Aircraft Composite Repair Decides the Future
The CACRC Commercial Aircraft Composites Repair Committee
Their task groups cover:
Composite repair materials, composite repair techniques, composite inspection, composite design, facility training, overhead line inspection, and composite repair conditions
This work is progressing.
It should be remembered that the CACRC focuses on current issues in service and therefore deals primarily with thin components, often sandwich constructions (ref, for example, SAE AE-27, the ‘Guide to Material Design durable, repairable and maintainable aircraft composites’ whose design case studies are mostly sandwich parts). As service confidence in composite materials increases, particularly in lightweight sandwich panels or adhesive bonded structures used inside aircraft cabins, more composite primary structures are being introduced. Today, for example, we find the vertical fins of Airbus and the horizontal tail planes of composite material, as well as the outer wings of the ATR72s of Aerospatiale. However, this construction is monolithic and generally thicker than secondary structures. Experience has shown that these structures are extremely tolerant of hazards in service, with the result that any increase in unscheduled maintenance costs due to more complex repair processes (compared to metals) is negligible considering the limited number. of occurrences. However, further efforts are needed to improve current repair techniques. Different emphasis is placed on various aspects of inter-airline compounds. For example, some encourage more widespread use of the composite primary structure, while others are more cautious. Some clients will carry out all of their compound maintenance in-house, others will outsource it to a third party. Paint removal concerns some, others not, and gluing or bolting repair techniques similarly have their supporters.
Threats in service to the use of aircraft composite materials:
As with most industries, commercial operators are subject to an increasingly competitive business environment, one of the consequences of which is intensive use of the aircraft themselves to maximize revenue. Aircraft response times of just 30 minutes ensure that gse aircraft impact hazards … catering trucks, passenger ladders, service trucks, tow vehicles, passenger buses, etc. are always present. Therefore, product reliability is of the utmost importance, as any unexpected downtime is directly reflected in airline profits. This has a greater impact than in years past, because today, a passenger can easily take a flight on a competing aircraft. However, aircraft maintenance, whether planned or not, is a cost that operators must incur to keep them in airworthy condition. It is the OEM’s responsibility to understand the aircraft’s working environment so that airworthiness requirements are met and maintenance costs for its products are kept to a minimum. The operator requires a design that is reliable and tolerant of threats in service.
There are many and varied threats to aircraft in service as they are used in a hostile environment.
The threats under consideration are
a) Impact of tire debris
b) Impact of engine debris (both large and small)
c) Engine fire
d) Lightning
e) High intensity radio frequencies
f) Local heating of the structure due to dry running of the fuel pump
g) Birdstrike impact
h) Impact of hot air due to rupture of the duct
i) Overall impact (hail, tool drop, ‘hangar eruption’, refueling nozzles, runway debris, etc.)
j) General environmental effects
Despite the evidence showing that thick monolithic laminates are extremely tolerant to damage, there will always be a potential requirement for a major repair.
There is very little airline experience of major repairs being carried out on thick composite monolithic structures where, for example, thicknesses may be greater than 1 inch in some areas of an Airbus type wingbox. While thin panels are typically repaired by bonding, this technique will encounter additional difficulties as the structural thickness increases. The steepness at the currently accepted angles will mean that the original damaged area can grow significantly in size and encounter adjacent structural features that will further complicate the repair process. Constant heat application and consolidation will be difficult unless carried out in multiple layers, however the downtime to complete successive cures can become unacceptably long. Lastly, the guarantee of bond line strength is still a frequently raised question and will have to be answered if applied to the wing box structure, which has the added complication of being exposed to prolonged contact. with fuel. Bolt-on patches (composite or otherwise) using metal-type processes may be the best method if major repairs are required, while minor damage can be restored using bonding techniques.
Corrosion at interfaces with metal components.
Corrosion of metallic structures causes the aeronautical industry great maintenance costs due to inspection, repair or prevention attempts. Without a doubt, one of the main advantages of composites is the enormous margin of reduction of this load, however, there will always be metallic components in the aircraft and the potential for corrosion at the interfaces with carbon must be taken very seriously. There are already many airframes that have completed high flight cycles and many years in service without suffering such damage. Adequate protection schemes are in place to prevent these problems and careful design and maintenance should be sufficient to achieve significant cost savings. Care must be taken with major interfaces, such as an outer and inner wing box seal, and the design should allow reasonable access for inspection.
Pickling for aircraft repainting.
Stripping and repainting is a regular maintenance operation for airlines, whether it is to renew the cosmetic appearance, to change the company logo or due to a change of ownership. An aircraft is typically repainted every 3 to 5 years, although it is quite common for the wings to be repainted every other service. Concern has been raised that this would be more expensive in a composite wing box, as abrasive methods are required rather than chemical stripping. The sophisticated equipment required for such abrasive methods (dry ice, wheat starch, waterjet, lasers, etc.) requires a large capital investment that operators are reluctant to spend.
Chemical stripping runs the risk of damaging the compound and is not a practical option today, however research continues to develop paints so that the top coat is easily removed with a mild stripper, but the primer remains in place. . Alternative solutions can be resin with color pigments or adhesive films that can be renewed without removing the paint.
Opinions on carbon composites within the aircraft industry are both positive and negative and are based primarily on components of thin laminates and / or sandwich construction. In order to gain service experience and therefore increase confidence in their performance, they have often been used to replace metallic secondary structures in locations that are vulnerable to impact. Consequently, damage occurs on a regular basis (as it would with metallic ones) and repairs are required. Ingress of moisture has caused problems with very thin laminates (typically 2-ply), however a good design, specifying a minimum of 3-4 layers, should provide the solution. It is recognized that these types of composite parts require more time to repair than their metal equivalents and therefore incur higher unplanned maintenance costs. This situation is made worse because the permissible damage limits of the compounds are conservatively small. These drawbacks have been recognized and the aviation industry is working together to improve the situation.
However, when thicker composite monolithic laminates are used, service experience shows that their tolerance to the service environment is excellent and, if used to fabricate wing boxes, they will be protected from most sources of impact. However, there will always be rare occasions when action is required to repair major damage, and the resulting unplanned maintenance cost can be higher than its metallic equivalent. However, when considering the overall expected reduction in LCC that operators will realize due to the benefits in fatigue, corrosion, reduced scheduled inspections, and fuel consumption (and these benefits are already appreciated by airlines), we must continue to make positive efforts to achieve the potential benefits of composites in large-scale commercial aircraft applications. The importance of LCCs is increasingly appreciated by the industry in general, but we are still in the early stages of having the tools available to assess and monitor them. Consequently, it is difficult to predict exactly the magnitude of financial benefits to be gained from replacing a metal component with a well-designed composite.
Manufacturers must recognize that their customers will only accept the large-scale application of composite technology if economic benefits can be demonstrated in both initial purchase price and life cycle costs. This is one of the challenges faced by OEMs today and it is up to them to meet the requirement through smart design and proper material application.
