AIRCRAFT
Safe operations with composite aircraft
Composite materials are increasingly used in aircraft design. The A350 XWB is the most recent illustration of this trend. Yet if the benefits of composite materials are not in doubt for airlines, some questions still remain as to their potential effects on safety.
Why composites in the first place?
The large increase of the fuel cost in the early 70s challenged aircraft manufacturers to improve considerably the fuel efficiency of commercial airplanes. This quest led designers to progressively replace aluminium by composites, as they typically weigh 20 per cent less than aluminium for an equivalent function. This aircraft weight reduction, in turn, led to a lower fuel consumption.
What are composites?
Composites are a particular kind of plastic. The majority of plastics in the world today are pure, and may be used to make things like toys or mineral bottles.
When additional strength is needed, plastics are reinforced with fibres and become comp, broadly known as reinforced plastics. The reinforcing fibres, or fabric, provide strength and stiffness to the composite, while the plastic resin, or matrix, gives cohesive properties, stability and environmental resistance.
In today’s aerospace industry, most applications use carbon as reinforcing fibres (fig.1). They are referred to as Carbon Fibre Reinforced Plastics (CFRP).
Fibres reinforced plastics are usually made into laminates, i.e. layered sheets. A layer or ply is made to the specified size and orientation and then more layers are added, playing on the orientation until the piece has the properties it needs to support the loads it will carry.
Reinforced plastics may be in monolithic or sandwich form. Monolithic if they are solid and sandwich if the laminate sheets are separated by a core of different material type, usually honeycomb or foam. (fig.2)
Heavily loaded structural components are usually monolithic, while lightly loaded fairings, interior components are usually of the sandwich type.
(fig.1)
Carbon/Epoxy Impregnation (Prepreg)
(fig.2)
The two main concepts of composite: sandwich and monolithic
Are there safety implications to the use of composites?
We are going to consider this question by looking at safety from three different perspectives:
1 – The certification process
Composite aircraft are certified according to the same rules as their conventional counterparts.
Since composites show, in some respects, a different behaviour when compared to metallic materials, the Airworthiness Authorities have developed new Acceptable Means of Compliance (AMC), allowing manufacturers to demonstrate that these
new materials meet the existing safety requirements.
In line with a conservative Airbus policy on the introduction of new technology, the manufacturer has carried out numerous tests, taking into consideration even the most exceptional scenarios, thereby exceeding the requirements defined in the above AMCs by a large margin.
2 – The behaviour of composites in the face of operational threats
In the course of its lifetime, an aircraft structure is exposed to a certain number of threats. Let us consider the main ones and the means put in place on the A350 XWB to mitigate these threats:
- Lightning strikes
Full scale tests have demonstrated that the A350 XWB fuselage is protected against lightning strikes. This is achieved by a proper dimensioning of the structure and by compensating the lower conductivity of carbon fibre composites by integrating a metallic mesh. (fig.3 and 4)
(fig.3)
Metallic mesh, general view
(fig.4)
Metallic mesh, close-up view
The typical lightning strikes led to no more than clearly visible burn marks and paint scrapes, as illustrated in figures 5 and 6. For very severe lightning strikes, despite more extensive damage, no detrimental effect was witnessed on the fuselage integrity.
- Bird strikes
Safe flight continuation was proven through residual strength analysis carried out after dedicated tests.
The objective was to verify the resistance of specific parts of the structure to damage caused by bird strikes. The most exposed composite areas are the radome and the leading edges of the wing and horizontal/vertical tail planes.
During these tests, damage from bird strikes was acceptable on secondary structure like aerodynamic fairings or leading edges but without detrimental effect on any load carrying primary structure.
One of these tests consisted of projecting an 8 pounds bird against the leading edge of the horizontal tail plane at a speed of 330 kt. The test demonstrated that the damage was limited to the leading edge, while the spar was unaffected by the collision. (fig.7)
(fig.5)
Impact of typical lightning strike (scale in centimetres)
(fig.6)
Impact of very severe lightning strike
(scale in centimetres)
(fig.7)
Damage to horizontal tail plane after bird strike test
- In-flight hail
The risk of in-flight hail was mitigated through design precautions, mainly by increasing the thickness of the structure on the most exposed areas, like the nose cone of the aircraft.
- Uncontained engine failure
A range of high velocity impact tests were performed on fuselage barrels to simulate the effects of an uncontained engine failure. The tests demonstrated no detrimental damage and no dynamic effect on the fuselage.
- Fire
Fire, Smoke and Toxicity requirements (FST) are applied for all aircraft interior elements. Composite materials are common to both structure and cabin, they therefore also have to fulfil the same smoke and toxicity requirements. Concerning the resistance to fire, it is interesting to note that CFRP is auto-extinguishable and that the thinner composite fuselage skin is more “burn through” resistant than a metallic equivalent.
- Corrosion
On their own, composite parts do not corrode and do not require specific protection against corrosion, while aluminium structures require continuous inspection and re-protection.
The risk of galvanic corrosion, which exists when composites are in contact with metal, has been mitigated on all Airbus programs by paying attention to the choice of metallic elements and by taking associated design precautions. Aluminium rivets for example, were replaced by titanium on the fuselage.
- Fatigue
Whereas aluminium structures require very specific attention, the composite structures do not require inspection for fatigue.
Composite structures are designed using static ultimate conditions, where the Materials and Design principle demonstrate no sensitivity to fatigue cycling.
Carbon-Fibre Components: Lighter, Stronger, Tougher
The use of carbon-fibre components in the aviation industry is becoming more and more common. But for Airbus, Carbon Fibre Reinforced Plastic (CFRP) components are nothing new.
Airbus has used carbon-fibre materials for years. starting with the A310-200 in 1983 when the spoilers, airbrakes and rudder were made of sandwich CFRP. Three years later, the A310-300 pioneered the introduction of composite on a primary structure with the vertical tail plane designed in monolithic CFRP. On the A320, carbon-fibre materials were used on flaps, ailerons, spoilers and on the vertical and horizontal tail planes. On the A340-600 the rear pressure bulkhead and keel beams were made of CFRP. On the A380, Airbus introduced them in the fuselage rear section and centre wing box connecting the two wings together, while the wing ribs moved to carbon-fibre. This evolution continued on the A350 XWB where the entire fuselage and wing skins – more than half of the structure – is made from carbon-fibre composites.
A350 XWB Composite Structure
3 – The assessment of impact damage
In case of impact damage, a composite structure may behave in a different way when compared to a metallic structure. As a consequence, in case of an impact with a foreign object, the internal damage on a composite structure might be larger than the visible external damage. This point was illustrated in a previous Safety first article (ref.A) as well as in an Airbus Operator Information Transmission (ref.B).
Thorough visual inspection of the aircraft exterior is therefore even more important on a composite aircraft than on its metallic counterpart. (fig.8 and 9)
Whereas a mechanic inspecting a metallic structure will typically look for dents or cracks and will determine whether action is needed based on the size of the damage, the same mechanic on a composite structure will rather look for any visual clue, more particularly for dents.
(fig.8)
External view of the damage
(fig.9)
Resulting internal delamination
- If damage is smaller than barely visible…
According to the Barely Visible Inspection Damage (BVID) concept, any dent whose depth is less than a certain threshold, defined in the Structural Repair Manual, is acceptable and does not require any action.The dimensioning of the aircraft panels takes into account the BVID criteria. In other words the panels have been sized with a margin corresponding to the loss of strength that the panel would incur when the damage is barely visible.
- If damage is larger than barely visible…
If a damage is visible and lies beyond the BVID threshold, a more detailed inspection, typically ultrasonic testing, may be required by the Structural Repair Manual (SRM) and carried out in accordance with the Non-destructive Testing Manual (NTM).
- The special case of high energy blunt impacts
Two types of events may be classified as high energy blunt impacts:
– Tire bursts
– Impacts from ground servicing vehicles
Above a certain energy threshold, a metallic structure sustains a permanent deformation and displays a dented area, whereas a composite panel deforms and then returns to its original shape with minor or no damage on its surface, but potentially important damage to its internal core.
On the A350 XWB, additional inspection tasks have been added to sections 05-51 of the AMM to deal with these types of impacts.
Whereas tire bursts are self-evident occurrences, impacts from ground vehicles require a high level of awareness among all ramp actors on the necessity to report these types of events.
The “line tool”
The “line tool” is an easy to use device developed by Airbus, which allows basic ultrasonic inspections to be performed by non NDT qualified personnel.
This tool will allow the release of an aircraft if no delamination is found. But if delamination is observed, a more detailed inspection must be performed by means of additional Non-Destructive Testing (NDT), which uses ultrasonic methods to determine the exact extent of that damage.
For abnormal events, such as lightning strikes or bird/hail impacts, reference should be made to the applicable Aircraft Maintenance Manual (AMM) 05-51 section.
These sections include relevant instructions for composite components inspections and checks.
Focus on training
Information on structure courses proposed by Airbus Training is available on: AirbusWorld and airbus.com
Extract from FAA Advisory Circular 65-33:
As more composite aircraft enter into operation, detailed and documented composite training should be developed to ensure that personnel performing composite maintenance on aircraft structures and components properly repair damage to meet the highest level of safety.
Extract from FAA Advisory Circular 20-107B and EASA Alternative Means of Compliance 20-29:
d. Damage Detection, Inspection and Repair Competency.
(2) Pilots, ramp maintenance, and other operations personnel that service aircraft should be trained to immediately report anomalous ramp incidents and flight events that may potentially cause serious damage to composite aircraft structures. In particular, immediate reporting is needed for those service events that are outside the scope of the damage tolerance substantiation and standard maintenance practices for a given structure…
The use of composites provides significant benefits to aircraft operators in the form of fuel savings, weight reduction, fatigue and corrosion resistance and extended in-service life. Composite aircraft are certified according to the same rules as their conventional counterparts. Due to the specificities of composites, Airworthiness Authorities have developed new Acceptable Means of Compliance (AMC) to adapt to this new technology and ensure an equivalent level of safety. In accordance with its policy on the introduction of new technology, Airbus has gone a long way beyond these AMCs.
Composite aircraft are designed to respond as well and in some cases, like fatigue and corrosion, better than traditional metallic airplanes to operational threats. Composites provide also some additional benefits in terms of behaviour to fire: Carbon Fibre Reinforced Plastic (CFRP) is auto extinguishable and more burn through resistant than aluminium. Composites, however, have a specificity that needs to be taken into account when assessing damage: the non-visible side deterioration might be larger than the visible external damage.
After visual inspections, the maintenance programs call for:
- No further action if the damage is barely visible
- A specific inspection if the dent lies beyond the Barely Visual Inspection Damage (BVID) threshold
This rule has two exceptions: tire bursts and impacts by ground servicing vehicles. Both types of events must always be reported and require appropriate inspection prior to returning the aircraft into service.
CONTRIBUTORS
Chantal FUALDES
Executive Expert Composite
Xavier JOLIVET
Director Flight Safety
Cédric CHAMFROY
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