Aircraft Composites Repair Decides the Future

Aircraft Composites Repair Decides the Future

The Commercial Aircraft Composites Repair Committee CACRC

It's task groups cover:

Composites repair materials, composites repair Techniques, composites inspection, composites design, Facilities Training, Airline Inspection and composites repair conditions
This work is progressing.

It should be remembered that the CACRC is focused on the current in service problems and so is primarily concerned with thin components, often sandwich constructions (ref for example SAE AE-27, the 'Guide for the design of durable, repairable and maintainable aircraft composites' whose design case studies are mostly sandwich parts). As in service confidence in composites grows, particularly in lightweight sandwich panels or adhesive bonded structures used in aircraft cabin interiors, more composite primary structures are introduced. Today, for example, we find the Airbus vertical fins and horizontal tailplanes of composite as well as the outer wingboxes of Aerospatiale's ATR72s. However this construction is monolithic and generally of a greater thickness than the secondary structures. Experience has shown that these structures are extremely tolerant of in service hazards with the result that any increased unscheduled maintenance costs due to more complex repair processes (compared to metals) are insignificant considering the limited number of occurrences. Nevertheless, efforts must be continued to be made to improve the current repair techniques. There is a different emphasis placed on various aspects of composites between airlines. For example, the more widespread use of composite primary structure is very much encouraged by some, others are more cautious. Some customers will carry out all their composite maintenance in-house, others will outsource it to a third party. Stripping of the paint concerns some, others not and similarly bonded or bolted repair techniques have their supporters.

In service threats to the use of aircraft composites:

As with most industries, commercial operators are subjected to a more and more competitive business environment, one consequence of which is the intensive utilisation of the aircraft themselves to maximise revenue. Aircraft turnaround times of only 30 minutes ensures that risks of impact from aircraft gse ... catering trucks, passenger stairs, service trucks, towing vehicles, passenger buses etc are ever present. Reliability of the product is therefore of paramount importance as any unexpected downtime reflects directly in the profits of the airlines. This has a greater impact than in years gone by, because today, a passenger is easily able to take a flight on a competitor's aircraft. However, maintenance of the aircraft, be it planned or unplanned, is a cost which the operators have to incur in order to keep them in an airworthy condition. It is the OEM's responsibility to understand the working environment of the aircraft so that both the Airworthiness Requirements are met and the maintenance costs of it's products are kept to a minimum. The operator requires a design which is reliable and tolerant of in service threats.

There are many and varied threats to the aircraft in service as they are used in a hostile environment.

Threats under consideration are
a) Impact from tyre debris
b) Impact from engine debris (both large and small)
c) Engine Fire
d) Lightning Strike
e) High Intensity Radio Frequencies
f) Local Heating of Structure from Fuel Pump Dry Running
g) Impact from Birdstrike
h) Hot air impingement from duct burst
i) General impact (hail, dropped tools, 'hangar rash', refuelling nozzles, runway debris etc)
j) General Environmental Effects

Despite evidence showing that thick monolithic laminates are extremely damage tolerant, there will always be the potential requirement for embodying a major repair.

There is very little experience at the airlines of major repairs being carried out on thick composite monolithic structure where, for example, thicknesses may be greater than 1 inch in some areas of an Airbus type wingbox. Whereas thin panels are normally repaired by bonding, this technique will meet with additional difficulties as the structural thickness grows. Scarfing at currently accepted angles will mean that the original damaged area may grow significantly in size and run into adjacent structural features which will complicate the repair process still further. Consistent heat application and consolidation will be difficult unless carried out in multiple lay ups, however, the downtime for completing successive cures may become unacceptably long. Finally the assurance of bondline strength is still a question which is often raised and will have to be answered if applied to wingbox structure which has the additional complication of being exposed to longterm fuel contact. It may be that bolted patches (either composite or otherwise) using metallic type processes are the better method if major repairs are required whereas minor damage can be restored using bonding techniques.

Corrosion at interfaces with metallic components.

Corrosion of metallic structures causes the aircraft industry vast expense in maintenance due to inspection, repair or attempts at it's prevention. Undoubtedly, one of the main advantages of composites is the enormous scope for reduction of this burden, however, there will always be metallic components on the aircraft and the potential for corrosion at the interfaces with carbon must be taken very seriously. There are already many airframes existing which have completed high flight cycles and many years in service without suffering such damage. Adequate protection schemes exist to prevent these problems and careful design and maintenance should be sufficient to realise a significant saving in costs. Particular care must be taken at major interfaces such as an outer to inner wingbox joint and the design should allow reasonable access for inspection.

Stripping for aircraft repainting.

Stripping and repainting is a regular maintenance operation for the airlines either to renew the cosmetic appearance, to change the company logo or due to change of owner. An aircraft is typically repainted every 3 - 5 years although it is quite common for the wings to be done at every other overhaul. Concerns have been raised that this will be more expensive on a composite wingbox as abrasive methods are required rather than chemical stripping. The sophisticated equipment which is required for such abrasive methods (dry ice, wheat starch, water jet, lasers etc) requires a large capital investment which the operators are reluctant to spend.

Chemical stripping risks damaging the composite and is not a practical option at present however research is continuing to develop paints such that the top coat is easily removed by a mild stripper but the primer remains in place. Alternative solutions may be resin with colour pigments or adhesive films which can be renewed without paint removal.

Opinions about carbon composites within the aircraft industry are both positive and negative and are based mainly on components of thin laminates and/or sandwich construction. In order to gain in service experience and hence increase confidence in their performance, they have often been used to replace metallic secondary structures in locations which are vulnerable to impact. Consequently damage occurs on a regular basis (as it would with metallics) and repairs are required. Moisture ingress has caused problems with very thin laminates (typically of 2 plies) however, good design, by specifying a minimum of 3 or 4 plies, should provide the solution. It is recognised that these types of composite parts are more time consuming to repair than their metallic equivalents and so incur higher unplanned maintenance costs. This situation is made worse due to allowable damage limits of composites being conservatively small. These drawbacks have been recognised and the aircraft industry is working together to improve the situation.

When thicker composite monolithic laminates are employed however, in service experience is showing that their tolerance to the in service environment is excellent and if used to manufacture wingboxes, will be protected from the majority of impact sources. Nonetheless, there will always be the infrequent occasion when action is required to repair major damage and the resulting unplanned maintenance cost may indeed be higher than it's metallic equivalent. However, when considering the overall expected reduction in LCCs to be gained by the operators due to advantages in fatigue, corrosion, reduced scheduled inspections and fuel burn(and these advantages are already appreciated by the airlines), we should continue to make positive efforts to realise the potential benefits of composites in large scale applications on commercial aircraft. The importance of LCCs is becoming more appreciated by the industry generally but we are still in the early stages of having the tools available for assessing and monitoring them. Consequently, it is difficult to predict exactly the magnitude of the financial benefits to be gained due to the replacement of a metallic component by a well engineered composite one.

The manufacturers must recognise that their customers will only accept large scale application of composite technology if economical benefits can be demonstrated both on initial purchase price and life cycle costs. This is one of the challenges facing the OEMs at present and it is their task to fulfil the requirement by intelligent design and the correct application of the material.