Composite repairs in the future

“Composite repairs now and in the future.”

(Reading time 10  minutes)
In September 2011, the US Government Accountability Office (GAO) released a report on the “status of the FAA’s actions to oversee the safety of composite aeroplanes” (GAO-11- 849 Aviation Safety). The study was necessary, it said, because “although composites are lighter and stronger than most metals, their increasing use in commercial aeroplane structures such as the fuselage and wings has raised safety concerns”. GAO, an independent agency that works for the US Congress and is mandated to investigate how the US federal government spends its money, released the composites report shortly after the certification of the Boeing 787 Dreamliner by the FAA and EASA in August 2011.

With regards to the FAA, GAO found that the organization had “followed its certification process in assessing the aeroplane’s composite fuselage and wings against applicable FAA airworthiness standards.”And had “applied five special conditions when it found that its airworthiness standards were not adequate to ensure that the composite structures would comply with existing safety levels”.

 

For its part, Boeing declared itself “pleased that the GAO report demonstrated confidence in the FAA and in the process through which the FAA certifies the safety of commercial aeroplanes”. A company spokesperson comments: “Regardless of the materials we use,

Boeing employs the same rigorous methods to deliver safe products for the flying public and is efficient for airlines. They test, analyze and demonstrate to themselves and the regulatory agencies that even in extreme conditions which never is experienced in an entire life of service, the aeroplane is safe and durable.”

Such safety approval” provides total reassurance in a world in which composite materials are be- Shortly after the heavily composite Boeing 787 was certified, a US government report looked into the safety concerns surrounding the use of composite materials coming increasingly crucial in aircraft manufacturing.

Note that earlier Boeing and Airbus aircraft are approximate.

Years ago, composite on Aircraft contained 10 to 15 % built of composite materials in terms of their total structural weights, the A380 approximately 20 to 25 % composite materials”.

The Boeing 787 Dreamliner and the Airbus A350 are approximately 50% to 55 % constructed of composite materials, including aircraft surfaces like fuselage barrels.
The GAO report also identified four central safety-related concerns with the repair and maintenance of composites.

These their limited information on the behaviour of aircraft composites structures, standardization of repair materials by the SEA CACRC task groups where owner /founder of EFC and this website is one of the founders of in 1991 ongoing till today like training and awareness; and technical issues related to the unique properties of composite materials.

These issues provide an exciting insight into the future of composite repairs, particularly the proficiency that needs to be built in the medium- to long-term to address them fully. GAO did not recognize these safety issues as posing extraordinary safety risks or as being insurmountable. Boeing points out that because composite materials have been used in commercial Aircraft for decades, albeit less intensely, there is already much repair expertise in the industry.

These issues are already being addressed through an industry-wide effort involving regulators, manufacturers, operators and maintenance and repair organizations,” the Boeing spokesperson adds. “This is a great example of how all stakeholders
work together to ensure safety continues at today’s high levels.”

Limited information

The GAO report admits that its concerns over the limited information on the behaviour of composite structures as they age or when they are damaged “are partly attributable to the limited in-service experience with composite materials used in the airframe structures of commercial aeroplanes”.

Herefore, “less information is available on the behaviour of these materials than on the behaviour of metal”. The report suggested that “more empirical data would help better predict the behaviour of damaged composite structures through more robust models or analytical methods”. Reliable damage behaviour predictions are vital as they help form the basis for a new aircraft’s design or maintenance programme.

The FAA has already issued an aircraft fatigue damage rule intended to help address “concerns related to limited information on how composite structures age and fatigue”.

The regulation Aging Airplane Program “Widespread Fatigue Damage, 75 Fed. Reg. 69746 (2010) require that all manufacturers take a proactive approach to managing risk related to widespread fatigue damage and demonstrating the validity of a structural maintenance programme.

Maintenance by test or service experience, to reduce the FAA’s current practice of issuing airworthiness directives after an incident,” according to GAO.  As a result of attempts to increase the predictability of composite structures’ behaviour, additional regulatory requirements are on the horizon.

“Imagine that the FAA is going to mandate more and longer testing on the static fatigue testing aircraft used for certification similar to the already performed fatigue testing requirements, but under more stringent rules, including expanding non-destructive testing (NDT) on the fleet leader aircraft,”
Another method for achieving more significant industry-wide availability of data on the behaviour of ageing and damaged composite structures might be the implementation of a composite behaviour data sharing and database development initiative as part of an industry collaborative decision-making effort. But this option faces considerable limitations.

The two main players are only going to share data when local authorities mandate it. Advanced know-how in composites manufacturing and behaviour represents a competitive advantage in the market and therefore sharing would not be very much appreciated by the manufacturers,”

“The operators will accumulate most of the information anyway as usual; the question will be how this information is:

• captured, funnelled back,
• validated,
• standardized
• and analyzed

for the most accurate results, to extrapolate the most accurate potential future results for the damage and fatigue behaviour of the surveyed aircraft structures.”

Limited standardization
GAO also reported that “composite materials and repair techniques are less standardized than metal materials and repairs”.
This is due, in part, to business proprietary practices and the relative immaturity of the application of composite materials in airframe structures.

As well as a repair technician potentially confusing materials or processes, which may result in improper repairs.

The GAO report cautioned that “less standardization can have a negative economic impact for airlines and repair stations because a repair facility might have to keep a large stock of repair materials and parts in house, which creates an inventory and storage challenge.
Composite materials generally need to be stored at a specific temperature, and the materials also have a shelf Live and expiration dates.

These issues are being addressed by the Commercial Aircraft Composite Repair Committee (CACRC), whose stated mission is “to reduce the cost of maintaining composite structures through standardization of materials technique and training”.

Earth & Flight Composites BV. (EFC) owner Bert Groenewoud was one of the founders of the CACRC  while working at the KLM training dept. set up the first task group meeting in 1990 with initiator Henk Lodewijck (former KLM Composite and Metal bonding Engineer KLM Engineering department

Together with KLM engineer Henk Lodewijck who arranged the sponsoring with ATA, IATA, we set up the first (CACRC) Commercial aircraft Repair Committee meeting in 1990 in Denver, Colorado. The first meeting ever was hosted by Richardt. Barret Engineer of  Continental Airlines

The Engineering members at Continental Airlines then developed the first training curriculum in the task group Present where Boeing members, Airbus members many Airlines, KLM/Airfrance, DELTA, and MRO stations and training centres.
All active committee members and some still attend meetings writing reports that a standardization effort supported by CACRC can significantly benefit the MRO industry.

During the pandemic, the training task group and other CACRC task groups have virtual task group meetings using  Webex or Zoom.

“An industry-standard guideline would decrease the amount of time necessary to obtain regulatory approval for repairs,” says a CACRC spokesperson. “This would then reduce the cost to an airline or MRO.

The” CACRC’s ‘Repair Technique Task Group’ has been  active in the development of standard repair process documents like AIRs (Aero Space information Report) and ARPs Aerospace Recommended Practises) from current best practices, and a number of these documents have already  been produced and approved by the all tasks groups members from Airlines or MROs like KLM Air France, Delta Airlines, manufactures like Boeing, Airbus by EASA in 2021

Another CACRC task group is the ‘Analytical Design Group’, which is active in developing a standard repair design and analysis document, “a guide of generally accepted stress analysis methods used for the design and evaluation of composite repairs for approval submission”.  “The document intends to provide best practices for the development of a repair to be submitted to an airworthiness authority,” says the Boeing spokesperson.

“This will be beneficial to aircraft maintenance and repair because it will provide a better understanding of repairs design requirements to the composite structure.”

The problem of limited standardization has also been, in part, proactively addressed at the aircraft manufacturing level. IFor example, in the Boeing 787 Dreamliner case, the Aircraft was “designed from the start with the capability to be repaired in the same manner that airlines would repair an aeroplane today with bolted repairs”.
Boeing states: The ability to perform bolted repairs in a composite structure is service-proven on the Boeing 777 and offers comparable repair times and skills as employed on metallic aeroplanes. In addition, airlines can perform bonded composite repairs, which offer improved aerodynamic and aesthetic finish. These repairs are permanent, damage tolerant, and do not require an autoclave.

 

Figure 3 carbon fibre Step Repair

While a typical bonded repair may require 24 or more hours of aeroplane downtime, Boeing has taken advantage of the properties of composites to develop a new line of maintenance repair capability that requires less than an hour to apply. This rapid composite repair technique offers temporary repair capability to get an aeroplane flying again quickly, despite minor damage that might ground an aluminium aeroplane.” 

Level of training and awareness
The GAO study’s training concerns whether repair technicians receive sufficient training and whether all those who come into contact with composites “are aware of and can appreciate the differences between metal and composite materials”. This issue is being addressed by the CACRC’s ‘Training Task Group’, which has developed standard curricula for technicians and engineers and is still under development for  NDT inspectors.
The curricula, AIRs and ARPs first developed in the mid-1990s” are reviewed every five years by the task groups and new ones balloted by all members.
The training task groups’ goal to modularise the current curriculum into manageable training blocks is already working.  The first is a primary repair training curriculum that is non-equipment specific and can be used to meet initial training requirements.  This developed curriculum document is the AIR 4938 Part, Part 2, Part 3, Part 4 and is used as the base for a composite repair training programme.

We have drafted a qualified cation/certification standard to give training providers a method to issue an industry-accepted certification of basic repair skills, knowledge, and abilities. The new document would refer to the curriculum document.”

The items developed are an advanced course curriculum that adds to the basic skill set, a metal bond repair curriculum, and a curriculum guide for the hardware-specific repair in an OEM’s structure repair manual (SRM) or other repair documents.

Airbus and Boeing are already implementing the AIR4938 curriculum, and Aerospace Recommended Practises in their manuals like the SRM.
The CCRC’s curricula appear likely to receive the endorsement of a relevant and influential regulator since “the task groups work closely with the FAA and European EASA concerning the availability of personnel competent enough to deliver the training on the contents proposed in these curricula, Boeing’ spokesperson says that “there are several training providers who would be able to provide the basic composite repair certificate training.

However, an industry-accepted basic repair certification would reduce the time required to train new employees.” On the differences between traditional metals and composites. In many cases, mechanics and line maintenance engineers still lack the necessary skills”, with “proficiency in this discipline not yet that well developed” and draws attention to a specific example. Most damages on Aircraft occur at the Ramp at  Airports by personnel and in-flight like hale or birdstrike.

Composite aircraft encounter lightning strikes quite often record small dark holes on the skin, and these need to be repaired as soon as possible; otherwise, there is an increased risk of moisture infiltration,” he says. “The problem is that even pilots are conducting. Ramp checks may not have the competence to detect such damages as threatening. Thus, there is an increased need for the know-how of behaviour of composites at the maintenance side of the business as well as in the piloting side.”

Technical concerns
Another category of concern raised in the GAO report relates to the “challenges in detecting and characterizing damage in composite structures, as well as making adequate composite repairs”.
The study noted that impact damage to composite structures is unique. It may not be visible or barely visible, making it more difficult for a repair technician to detect than damage metallic structures.
A long term solution could involve applying evolved protective skins.  The US National Aeronautics and Space Administration (NASA) is funding a research programme whose goal is “developing potential concepts for protective skins that enable natural laminar flow and a significant weight reduction in the aircraft’s primary structure”.

A protective skin is needed to absorb impact damage and to provide environmental protection. The NASA contract led to the development of the ‘STAR-C2’ concept. It should be responsible for smoothing out bumps or gaps, providing thermal insulation, absorbing impact and acoustic energy, reflecting ultraviolet and infrared radiation, conducting large amounts of electrical current (for lightning strike), and providing a decorative or appealing surface”.
Composite structures are currently “overdesigned” to handle impact and hot, humid conditions in addition to carrying the loads. “On the inside of the fuselage, acoustic (and sometimes thermal) insulation is present.

The outside of the fuselage uses filler and fairing to provide a smooth and cosmetically pleasing surface. A surface and a layer of lightning strike protection material and paint provideThey have developed a “nano stitching” technique to make composite aircraft skins stronger, reinforcing carbon fibres’ plies with nanotubes aligned perpendicular to the plies. “Because the nanotubes are 1,000 times smaller than the carbon fibre, they don’t detrimentally affect the much larger carbon fibres, but instead fill the space around them, stitching the layers together”, reports the MIT.

This technique advantages currently used in advanced manufactured composites. It allows composites to be one million times more electrically conductive than their counterparts without nanotubes, thus granting greater protection against damage from lightning.
An additional advantage is in the area of damage detection: composites reinforced with nanotubes “can be subjected to infrared thermography without the need for an outside heat source”.

When a small electric current is applied to the surface, the nanotubes heat up. It means that abnormal heat flow is then clearly visible to an inspector equipped with a thermographic camera or goggles and a simple handheld device to supply the electric current, the MIT states.

The future of the MRO Industry
The increased presence of OEMs in the aircraft aftermarket business could lead to more consolidation of the MRO industry. In turn, it will impact the future of composite repairs. “
In the coming twenty years, a further concentration of the MRO industry is expected similar to the current attention being witnessed with airline operators,” “New skills will be demanded from maintenance personnel, whereas before sheet metal repairs where common trades.

We, as a training centre, convert Sheetmatal technicians partly into Composite repair technicians
The original sheet metal technician has an advantage over a composite technician in removing  fasteners  from composites, drilling in composite parts, attaching repair patches and doublers to the Aircraft

Composite repair skills are already in high demand, with all the necessary background education such as:

• bonding carbon,
• Prepregs Repairs,
• tooling fabrication,
• know-how to apply vacuum
• heat treatment techniques for curing of repairs
• more NDT skills using a ramp damage checker,
• Ultrasonic devices like the Dolphicam
• Tap hammers steel and aluminium
  Click this link https://www.efcomposites.com/product/b-aluminium-tap-.
• Tap hammer instrument the Woodpecker

“This change will demand that primary maintenance personnel’ education will need to address this. The aircraft manufacturers need to have to help in developing field repairs for minor repairs. When it comes to major repairs, this could be not easy and either only possible for the aircraft manufacturers or huge organizations that can afford to build up the skills and invest in the tooling. Aircraft repairs will become more expensive, as they always do, and the labour cost will be under even more pressure.

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