Part 1 of this article introduced the concept of composite patch repair and discussed techniques studied under the European AeroPlan programme. Part 2 outlines composite repair techniques being used by Boeing and Airbus.
Boeing and Airbus solutions
| Composite patches are increasingly preferred because they can be bonded rather than bolted, forming a superior repair. |
While taking into account the R&D programmes brought together in AeroPlan, the leading airframers have, unsurprisingly, been developing their own patch repair solutions. Both Boeing Commercial Airplanes and Airbus have sought to address the issue of ‘ramp rash’ – the sort of generally modest damage caused when service vehicles and equipment bump into aircraft, particularly their fuselages. Traditionally, these ‘dings’ have been fixed with bolted metal patches that are familiar on aluminium aircraft. These have been of aluminium or titanium, although today composite patches are increasingly preferred because they can be bonded rather than bolted, forming a superior repair.
For the new breed of passenger jets having composite fuselages, composite patches are recommended due to the more effective bonds that can be formed between the similar materials. Boeing has developed for its B787 Dreamliner, the first composite-fuselaged passenger jet to enter service, a quick-cure repair patch. This patented system comprises a pre-cured carbon fibre/epoxy patch which, when heated with a chemical heat pack, enables a typical repair to be carried out ‘in an hour,’ according to the company. Because it utilises an adhesive that cures at relatively low temperature, the repair is intended as temporary and sufficient to restore enough strength to permit continued service until a full restorative repair can be undertaken. A number of quick-cure repairs have already been made to in-service aircraft.
Airbus has sought to make patch repairing a more automated procedure, having developed a small mobile system able to tailor pre-cured composite patches for specific repair sites. Its automated system first gathers 3D surface data of the damaged area using a stereoscopic camera, then ultrasonically tests the area to determine the damage extent. Next, it applies robotic milling to remove damaged material. The repair patch is produced using a 3D scan of the milled repair area, and then applied to the fuselage with a film adhesive. After being vacuum bagged, the repair is cured under heat and pressure. Infusion option
German aerospace research organisation DLR reminds us that a pre-impregnated, pre-cured (hard) patch is not the only patch option and champions a solution based on infusing a dry preform in situ. DLR says this method produces results as good as those achievable with hard patches. It is said to suit curved surfaces better, avoiding the need to produce a tool in which to mould a hard patch, with the associated time and expense.
This infusion method is part of an automated system DLR has developed that can make repairs which are flush to the repaired surface rather than formed over the top. It would therefore suit more permanent repairs made at times when base maintenance is scheduled. The automated cell uses NDE tools to survey the damage and produce 3D digitised imagery, then cuts out damaged material under computer control to precise dimensions derived from the survey. It then manufactures a preform to match those dimensions, inserts this into the excised space, infuses resin into the preform and applies heat and pressure to effect a cure.
| Automated repair technology 'virtually essential' to the profitable operation of composite aircraft. |
In repairs that extend the full depth of the laminate, the infused preform is more a plug than a patch. Either way, the patch/plug has chamfered edges that sit precisely on a matching edge profile lasered into the repair site. This ensures a flush finish that renders the final scarfed repair virtually invisible. It is more integral and should be inherently stronger than an overlain patch as well as aesthetically superior.
Other automated repair cells are being developed outside Europe, including one in the USA under development by a consortium led by American GFM, a branch of Austrian machine tool maker GFM. Proponents of this and other systems see automated repair technology as being virtually essential to the profitable operation of composite aircraft, particularly as these begin to populate the skies in significant numbers. Automated cells should reduce labour costs and the chances of human error as well as that crucial MRO metric, time to repair.
Largest patch
Demonstrably the largest composite patch so far implemented for repairing primary aerostructure is that produced by Boeing for the Ethiopean Airlines B787 that suffered a fire while parked, empty, at London’s Heathrow Airport in July 2013. The fire, caused by a faulty emergency locator beacon, burned out a sizeable section of the crown of the monolithic carbon/epoxy fuselage laminate.
Reports describe how, to make the repair, Boeing accessed an entire rear fuselage barrel in the factory and cut out a ~25 ft long area of its crown corresponding with the damaged area on the Ethiopean jet. This became the repair patch. This large patch, complete with sections of the attached frames and co-moulded stringers, was then flown to Heathrow where it was lowered into the same-sized aperture left by the removal of damaged composite from the airliner. The small gap left around the insert’s perimeter was filled with a sealant.
Next, technicians working inside the fuselage glued a splice plate to the inside surfaces, overlapping the joined edges by a few inches on each side. After vacuum bagging the repair, technicians applied thermal blankets to heat and pressurise the site for the duration of a carefully monitored and controlled cure. Heatcon controllers were used. Some joining and making good of the stringers and frames was also necessary. Mechanical fasteners were used to complement the bonding.
The cured repair was carefully evaluated, both on the ground using ultrasonic NDE and in the air with strain and other sensors during flight tests. Test results enabled Boeing to declare the repaired fuselage to be as sound as the original. Only time will tell whether this will be justified by continued integrity of the repair in the harsh conditions of a high-flying airliner constantly cycling between environmental extremes over many years.
Meanwhile, Boeing has gained valuable experience from the development and application of the ultra-large pre-cured repair patch used for this ambitious repair. Its engineers certainly knew they were breaking new ground well outside the normal scope of the SRM.
Composite patch repairs to airframes have already come some way. Aircraft manufacturers and operators, MRO specialists and research bodies will all help ensure that this crucial enabler for the economic success of extensively composite aircraft develops further yet. ♦
This article was published in the September/October 2014 issue of Reinforced Plastics magazine.
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