Testing equipment adapts to aerospace industry's move to composites

You might be worried going around British Airways Engineering Heathrow maintenance and repair facility inside Heathrow airport. There are parts everywhere from flaps to engine covers to doors all in need of attention by the facilities skilled maintenance engineers and technicians. Its state of the art workshops make sure the airline's 256 aircraft, and some third party aircraft, operate safely, economically and with the least amount of downtime.

Seeing up close, however, a leading edge with a 30cm2 hole or a flight surface with such bad delamination that it looks like an air bubble is stuck under the metal, does make you feel slightly vulnerable. It highlights that aircraft are not invincible. It seems obvious, but aircraft get damaged, and most – if not all older aircraft - carry some sort of repair which is often pretty extensive.

Perhaps the two biggest causes of damage are from foreign object debris - most commonly bird strike - and also damage caused by water ingress. The first is fairly self explanatory, hitting any object with a significant mass at 500mph is going to cause a mighty impact. And it does. Though bird strikes on engines have been well publicised for good reason, what goes largely unreported are the prolific number of impacts on aircraft structures.

The other major area for repair is much more subtle. Most in-service aircraft by Boeing and Airbus use a sandwich panel construction, usually a thin aluminium or carbon fibre skin that is bonded on to some kind fibreglass, aluminium and even paper honeycomb core. Water finds its ways in to the panels which freezes and expands. This can force the skin of the sandwich panel to delamination from the honeycomb.

This damage however, is not as obvious as impact damage and is not always easy to spot. So as composites become an increasing part of the structure of aircraft, so too does non destructive testing (NDT) tooling to engineers and technicians.

"A lot of what we do is around aluminium as that has traditionally been the material aircraft structures have been made from," says Carl Sheppard, NDT manager at British Airways Engineering. "Becoming more common, however, is inspection of composites."

British Airways is due to soon take delivery of the Boeing 787 Dreamliner that is made from 50% composite. Carbon fibre based composites do not tend to deform in the same way as aluminium, so you don't get dents.

Dents can actually quite useful in many respects as they are a visual indictor of damage. Composites, however, show little or no surface damage, yet internal fibres can be broken often feet away from the point of impact. This can weaken the structure hugely. NDT is vital as future structures may have no visual indicators that they are damaged.

The British Airways Engineering NDT team identify and assess all types of damage to aircraft. It has at its disposal one of the best stocked tool boxes in the world when it comes to NDT, so what are the tools and methods used when it comes to doing an inspection of an airliner?

1. Eddy Current instruments (aluminium alloys / various metals)
These tools work by passing alternating magnetic current through a coil which sets up a magnetic field. When that magnetic field is put near a conductive material, it is induced in to it and creates eddy currents. By establishing a normal signal – known as the equilibrium – when engineers come across a crack the signal changes, which is visualised on a screen. The Eddy Currents can be induced using normal probes on flat surfaces, but also rotating probes to go inside fastener holes for inspection.

Using high frequency eddy currents can find surface cracks. This is useful as it can be difficult to tell if the paint has a tiny crack or the metal underneath. Using a low frequency can find cracks in the structure down around an 1/8th of an inch. It can also carry out paint thickness measurements, which can affect weight and balancing.

"It wouldn't be used to scan the whole aeroplane, just specific parts," says Sheppard. "If you have an aircraft that is hit by the boarding stairs or a baggage ramp, for example, you would then want to inspect it to make sure there are no cracks."

Used for: looking for cracks and corrosion on aluminium structures.

2. Tap tester - coin tap test (bonded materials)
This instrument has been developed from when engineers used a coin and listened for any change in pitch and resonance. The change would point to a delamination between a skin and honeycomb core. The skill is knowing what is filler, or a repair, and what is delamination.

British Airways use the Mitsui Woodpecker automatic tap tester that – like the name suggests – repeatedly taps the surface. Engineers run the instrument over a surface and if there is any change the machine beeps. Working environments can be loud and often requiring ear defenders, this machine takes out human error.

Used for: checking for delamination.

3. Mechanical Impedance Analysis (composites / metals)
British Airways use the BondMaster 1000E+, a versatile NDT instrument that offers mechanical impedance analysis (MIA), as well as sonic pitch-catch and resonance mode technologies for inspection. This allows the engineers to select the best method for a particular application, to test bonds and inspect a wide variety of composite materials. It is commonly used by BA to test D-Duct engine surrounds from the Boeing 777. Defects have been found on the structure as it has overheated, which has affected the bonds.

Used for: detecting internal defects or delaminations.

4. Ultrasonic pulse echo inspection (any solid material)
Each probe uses a piezo-electric crystal. When an AC current is passed through it, the crystal is exciting and resonates creating a sound wave. By 'coupling' the probe to a solid material the sound wave passes through it, travels to bottom surface, and then bounces back.

The time it takes for the signal to reflect back for any material is known, so a crack creates interference and affects the signal, and the device will visually display the result. The sound wave will also not pass through multiple materials or an air gap.

Used for: identifying internal flaws and to measure the thickness of a material.

5. Airborne Ultrasonic Inspection (any material)
This device also works on the principle of ultrasonics, with the main difference being that it can travel through air and multiple materials. It can be used for relatively quick – and fairly crude – inspection of large areas to detect skin to core delaminating. It works at a very high frequency to allow the ultrasonic sound wave to pass through different media. The device called the Curlin Air can be a bit cumbersome due to its size and can detect defects around a square inch. Once found other instruments are used for more in-depth further assessment.

Used for: quick and crude inspection of large areas to detect defects and delaminations.

6. Phased Array Ultrasonics (any material)
Pulse echo is one crystal in isolation, where as Phased Array Ultrasonics use 64 individual crystals. The probe containing the crystals is rolled over a surface and builds up a visual display on the instrument as each element sends and receives sound waves.

"This is the future of NDT," says Sheppard. "It also tells you dimensions and what is lying below the surface. That is vital for new composite aircraft coming through as we can see impact damage which visually might look tiny, but on the inside is much larger. You might not even see an indication on the surface but the plies on the back can be shot. We can now measure the extent of this damage and make an assessment that forms the basis for a repair action."

Used for: Assessing the condition of composite materials and measuring thicknesses.

7. Thermography (any material)
Thermography works on the principle of making infrared (IR) radiation visible on a screen. It essentially shows the heat signature of objects. For composite structures it is useful for identifying water ingress and if any moisture is trapped in a composite material.

If an aircraft has just landed, the team can carry out a thermographic inspection of the wing. Any water will stay as ice for about 20mins. That will emit a different IR signal to the rest of the wing and the team look for cold spots with the camera.

"We can also simulate this effect in the workshop by putting components in the freezer, or in the oven," says Sheppard. "In the oven, any water trapped will heat up and be in equilibrium with the structure. When it's removed the water will hold on to the heat longer than the surrounding structure so we let it cool for a few minutes and then look for the hot spots using the camera."

Used for: identifying water ingress inside of composite panels

8. Fluorescent Dye Penetrant (all non-porous materials) and Magnetic Particle Inspection (metals)
Fluorescent dye penetrant is a chemical process that uses a penetrant on a material to seek out any tiny flaws or cracks and seep in to them. The excess is then wiped away and a developer is added which is able to draw out the penetrant. A UV light then shows any areas of concern.

Used for: Identifying any hairline crack and fatigue crack on a surface.

Magnetic particle Inspection is a process that detects surface and slightly subsurface discontinuities in ferrous alloys. The process puts a magnetic field into the part, which is then magnetised. The presence of a surface or subsurface discontinuity in the material makes the magnetic flux leak, since air cannot support as much magnetic field as the metal. Ferrous iron particles are then applied and if an area of flux leakage is present the particles will be attracted to this area. The particles will build up at the area of leakage. This indication of a crack can then be assessed.

Used for: Identifying any hairline crack and fatigue crack on a surface

9. X-ray Inspection (any material)
X-ray is used to assess the internal quality of a part. It shows up cracks, and also water ingress and delamination. It works using the same principal as medical x-ray machines, exposing and producing the results on a film. Though ultrasonic and thermographic cameras have largely replaced x-ray inspection, it is still a vital capability.
Used for: Assessing a parts in excellent resolution

10. Repair facility - Radomes
The nose cone of an aeroplane, known as the radome, is vital for the aerodynamics of the aircraft and for housing its weather radar. However, these are frequently damaged. Repairs are tricky, not just because of the geometry but also because they need to be the right thickness to allow the radar to get through.

Repairs use a flexible honeycomb core that can be bent in different directions to enable the aerodynamic shape. The repair usually consists of a flexible core being covered by fibreglass and covered in prepreg or resin.

Whatever damage there is, the structure and the geometry need to be made back to the original specification, and the repairs are then fully tested.

British Airways has its own in house design authority that gives it leeway to develop improved inspection and repair processes. This is feedback to the OEMs and sometimes is incorporated by them as standard processes that they recommend.

Despite seeing the damage that is caused to aircraft, it has actually made me feel even safer about flying. Seeing all the work it does and the repairs to the aircraft – and the skill involved - has insured one of the best safety records in its industry, and of transport method.

Justin Cunningham

This material is protected by MA Business copyright
See Terms and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the sales team.


Supporting Information
Do you have any comments about this article?

Your comments/feedback may be edited prior to publishing. Not all entries will be published.
Please view our Terms and Conditions before leaving a comment.

© MA Business Ltd (a Mark Allen Group Company) 2021