Safe operations with composite aircraft

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.

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

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 behavior 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.

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