FEATURE

The challenge of
composite fuselage
repair
Freelance journalist George Marsh explains why the increasing use of
composites in commercial aircraft is creating debate about the best
way to repair these materials.

R
epairing the composite parts that they are engineered to withstand constantly • the level of training and awareness of
have been produced for jet airliners repeated cycles of elevated internal workers handling composites; and
up to now is a well established art, pressure. • a dearth of information on the longer-
but repairing reinforced plastic fuselages is term behaviour of aircraft composites due
likely to be, as they say, a whole new ball Clearly, with structures that are protecting to limited in-service experience so far.
game. With both leading commercial jet occupants flying six miles high, repairs must
airframers now having adopted compos- be qualitatively beyond reproach, offering Significantly too, the airworthiness authori-
ites for the fuselages of their latest aircraft the highest levels of assurance. However, ties remain wary of bonded repairs and
– the Boeing 787 and Airbus A350 XWB given the variability inherent in this have been loath to certify them.
(although Hawker Beechcraft has shown supremely tailorable class of materials, plus
the way with the Hawker 4000 [formerly the diverse skill levels of repair technicians, Boeing and Airbus may be somewhat over-
Horizon] business jet) – airframers, regula- achieving this consistently is difficult. optimistic in claiming that they either have
tors and repairers are facing a major repairs answers to all these points or are well on
challenge. the way to having them. Both assert that
Answers not yet in place
composite fuselages are in any case tougher
There are uncertainties surrounding the Despite positive spin from the airframers, than those of metal and will be more
repair of these internally-pressurised, the industry is to an extent feeling its way resistant to damage. However, experience
humanity-containing tubes that are arguably on this issue and experts agree that not all suggests that some damage will inevitably
the most safety-critical part of an aircraft the answers are yet in place. occur, especially to lower fuselage sections
and simply cannot be allowed to fail or be which are vulnerable to impacts from
compromised. Fuselages are very different Recently, for instance, the General baggage loaders, catering carts and other
from the planar thin-skinned, honeycomb- Accounting Office (GAO), the influential service vehicles. When this does happen,
cored structures that have been the watchdog over official policy in the USA, aircraft operators will require access to repair
mainstay of aircraft composite repair shops issued a report expressing concerns about schemes that are fully developed, approved
to date. They have pronounced curvature the accelerating use of composites in and certified.
and utilise monolithic laminate that can aircraft structures. Highlighting repair issues,
be thick in some places. Being gener- the GAO report cited: Boeing, whose B787 Dreamliner entered
ally filament wound or tape laid they rely, • the difficulty of detecting damage; service with Japan’s All Nippon Airlines
for at least part of their integrity, on long • limited standardisation of composite mat- (ANA) last year, says that maintainers can
continuous fibres. And, most significantly, erials and repair techniques; get an aircraft flying again after suffering

30 REINFORCEDplastics May/June 2012 0034-3617/12 ©2012 Elsevier Ltd. All rights reserved
 FEATURE

accepting that bolt-on repairs may be

familiar and quick, we would prefer bonded
repairs if the regulatory hurdles could be
overcome. It’s certainly a shame to have to
make holes [for bolts] in nice continuous
fibre lay-ups. But bonded repairs are not yet
accepted by the airworthiness authorities
except, essentially, as a cosmetic fix.”

Acknowledging this bonding preference,

Boeing has included an elementary type
of bonded repair in its Structural Repair
Manual (SRM) for the B787. Its temporary
‘band aid’ repair system can get an aircraft
flying again quickly after suffering minor
damage that might otherwise cause a
grounding. Essentially, a composite patch is
used instead of metal and through-bolting
is avoided. Boeing says its system enables
patch repairs to be made in as little as half
an hour, compared with the 24 hours or
more a classic bonded repair might take.

In use, a pre-cured composite patch

is epoxy bonded to the outside of the
damaged area and a chemical heat pack
is applied to cure the resin – a formulation
that has been selected for its ability to cure
at relatively low temperature. The repair
is designed to be applied at the gate if
necessary and to restore sufficient strength
to enable the aircraft to be cleared for
flight. It is primarily intended for instances
of light damage and not for more serious
cases where load path integrity is at stake. A
more permanent repair can be made later,
typically when the aircraft is due for a major
maintenance check inside a hangar where
conditions can be controlled.
Composite fuselages are tougher than those of metal and will be more resistant to damage, but some
damage will inevitably occur, especially to lower fuselage sections which are subject to impacts from That later more permanent solution is
numerous service vehicles. (Picture used under license from Shutterstock.com © Rob Wilson.)
likely to involve a bonded material insert
or ‘plug’, as distinct from a patch. While
ramp damage by using the same repairs Bonded vs bolted favoured by the composites community,
they have traditionally used on metal While bolted patch repairs may appeal such repairs take longer to perform and
aircraft – basically bolted metal patches. to airframe repair technicians throughout are likely to result in unscheduled down
Bolted repairs have the advantage of being the world who have been brought up time. Flush repairs having scarfed joints,
quick to execute so that they can often with metals, they are anathema to many in which tapered edges spread loads over
be completed within an aircraft’s normal composites specialists. a substantial contact area, are inherently
turnaround period on the ground between time consuming. They require painstaking
flights. They are also thoroughly familiar to As one expert, Dr Christian Sauer, manager preparation, careful tailoring and lay-up
airframe repair technicians, who can carry engineering, airframe related component of composite plies, a period of heating
out repairs using their normal tools and services at leading maintainer Lufthansa and adequate cure time. However, for
equipment. Technik (LHT), told Reinforced Plastics: “While the Federal Aviation Authority (FAA) and

www.reinforcedplastics.com May/June 2012 REINFORCEDplastics 31

 FEATURE

European Airworthiness and Safety Admin- on aircraft, individual technicians currently Another counter to variability is the provi-
istration (EASA), the main reason for with- do not have to be. sion in that airframe maintainer’s ‘bible’, the
holding certification of bonded repairs is SRM, of a range of approved repairs that are
the lack of certainty over bond quality. Great efforts are being made to over- described in detail. For the B787, Boeing is
come these disadvantages so that bonded including solutions ranging from bolt-on/
As one industry insider succinctly put it: repairs to fuselages can meet airworthiness glue-on patch repairs, through simple
“The difficulty at present with a repair bond criteria. Acknowledging the difficulty of bonded repairs, to large (up to 1 m²) repairs
is knowing exactly what strength you’ve matching precisely in a repair the original involving replacement material bonded at
got. There‘s no sure way of testing a bond’s ‘as new’ composite, Boeing has researched high temperature. The latter would normally
strength without breaking it, and one has and specified a single compatible carbon need to be carried out under controlled
to rely on coupon or sample tests, which prepreg repair composite which it says can environmental conditions in base facilities.
might not be fully representative.” be applied throughout the B787 fuselage. More repairs will be added to the SRM as
This, it says, avoids the need for main- they are developed and approved.
tenance departments to stock and manage

The difficulty at present a range of materials.

Technology prospects

with a repair bond is It is harder still to overcome the vari- Despite systemic improvements, variability
ability associated with manual processes and lack of certainty about repair quality
knowing exactly what carried out by human beings. Many in the are likely to remain difficult issues for some

strength you’ve got. industry believe that improved training

is a key part of the answer to this. One
time. Indeed, there is a growing belief that
manual processing cannot be the answer

overcome. There’s no such is Michael Hoke, president of Abaris

Training Resources Inc, who identifies the
and that ultimately it will have to be super-
seded by automated solutions. Only these, it
way of testing a bond’s incorporation of thick solid laminates in is argued, can deliver repairs that meet the
aerospace structures (including door and required standards consistently and reliably.
strength without window surrounds in fuselages) as one

breaking it. of the drivers for this need. Dealing with

solid laminates that may be up to an inch
Matthew Beaumont, Head of Operations,
Composite Technologies, at EADS Innovation
and 75 to 100 plies thick is, he notes, very in Germany sees automation as a way to
different from repairing the more usual face alleviate concerns about bonded structural
Similarly, LHT’s Sauer, while applauding sheets, perhaps 0.05 inch thick, on flatter repairs. He believes that present practice
airframer efforts to validate large bonded sandwich structured components. Techni- relies excessively on bolted repairs and, in
repairs on primary aerostructure, has cians will need to master heat application a telling quote, has said: “As a developer of
questioned whether such repairs, no matter methods requiring special hot bonders advanced composite structures, it hurts me
where in the world and by whom they are and backside access for thermal curing, to even think about drilling myriad holes
carried out, can uniformly meet the required plus slower ramping to ensure heating through these very optimised and precisely
standard. This, he told us, is the crux of the throughout the laminate. designed and manufactured components.”
difficulty in certifying them.
A related issue is the size of the repair. Even EADS Innovation has been working on auto-
At the heart of the matter is variability, in a small area of damage might require an mation that might eventually carry out an
terms both of the materials used and of extensive repair because of the need with a entire repair cycle encompassing damage
technician competence. Because composites scarfed joint to ‘step out’ from the damage detection, surface preparation, repair patch
are, by definition, a mix of different materials site in order to achieve the required creation, patch application and finally
drawn from an ever expanding palette of through-thickness chamfer gradient. Techni- quality assurance checking. Partners in
possibilities, precisely matching the original cians will need considerable training and this programme to develop repair robotics
lay-up in the repair might not be pract- practice before they have the skill and include Lufthansa Technik, Eurocopter and
ical. Maintenance organisations are rarely finesse required to prepare and carry out Cassidian Air Systems.
equipped to manage the diverse order lead scarfed repairs that are both strong and
times, storage requirements, processing leave an aesthetically pleasing flush surface Meanwhile, the German Aerospace Research
regimes etc to do this. At the same time, finish. Many believe that, to convince the Centre DLR has been investigating the
repair technicians vary widely in their back- airworthiness authorities of the efficacy of automation of resin infused repairs. The
grounds, experience, training and aptitude bonded repairs, training will have to be aim is to develop scarf repair capability
for the work. And, while repair shops must accompanied by a system of certification of including damage removal by computer-
be certified to carry out composite repairs individual repair technicians. controlled milling, impregnation of a dry

32 REINFORCEDplastics May/June 2012 www.reinforcedplastics.com

 FEATURE

preform laid into an excised site, and

subsequent cure. DLR claims the method
is particularly appropriate for curved areas,
reducing complexity and avoiding the need
to produce special tooling.

Other companies are focusing on particular

parts of the repair cycle. Laser specialists
cleanLASER and SLCR, also in Germany, are
separately working on systems to prepare
repair sites. CleanLASER’s bond surface pre-
treatment for carbon fibre composite is said Japanese airline ANA has become the first commercial operator of the B787 Dreamliner, which has a
to result in superior bonded repairs without composite fuselage. (Picture © Boeing.)

recourse to abrasives or chemicals, a laser-

based optical machining system being used
to remove damage ply by ply. SLCR, mean-
while, is working with UK concern GKN
Aerospace to develop its own automated
facility for preparing a repair site.

GKN Aerospace’s John Cornforth, Vice

President Technology, describes their laser
ablation solution as a non-manual, contact-
free process able to remove material from
a repair site by ablating away the resin,
leaving brittle fibres that can be brushed
out afterwards. Damaged material is
removed forensically, leaving a site prepared
to precise dimensions for a tailored replace- Fuselages are typically monolithic wound or tape laid structures with various thicknesses and may have
ment plug or patch. No force or vibration complex curvatures, as in this B787 nose section. This constitutes a new level of challenge for composite
repairers. (Picture © Boeing.)
is applied to the structure during material
removal so there is no adverse effect on
its strength or integrity. Cornforth reports
promising results from the programme’s
first phase, in which the partners produced
prototype equipment able to prepare a
repair site. This prototype is now installed
at GKN Aerospace’s Composite Research
Centre at Cowes, Isle of Wight, UK.

A second programme phase, now under

way, will see the equipment developed
further to cope with more complex damage GKN Aerospace’s prototype laser ablation machine used for repair site preparation.
sites where there may be underlying
obstructions, such as frames and stringers. linked to CAD imagery of the original part those produced by HEATCON Composite
The laser will be able to prepare sites for so that pre-manufacture repairs could be systems and Zymac Fabrications. The latter,
stepped or constant angle scarfed repairs, prepared. Various non-destructive evaluation for instance, has produced heating blankets
taking account of these features. (NDE) technologies could be included for that incorporate separate small thermal cells
rapid damage detection and assessment. so that, irrespective of what structural heat
Cornforth told Reinforced Plastics that, sinks lie behind a large repair site, more
although the technology is in its infancy, Another area ripe for technological advance even heating is provided throughout the
he envisages the eventual incorporation of is the heating and cure part of the cycle. repair material. Meanwhile, HEATCON has
a mix of repair capabilities within a single One particular focus is the further develop- developed a stretchable heating blanket for
automated cell. Laser ablation would be ment of conventional hot bonders, such as use around curved surfaces.

www.reinforcedplastics.com May/June 2012 REINFORCEDplastics 33

 FEATURE

used on metal aerostructures are apt to material to reveal defects within the
attack not only the paint, but also the laminate. The partners aim to embody a
underlying composite. Paint manufacturers scanning camera system within a practical
have sought to counter this with finishes mobile NDE tool.
that include between the primer and top
coats an intermediate layer that will dissolve A solution that combines monitoring
in mild strippers that do not degrade either with the induction heating of composite
the primer coat or the composite. Even so, repairs has been the subject of a European
where damage has occurred, repair sites Union funded programme called INDUCER
will still require manual sanding. (Induction Heating and Health Monitoring
Solutions for Smart Aircraft Maintenance
Once more, however, lasers might offer Using Adapted Composite Patches). Within
a solution. Lufthansa Technik has been the programme, TWI (formerly the Welding
Multi-axis articulating robotic machines like this, investigating the use of a laser on a control Institute in the UK) and GMI Aero have
currently being developed for manufacturing head that moves over the fuselage surface. collaborated on the concept of smart
operations, could be adapted for automated repair
use. (Picture courtesy of Coriolis Composites.) Modern lasers, says LHT, can strip paint patches in which a magnetostrictive sensor
from large areas at hitherto unachievable mesh embedded within the repair lay-up
rates while recognising which layer is being serves both to heat the repair for cure
removed. This raises the prospect that purposes and, in later monitoring mode,
future robotic systems will be able to strip to react magnetically to stresses. Thus, it
anything from a modest repair site to an is likely that stresses incurred during cure
entire composite aircraft. Although LHT has and then the on-going structural integrity
been obliged to put this work on hold for of repairs during service can be monitored
the moment due to a company decision using the single mesh ply approach. .
to close its Hamburg painting facility, the
Premium AEROTEC manufactures fuselage panels stripping concept is still promising and may Other developments address the difficulty
for the Airbus A350XWB using MAG tape laying be taken up elsewhere. of knowing, should damage occur, whether
machines. Basing the fuselage on joined panels
rather than wound barrel sections is said to confer the affected laminate is ‘clean’ or has been
a maintainability advantage since, in the event of contaminated, for example by hydraulic
major damage, it might theoretically be possible Detecting the damage
fluid or fuel. Even slight contamination can
to replace a panel. (Picture © Premium AEROTEC
GmbH.) A particular issue with composites is the seriously impair a bond, and a number of
difficulty of establishing, after an impact, companies, including Lufthansa Technik, are
whether the laminate has suffered damage working to develop reliable contamination
Alternative heating methods, such as without there being any sign of it at the detectors.
induction heating and incorporation of surface. Unlike metals, which show they are
heating elements within composite repair ‘hurt’ by exhibiting dents, composites can Still, technology cannot provide all the
patches, are under investigation. Ground spring back from low-energy impact such answers. Probably there will always be times
support specialist Sunaero Aviation Inc, for that damage several plies down within the and instances where skilled manual skills
instance, is developing a hand-held infrared laminate can be hidden behind an appar- are required, placing a premium on effect-
device intended to speed up repair cure ently unharmed surface. ive training and hands-on practice. Nor
without subjecting the material to exces- will the need for education apply only to
sive heat. A version being developed for Reliable and speedy detection of damage composites technicians. It will, for example,
carbon fibre composite could supplant the on the flight line will require compact, be important to impress on both air and
use of heating blankets in certain applica- portable NDE equipment. Boeing has ground crews the importance of reporting
tions. Other possibilities include the use of been developing a hand-held ultrasonic any accidental contacts, no matter how
microwave-coupled heating blankets and ramp damage checker which, according apparently slight, between the fuselage and
repair patches that incorporate a conduc- to the company, will detect sub-surface other objects so that the impacted area
tive mesh so that the patch itself can be laminate damage with sufficient clarity to can be inspected for damage. In terms of
heated directly. enable technicians to say whether or not ground personnel, this will be difficult as
an aircraft can be cleared to fly. Similarly, there will always be drivers of service vehi-
A further technological focus is that of Norwegian ultrasound specialist DolphiTech cles in far corners of the world who neither
managing paint finishes. Removing paint is working with EADS Innovation Works on know nor care whether the fuselage is
from large composite surfaces is tricky a hand-held ultrasonic sensor said to be composite or metal and may fail to report
because the chemical strippers routinely able to see through 8-10 mm of composite ‘bumps’ that result in no visible damage.

34 REINFORCEDplastics May/June 2012 www.reinforcedplastics.com

 FEATURE

For the present at least, uncertainties are hundreds of aircraft with composite at altitude. The dreadful consequences
remain. A big question is what happens fuselages in service. of this became all too evident to British
when a major repair is needed that is planemaker DeHavilland at the dawn of the
outside the scope of the SRM? Almost The airframers expect that the evolution of passenger jet age more than half a century
inevitably, this will be an aircraft-on-ground repairs, along with the associated infra- ago. In that case it was metal fatigue that
(AOG) situation and therefore time critical. structure, will be sufficiently rapid to keep led to the catastrophic loss of several
One can be sure that, in the early stages of pace with fleet growth. Only time will tell of DH’s boldly conceived Comet aircraft.
a new aircraft’s service when overall fleet if this view is justified. Clearly, the move to Admittedly, a composites failure is likely to
size is small, airframers and their partners composite fuselages for commercial jets be more progressive and less catastrophic,
will be highly proactive in supporting is a bold one that should reap dividends though with high-flying pressurised fuse-
operators which face this situation. They through reduced aircraft weight and fuel lages one can never be quite sure.
will bring all their forces to bear in fielding usage – but only if maintainability risks can
repair schemes as rapidly as possible. be managed successfully. Fortunately, risk management has moved
on since the Comet days. Nevertheless,
Independent maintainers will, ideally, be minimising risks associated with fuselage
Biggest risk
given open access to the manufacturer’s repairs will, over the next several years, be
stress and other calculations as well as Arguably, the biggest risk of all is that the one of the most significant challenges for
the relevant experts, so that they too structurally sound fuselage that emerges composites. Clearly, it behoves all concerned
can develop viable repairs. No doubt the newly built from the aircraft factory can – particularly airframers and their partners,
communication channels between main- have its integrity progressively degraded aircraft operators, airworthiness regulators
tainers, the design authority (airframer) and over a history of successive repairs. No-one and maintenance/repair organisations –
perhaps the airworthiness authorities will knows how many repairs of various types a to tread carefully, maintain open lines of
be relatively unobstructed in these early fuselage can sustain before its strength is so communication and collaborate closely so
stages. But this situation could change compromised that there is a risk of failure that the high reputation of this remarkable
as fleet size builds until, eventually, there – at worst an explosive decompression class of materials is maintained. ■

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