The headaches that accompany new aircraft materials are nothing new to the maintenance industry. In the Battle of Britain, the all-metal Spitfire outperformed the Hurricane, but the latter could be repaired much more easily because of its traditional wood-and-canvas construction and, as a result, could return to service more quickly.
Today, operators are not willing to pare every last gram of weight from an aircraft just for the sake of building it with a new material; damage resistance/tolerance and repairability will be of primary interest. The use of composite is not always better and such materials may be used more for fashion reasons than engineering ones.
Aircraft must now work hard for their living and excessive downtime for repairs or parts replacement leads to vociferous complaints. Manufacturers, however, are much more attuned now to their customers' needs so that parts and structures made in advanced materials are less likely to need frequent or difficult repairs. Also, repair techniques have improved, particularly for composite materials. But there is still room for improvement in the selection of materials with maintenance procedures and costs in mind.
Jens Hinrichsen, director of structures on the A380 at Airbus Industrie, says that airlines complain about difficult-to-repair honeycomb structures.
"Our intention is to drop the use of honeycomb where we can," he says. "There is a move to solid, laminated structures." On Airbus types honeycomb is found in the belly fairing, the leading edge of the fin and tailplane, plus landing gear doors - about 2.5t of honeycomb per aircraft.
"We don't take any decisions on materials or design without the airlines' agreement," says Hinrichsen. "We are in dialogue with the airlines about where we can replace honeycomb with laminated structures and where we will still use honeycomb."
The belly fairing is prone to damage because of debris thrown up by the landing gear. But Hinrichsen says that it's not possible to build a lightweight fairing with anything other than honeycomb. "There is no alternative at the same weight," he says. Moisture ingress is really caused by design faults, but was especially a problem with aramid fibres, so they are no longer used at Airbus. "We had strong requests from the airlines not to use aramid composite any more," says Hinrichsen. "It was used in blade containment structures, engine pylon fairings, radomes, etc. For Airbus now it's gone."
Carbonfibre solid laminates account for most Airbus composite structures - 16% by weight in the A340 and A380. It might reach 20% in later types, but no more because advanced aluminium and titanium alloys will give stiff competition. The all-one-material aircraft will remain a dream because different materials work better in different areas.
In designing repair schemes, manufacturers and maintenance staff have to consider the skills that repair people already have. It is no use coming up with a highly effective repair if the workforce does not have the skills needed to implement it. Airbus has a maintenance staff training programme using experts from partner companies who all meet in Toulouse.
Airbus approves riveted repairs of composite structures as both temporary and permanent solutions. Airline maintenance staff can easily rivet stainless steel, or an aluminium or titanium alloy patch. Bonded repairs are better for cosmetic fixes and this can be achieved with heating below 80°C and without an autoclave. Repair schemes needing an autoclave are best avoided because even if a repair shop has one, the structure may not fit inside it. Carbonfibre wing tanks cannot be repaired with metal items in case there is a lightning strike and it follows the metal items into the tanks.
Hailstones, birdstrikes and debris from the runway all cause damage. Attaching leading edges in sections allows one section to be changed, if it's badly damaged, or patch repaired otherwise.
Cesar Bautista, working on empennage design at Airbus, says that a proportion of thermoplastic is being put in the matrix of some carbonfibre composites to improve impact behaviour. About 10-12% of the matrix is thermoplastic in small bubbles. The rest is epoxy. This gives better damage tolerance because the thermoplastic absorbs energy well and halts crack propagation. After a 50-joule impact, a structure must still withstand the ultimate load which it was designed to take. The thermoplastic additive helps.
Pure thermoplastic resins may be used. "We've having a thermoplastic wing D-nose, " Hinrichsen says, "below the high lift devices on the wing leading edge on the A340-600. Repair is easy with the application of heat." Airbus is going for PEI thermoplastic from Ten Cate.
Glassfibre is used for RF transparency. It is also used to isolate carbonfibre parts from metal ones to avoid galvanic corrosion. Aircraft radomes are often made of glassfibre reinforced epoxy and repairing them can be tricky, because there are conflicting structural and electrical requirements. Some companies which specialise in repairing radomes which have a tendency to suffer grounding damage because they are often on the nose.
The A380 will have carbonfibre upper floor beams, flaps, elevons, spoilers and fairings. The next step may be carbonfibre flap tracks, and Airbus is making a final decision on this soon. The A400M will have a carbonfibre wing box. A composite wing box is easier in such a design because the landing gear is not attached to the wing, so the landing loads do not go through it.
All-carbonfibre wings may appear on civil aircraft in 10 years, says Hinrichsen. Resin transfer moulding will have to be adopted - Airbus has produced a tailplane demonstrator for the A380. At 260m² (2,800ft² ), the tailplane is the size of an A310 wing.
Boeing has been using composite materials on its aircraft for more than 25 years. The 777, introduced into commercial service in 1995, has the largest application of composite materials - about 10% of its structural weight. This is a significantly higher percentage than on any other Boeing model.
"With performance and life cycle cost benefits in mind, it is certain that more composites will be used. But metals may still be the best material choice for some applications.
"Technology improvements with new alloys and manufacturing techniques continue to be made. Boeing is committed to producing aircraft that reduce total operating costs and increase revenue for airlines. Any material or structural technology, including composites, must provide the aircraft owner with value, safety, performance and durability prior to implementation," says Boeing.
"Reductions in raw material and manufacturing costs are needed to achieve breakthrough reductions in total life-cycle costs; maintenance-related issues of new applications must be addressed if airline acceptance is to be gained. And advances in technology must be production ready."
Composite structures are often formed by putting down one layer of material on top of another (laying-up). This makes material storage and handling easier, but can lead to a failure mode not seen in metal structures - delamination. Each laminate may be very strong, but the material's limit will be determined by the strength of bonding between the layers.
Laminations can come apart in severe service or conditions. About 10 years ago, Boeing developed and implemented a resin injection method of repairing delaminations. This technique augments bolted repair approaches and can repair delaminations that would ordinarily cause scrapping of the hardware. The approach is for repair of delaminations caused by impacts, routing, drilling, and fastener installation/removal in cloth and unidirectional reinforcements of carbon, glass or Kevlar.
It involves drilling holes at appropriate locations and injecting resin to fill the delamination. Boeing says this method is superior to vacuum and syringe techniques, and can be used for almost all delaminations regardless of size or location.
Lufthansa Technik supports both Boeing and Airbus aircraft. Michael Witt, manager customer support, airframe-related components, says that sometimes airframers make structures and parts in advanced composites without considering the increase in part or repair costs.
He compares, for example, the radome of the A320 which is made of aramid fibre reinforced plastic with that of the Boeing 737, which is glassfibre. They are similar in size. However, the A320 radome is 600 grams lighter and nearly five times more expensive than that of the 737. Lufthansa's support of its fleet of A320s, results in additional spares costs of $320,000 and repair costs of $20,000 per year - hardly worth the weight reduction.
Witt thinks that some original equipment manufacturers' keenness for weight saving at any price comes from a lack of understanding of costs, and the historic expectation that fuel prices would soar in real terms. Fuel prices were high until 1985, but have fallen to pre-1978 levels. Expensive fuel would make airlines very weight conscious. Boeing chose not to use composites extensively in the 747-400, which Witt believes was a wise decision.
Another problem is the large number of different branded materials that have to be kept and their short storage lives. Some aircraft may have more than 200 adhesives, prepregs and films. These have to be refrigerated and, even then, shelf life may only be three months. Much more standardisation of composite materials is needed.
Damage tolerant designs are particularly desirable in short-haul aircraft which spend a much higher percentage of their time on the ramp where they might get hit by jetways, towing vehicles, etc. In addition, their under-wing and under-tail heights are less, which also increases the probability of collisions.
An impact that might put a minor dent in a metal structure might crack a composite one, and require replacement. In addition, damage to composites can be less obvious on the surface than it would be with metals. So composite structures often need more frequent non-destructive testing/evaluation (NDT/E).
In the air, hail can cause elevator composite skins to be written off because it can penetrate the outer layer, rebound off an internal one and tear the first skin outwards, beyond economic repair.
Fan cowl doors can suffer overheat damage from engine hot air leaks and, again, composite ones may be worse affected than metal ones. Witt says that four metal doors were required as spares to support the 100 which Lufthansa had in service, whereas six composites ones were needed to support 52. In the mid-1980s, a metal door cost $30,000 whilst a composite one cost $100,000. Today, the composite doors cost over $200,000.
Repairs to composites can also take much longer than to equivalent metal parts. Repairs to metal structures which take days can run to weeks for composite parts, and their repair costs can be up to seven times as high.
It is ironic that composite parts are often found in high damage areas. A study by Witt showed which parts are most likely to be damaged and it found that composites are often used in areas such as wing tips, leading edges, trailing edges and wing-to-body fairings.
Sandwich construction leads to different composite materials being joined. If they have different coefficients of thermal expansion, heating and cooling will inevitably cause microcracking and this absorbs water. If the structure were then to be struck by lightning, the water could vapourise and explode.
Microcracks can also be induced in a composite structure when it is physically stripped of paint by sanding or blasting. Chemical strippers cannot be used, however, because they would react with and damage the composite. So stripping a composite component can take up to five times as long as a metal one. Yet paint has a higher coefficient of thermal expansion than composites, leading to crazing and a more frequent need for repainting.
The big advantage of composites is the major reduction in corrosion they offer and their virtual immunity to fatigue. Several composite task forces have been set up, and they resulted in SAE/AE-27:27, a document which provides guidance on producing durable, repairable and maintainable composite structures.
Sometimes old materials are used if service experience warrants it. For example, new Boeing 737s have aluminium alloy inboard flaps, replacing the composite ones of earlier models. These items are more likely to suffer foreign object and tyreburst damage and the metal ones survive better. Similarly, the A340-500 and -600 have aluminium inboard flaps.
However, these aircraft do have thermoplastic/carbonfibre fixed wing leading edges (behind the slats). The Boeing 777 has an all composite empennage and passenger floor beams. In today's cost-conscious world, new materials must buy their way onto an aircraft.
Source: Flight International