| The Commercial Aircraft Composites Repair | | | | composite or otherwise) using metallic type |
| Committee CACRC | | | | processes are the better method if major repairs are |
| It's task groups cover: | | | | required whereas minor damage can be restored |
| Composites repair materials, composites repair | | | | using bonding techniques. |
| Techniques, composites inspection, composites | | | | Corrosion at interfaces with metallic components. |
| design, Facilities Training, Airline Inspection and | | | | Corrosion of metallic structures causes the aircraft |
| composites repair conditions | | | | industry vast expense in maintenance due to |
| This work is progressing. | | | | inspection, repair or attempts at it's prevention. |
| It should be remembered that the CACRC is focused | | | | Undoubtedly, one of the main advantages of |
| on the current in service problems and so is primarily | | | | composites is the enormous scope for reduction of |
| concerned with thin components, often sandwich | | | | this burden, however, there will always be metallic |
| constructions (ref for example SAE AE-27, the 'Guide | | | | components on the aircraft and the potential for |
| for the design of durable, repairable and maintainable | | | | corrosion at the interfaces with carbon must be |
| aircraft composites' whose design case studies are | | | | taken very seriously. There are already many |
| mostly sandwich parts). As in service confidence in | | | | airframes existing which have completed high flight |
| composites grows, particularly in lightweight sandwich | | | | cycles and many years in service without suffering |
| panels or adhesive bonded structures used in aircraft | | | | such damage. Adequate protection schemes exist to |
| cabin interiors, more composite primary structures | | | | prevent these problems and careful design and |
| are introduced. Today, for example, we find the | | | | maintenance should be sufficient to realise a |
| Airbus vertical fins and horizontal tailplanes of | | | | significant saving in costs. Particular care must be |
| composite as well as the outer wingboxes of | | | | taken at major interfaces such as an outer to inner |
| Aerospatiale's ATR72s. However this construction is | | | | wingbox joint and the design should allow reasonable |
| monolithic and generally of a greater thickness than | | | | access for inspection. |
| the secondary structures. Experience has shown that | | | | Stripping for aircraft repainting. |
| these structures are extremely tolerant of in service | | | | Stripping and repainting is a regular maintenance |
| hazards with the result that any increased | | | | operation for the airlines either to renew the |
| unscheduled maintenance costs due to more complex | | | | cosmetic appearance, to change the company logo |
| repair processes (compared to metals) are | | | | or due to change of owner. An aircraft is typically |
| insignificant considering the limited number of | | | | repainted every 3 - 5 years although it is quite |
| occurrences. Nevertheless, efforts must be continued | | | | common for the wings to be done at every other |
| to be made to improve the current repair techniques. | | | | overhaul. Concerns have been raised that this will be |
| There is a different emphasis placed on various | | | | more expensive on a composite wingbox as abrasive |
| aspects of composites between airlines. For example, | | | | methods are required rather than chemical stripping. |
| the more widespread use of composite primary | | | | The sophisticated equipment which is required for |
| structure is very much encouraged by some, others | | | | such abrasive methods (dry ice, wheat starch, water |
| are more cautious. Some customers will carry out all | | | | jet, lasers etc) requires a large capital investment |
| their composite maintenance in-house, others will | | | | which the operators are reluctant to spend. |
| outsource it to a third party. Stripping of the paint | | | | Chemical stripping risks damaging the composite and |
| concerns some, others not and similarly bonded or | | | | is not a practical option at present however research |
| bolted repair techniques have their supporters. | | | | is continuing to develop paints such that the top coat |
| In service threats to the use of aircraft composites: | | | | is easily removed by a mild stripper but the primer |
| As with most industries, commercial operators are | | | | remains in place. Alternative solutions may be resin |
| subjected to a more and more competitive business | | | | with colour pigments or adhesive films which can be |
| environment, one consequence of which is the | | | | renewed without paint removal. |
| intensive utilisation of the aircraft themselves to | | | | Opinions about carbon composites within the aircraft |
| maximise revenue. Aircraft turnaround times of only | | | | industry are both positive and negative and are |
| 30 minutes ensures that risks of impact from aircraft | | | | based mainly on components of thin laminates and/or |
| gse ... catering trucks, passenger stairs, service | | | | sandwich construction. In order to gain in service |
| trucks, towing vehicles, passenger buses etc are | | | | experience and hence increase confidence in their |
| ever present. Reliability of the product is therefore of | | | | performance, they have often been used to replace |
| paramount importance as any unexpected downtime | | | | metallic secondary structures in locations which are |
| reflects directly in the profits of the airlines. This has | | | | vulnerable to impact. Consequently damage occurs on |
| a greater impact than in years gone by, because | | | | a regular basis (as it would with metallics) and repairs |
| today, a passenger is easily able to take a flight on a | | | | are required. Moisture ingress has caused problems |
| competitor's aircraft. However, maintenance of the | | | | with very thin laminates (typically of 2 plies) |
| aircraft, be it planned or unplanned, is a cost which | | | | however, good design, by specifying a minimum of 3 |
| the operators have to incur in order to keep them in | | | | or 4 plies, should provide the solution. It is recognised |
| an airworthy condition. It is the OEM's responsibility to | | | | that these types of composite parts are more time |
| understand the working environment of the aircraft | | | | consuming to repair than their metallic equivalents and |
| so that both the Airworthiness Requirements are | | | | so incur higher unplanned maintenance costs. This |
| met and the maintenance costs of it's products are | | | | situation is made worse due to allowable damage |
| kept to a minimum. The operator requires a design | | | | limits of composites being conservatively small. These |
| which is reliable and tolerant of in service threats. | | | | drawbacks have been recognised and the aircraft |
| There are many and varied threats to the aircraft in | | | | industry is working together to improve the situation. |
| service as they are used in a hostile environment. | | | | When thicker composite monolithic laminates are |
| Threats under consideration area) Impact from tyre | | | | employed however, in service experience is showing |
| debrisb) Impact from engine debris (both large and | | | | that their tolerance to the in service environment is |
| small)c) Engine Fired) Lightning Strikee) High Intensity | | | | excellent and if used to manufacture wingboxes, will |
| Radio Frequenciesf) Local Heating of Structure from | | | | be protected from the majority of impact sources. |
| Fuel Pump Dry Runningg) Impact from Birdstrikeh) | | | | Nonetheless, there will always be the infrequent |
| Hot air impingement from duct bursti) General impact | | | | occasion when action is required to repair major |
| (hail, dropped tools, 'hangar rash', refuelling nozzles, | | | | damage and the resulting unplanned maintenance cost |
| runway debris etc)j) General Environmental Effects | | | | may indeed be higher than it's metallic equivalent. |
| Despite evidence showing that thick monolithic | | | | However, when considering the overall expected |
| laminates are extremely damage tolerant, there will | | | | reduction in LCCs to be gained by the operators due |
| always be the potential requirement for embodying a | | | | to advantages in fatigue, corrosion, reduced |
| major repair. | | | | scheduled inspections and fuel burn(and these |
| There is very little experience at the airlines of major | | | | advantages are already appreciated by the airlines), |
| repairs being carried out on thick composite monolithic | | | | we should continue to make positive efforts to |
| structure where, for example, thicknesses may be | | | | realise the potential benefits of composites in large |
| greater than 1 inch in some areas of an Airbus type | | | | scale applications on commercial aircraft. The |
| wingbox. Whereas thin panels are normally repaired | | | | importance of LCCs is becoming more appreciated by |
| by bonding, this technique will meet with additional | | | | the industry generally but we are still in the early |
| difficulties as the structural thickness grows. Scarfing | | | | stages of having the tools available for assessing and |
| at currently accepted angles will mean that the | | | | monitoring them. Consequently, it is difficult to predict |
| original damaged area may grow significantly in size | | | | exactly the magnitude of the financial benefits to be |
| and run into adjacent structural features which will | | | | gained due to the replacement of a metallic |
| complicate the repair process still further. Consistent | | | | component by a well engineered composite one. |
| heat application and consolidation will be difficult unless | | | | The manufacturers must recognise that their |
| carried out in multiple lay ups, however, the | | | | customers will only accept large scale application of |
| downtime for completing successive cures may | | | | composite technology if economical benefits can be |
| become unacceptably long. Finally the assurance of | | | | demonstrated both on initial purchase price and life |
| bondline strength is still a question which is often | | | | cycle costs. This is one of the challenges facing the |
| raised and will have to be answered if applied to | | | | OEMs at present and it is their task to fulfil the |
| wingbox structure which has the additional | | | | requirement by intelligent design and the correct |
| complication of being exposed to longterm fuel | | | | application of the material. |
| contact. It may be that bolted patches (either | | | | |