared to metals) given the limited number of incidents and any increased scheduled maintenance costs are negligible. However, efforts must continue to improve current repair techniques. Various aspects of composites are emphasized differently among airlines. For example, the widespread use of composite primary patterns has been highly encouraged by some, while others are more cautious. Some clients will do all of their overall maintenance in-house, others will outsource it to a third party. Stripping of paint worries some, not others and similarly bonded or bolted repair techniques have their proponents.

In service hazards in the use of aircraft composites:

Like most industries, commercial operators are subject to an increasingly competitive business environment, one result of which is intensive utilization of aircraft to maximize revenue. An aircraft turnaround time of only 30 minutes ensures that the aircraft gse … the risk of impact from catering trucks, passenger ladders, service trucks, towing vehicles, passenger buses etc. are always present. Product reliability is of paramount importance as any unexpected downtime directly reflects on airline profitability. This has had a greater impact than in previous years, as today, passengers are able to easily take a flight on a competitor’s aircraft. However, aircraft maintenance, whether planned or unplanned, is incurred by operators to keep them in airworthy condition. It is the OEM’s responsibility to understand the aircraft’s operating environment so that airworthiness requirements are met and maintenance costs of its products are kept to a minimum. The operator needs a design that is reliable and tolerant of service hazards.

Aircraft in service have many and varied threats as they are used in hostile environments.

Threats are pending

a) Impact from tire debris

b) Impact from engine debris (both large and small)

c) Engine fire

d) Electric shock

e) High Intensity Radio Frequency

f) Local heating of the structure from fuel pump dry running

g) Effects from Birdstrike

h) Blockage of hot air from duct bursts

i) General effects (hail, falling equipment, ‘hangar rash’, fueling nozzles, runway debris etc.)

j) General environmental effects

Despite evidence showing that thick monolithic laminates are extremely damage tolerant, there will always be a potential need for major repairs.

There is very little experience in airlines performing major repairs on thick composite monolithic structures where, for example, Airbus-type wingboxes may exceed 1 inch in thickness in some areas. Whereas thin panels are usually repaired by bonding, this technique will face more difficulties as the structural thickness increases. Scarfing at currently accepted angles means that the actual damaged area can increase significantly in size and run into adjacent structural features which will further complicate the repair process. Continuous heat application and consolidation will be difficult unless done in multiple lay-ups, however, as downtime to complete successive cures can be unacceptably long. Finally the assurance of bondline strength is still a question that is often raised and will have to be answered if applied to a wingbox structure that has the added complexity of long-term fuel contact. It may be that bolted patches (either composite or otherwise) using metal type procedures are a better method if major repairs are required while minor damage can be restored using bonding techniques.

Corrosion at the interface with metal components.

Corrosion of metal structures causes huge maintenance costs to the aircraft industry due to inspection, maintenance or its prevention efforts. Of course, one of the main advantages of composites is the great scope to reduce this burden, however, the aircraft will always have metal components and the possibility of corrosion at the interface with carbon must be taken very seriously. There are already many airframes in existence that have completed high flight cycles and are suffering such damage over many years of service. Adequate safety plans exist to prevent these problems and careful design and maintenance should be sufficient to realize significant savings in maintenance costs. Special care should be taken at major interfaces such as the outer to inner wingbox joint and the design should allow reasonable access for inspection.

Stripping for aircraft repainting.

Stripping and repainting is a routine maintenance task for airlines either to renew the cosmetic appearance, change the company logo or due to a change of ownership. An aircraft is usually repainted every 3 – 5 years, although it is common for the wings to be done every other overhaul. Concerns have been raised that this would be more expensive in composite wingboxes as abrasive methods are required rather than chemical stripping. The sophisticated equipment that is required for such abrasive methods (dry ice, wheat starch, water jets, lasers etc.) requires large capital investments that operators are reluctant to spend.

Chemical stripping risks damaging the composite and is currently not a practical option but research is ongoing to develop paints such that the top coat can be easily removed with a light stripper but the primer remains in place. Alternative solutions can be resins with color pigments or adhesive films that can be renewed without removing the paint.

Opinions about carbon composites within the aircraft industry are both positive and negative and are mainly based on components of thin laminate and/or sandwich construction. In order to gain service experience and thereby increase confidence in their performance, they are often used to replace metal secondary structures in places vulnerable to impact. Consequently damage occurs regularly (as it does with metal) and maintenance is required. Moisture penetration has caused problems with very thin laminates (typically 2 plies), however, good design, specifying a minimum of 3 or 4 plies, should provide a solution. It is recognized that these types of composite parts will take longer to maintain than their metal counterparts and therefore incur higher unplanned maintenance costs. This situation is made worse by the fact that the allowable damage limit of composites is conservatively small. These weaknesses have been identified and the aircraft industry is working together to improve the situation.

When thicker composite monolithic laminates are used, however, service experience shows that their tolerance in service environments is excellent and, if used to construct wingboxes, will be protected from most impact sources. However, there will always be the rare occasion when work is required to repair major damage and the resulting unplanned repair costs may actually exceed its metal equivalent. However, considering the overall expected reduction in LCCs being acquired by operators due to the benefits of fatigue, corrosion, less scheduled inspections and fuel burn (and these benefits are already appreciated by the airlines), we must continue to make positive efforts. Realize the potential benefits of composites used extensively in commercial aircraft. The importance of LCCs is becoming more appreciated by the industry in general but we are still in the early stages of having tools available for their assessment and monitoring. Consequently, it is difficult to accurately estimate the economic benefits of replacing metallic components with well-engineered composites.

Manufacturers must recognize that their customers will only accept large-scale applications of composites technology if the economic benefits in both initial purchase price and life cycle cost can be demonstrated. This is one of the challenges faced by the current OEMs and it is their task to meet the requirement through intelligent design and correct application of materials.

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