Repairs on high: engineering innovation in aircraft maintenance

How are aircraft returned to active service as soon as possible? The answer lies with the MRO industry. Stuart Nathan reports.

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Drones for inspection of inaccessible places such as the tailfin of an aircrafty are under test at Lufthansa Teknik

While much of The Engineer’s coverage of the aerospace sector involves the research, development and manufacture of new aircraft, this is hardly the whole story of engineering in aerospace. The average lifetime of any aircraft can be measured in decades, and the effort – and innovation – that goes into keeping them in the air is considerable. And like so many aspects of the engineering world, it has its own set of acronyms.

The most important of these is MRO – maintenance, repair and overhaul – which refers to the activity and the companies that provide this service. The MRO industry is as varied as any other part of the engineering landscape, consisting of companies both large and small. At the top end of the spectrum, many of the large airlines run MRO operations on behalf of smaller players, something that has grown with the increasing prevalence of low-cost airlines that find it cheaper to contract in services than to provide their own. At the small end of the market are operations that tend to service ‘executive airlines’ that run a few business jets.

It would be fair to say that virtually all innovation in the aerospace sector is driven by efficiency, although different parts of the sector attach their own meaning to this. For aircraft manufacturers, efficiency means extracting the maximum possible performance from their vehicles for the minimum input of fuel: flying further for less cost and producing low emissions of greenhouse gas and noise. The obvious consequence of this is aircraft becoming lighter and using composites in place of heavier metals.

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For MROs, efficiency has a slightly different meaning. It’s to do with the most efficient use of the aircraft in service of its owner: in practical terms making sure it is earning money for the largest-possible fraction of its working life. An aircraft being maintained is not carrying passengers and therefore not earning fares – so in almost all cases, innovation in MRO is to do with making the process faster and returning the aircraft to active service in the air as soon as possible, rather than it being stuck in a hangar with components strewn over the floor.

No alarms and no surprises, please

Prof Ian Jennions, director of the IVHM (integrated vehicle health management) centre at Cranfield University, explained that airlines have scheduled maintenance, where the aircraft comes in for various kinds of checks at set periods, and unscheduled maintenance, which is fixing things that have gone wrong. “That’s the expensive stuff, where the pilot comes into the cockpit turns the key [figuratively] and nothing happens. For the airlines, it’s a case of how long will it take for us to turn that around? And because they don’t know what’s wrong, they don’t know how long the plane will be the ground or how much it will cost. So, increasingly, the goal of scheduled maintenance is to, as far as possible, stop that from happening. When you ask airlines what they want from maintenance, the answer is that they want 100 per cent availability of their asset. Of course, that’s not really possible: what they want is to be able to use their aircraft when they need it. And that means ‘no surprises’.”

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Exoskeletons can assist staff in carrying out physically taxing overhead tasks

In common with many sectors of industry, that means preventative maintenance is the priority during those scheduled maintenance periods. “Scheduled maintenance is called A, B, and C checks,” Jennions explained. “A checks are like ‘check the oil and the tyres’. Bs are where you might also check the batteries and things like that. C is more the heavy maintenance where you strip the aircraft down, as far as taking the engines and even wings off, and checking for corrosion. During those periods, a lot of activity is about playing safe. If you have a component that you know will last for about 1,000 hours of flying time, you’ll give it 950 hours and then pull it for replacement. That’s simple, of course, but the area I work in is moving towards condition monitoring. And that’s to do with informing maintainers how components are behaving. So, we fit sensors, we take data, we perform algorithms, and look at how those different parts are degrading to avoid this unscheduled maintenance.

Anticipate, don’t react

“One kind of algorithm is a diagnostic; that will tell you what has gone wrong with something. The other is a prognostic, which is saying, ‘something is not quite right and it will ultimately fail in the future’. It’s giving you a time, saying that if you continue to use this vehicle in this way, you have a month, two months, three months in which to repair it before it can fail. That’s kind of where it’s at. So, you can inform the airline that they need to take action. They can arrange that with their maintenance staff, so they do it when they want to rather than when the plane breaks.”

A variety of sensors are used for these purposes, Jennions said. Depending on the type of component or subsystem in question, they might be temperature or vibration sensors, for example. And this is bringing about a change in the way that maintenance is carried out, he added. “Aircraft have a maintenance computer and when they land, a first-line maintenance engineer will go into the cockpit and it will tell them what to do. That’s quite a big change in culture, and you often get a response of ‘that’s not gone wrong, it’s never gone wrong before.’ But once people check and see that it’s right, they quickly start taking the systems much more seriously.”

Internet of flying things

Such scenarios are likely to be familiar to engineers in manufacturing sectors, where factories are also becoming increasingly sensor-equipped and oriented towards condition monitoring and preventative maintenance. But the MRO sector is also turning towards new technologies to help maintenance engineers do their job more effectively. For one of the biggest MROs, Lufthansa Technik, which provides services for many smaller and budget airlines, there is a dedicated business function for developing, assessing and forming usage procedures for such new technologies. Kevin Guelck, who works in the base maintenance product division (which handles mainly what Jennions referred to as C checks, the large-scale strip-down activities that occur at relatively long periods in the aircraft’s duty cycle, typically every three years or so depending on the aircraft’s flight hours), explained what happens in Lufthansa Teknik’s Innovation Bay.

The bay is a hangar at the business’s base in Malta, he explained. “We came up with the idea in 2018,” he said. “It’s a lot about internal process optimisation and improvements. We are not aiming for a theoretical improvement idea. It’s really practical, next to the aircraft, interacting with the customer, interacting with the experts on site and trying to get things moved there.” Some of the technologies being tested are on the cutting edge of engineering techniques, said Guelck. “One very good example is 3D printing, which we are using to make small tools and fixtures. It’s a very fast way for us to prototype these tools, and we’ll work up towards printing them on demand to be used during actual operations, rather than having to have them manufactured off site.”

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3D printing small tools can save valuable hours in the maintenance workshop

Another technology that has been trialled in the Innovation Bay is the use of drones in aircraft inspection. “Today, that’s all done by the human eye. A maintenance engineer inspects the surface of the aircraft for scratches, corrosion and dents. A camera mounted on a drone system with machine vision checking the camera output is potentially faster and more able to reach some of the difficult areas, such as the vertical tail fin, which can be very high off the ground, so getting a person up there on a gantry and getting them close enough to inspect properly can be a problem.”

UK company Blue Bear is among those developing drones for this activity. Operations director Gavin Goudie told The Engineer that drone inspections are intended to supplement rather than replace altogether visual inspections by maintenance engineers. “However, we are looking at installing different types of sensor on the drone that could potentially detect the kind of defects that would not be visible even to the experienced human eye,” he said. “As with all technology in this space, the goal is to improve the thoroughness of the inspection and to reduce the time it takes.”

Guelck also explained how Lufthansa Technik is looking at the introduction of augmented reality systems to help maintenance engineers perform their role. “We’ve looked at one system that helps the operator to get a direct visual feedback on the dimensions of a dent or scratch. The more ‘classical applications’ of augmented reality, with systems such as Google Glass or Microsoft HoloLens, have different options where they are used in training or to help guide people through the various steps of inspection or repair processes.”

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Augmented reality helps engineers assess and record the size of surface defects

Aircraft manufacturers are looking at ways of augmenting engineers themselves to help with their jobs. For example, Airbus now uses partial exoskeletons to help engineers perform repetitive operations that require awkward physical positioning, such as lifting heavy tools many times to fix fasteners in position. Lufthansa Technik is also looking at exoskeleton usage, said Guelck. “Base maintenance is very man-hour intensive. The people working on the aircraft have to carry out tasks that are sometimes very, very heavy and take a lot of energy. And that gave us the idea to bring the problem side and the technology side together and trial exoskeleton systems in the Innovation Bay.”

Two different types of exoskeleton have been tested, said Guelck. One of these is a ‘chairless chair’, a device that supports operators in a relaxed sitting position when they have to work on something at a lower level. Another is a device called the Airframe, which supports the operator’s arms when they lift heavy items. Eventually, the company decided not to implement either solution. “We got some positive feedback in that they do help with support, but operators also found that they limited flexibility of movements. We continue to work closely with suppliers to develop such systems – in these cases they came from the medical sector to help people rehabilitating from injury and from prosthetic technology, but we decided they weren’t for us in routine operations.”

To the moon and back, 20 times

Much of Guelck’s work is with airframe maintenance; for Lufthansa Technik, at least, engine maintenance comes under a different division. But here, new technology is again developing, with the aim of making engine maintenance faster. At Rolls-Royce, a suite of robotic systems is being developed to maintain engines without having to take them off the wing of the aircraft.

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Flame-spraying to repair the coatings inside combustion chambers. Image: Rolls-Royce

Rolls-Royce joining and addition specialist Chris Heason explained that taking an engine off a wing makes working on it much easier, as all the systems are more accessible, but it means that the aircraft will be out of operation for weeks. If the maintenance and repair operations can be carried out with the engine in place, that could potentially be shortened to days. “We only make an engine once, but it will be fully serviced four or five times, and that tends to be every 10 million miles of flight service – that’s equivalent of flying to the Moon and back 20 times. When they come in for overhaul, many if not most of the parts look misshapen compared to when they came out of the factory.”

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Swarm robots map the interior spaces of an engine core. Image: Rolls-Royce

Rolls-Royce is developing three different on-wing robotic maintenance systems. Flare is a snake-robot-based system that can go inside engine combustion chambers and carry out in-situ repairs. The interiors of combustion chambers are coated with a specialist ceramic that helps them resist the high temperatures of burning fuel, but these coatings can, over time, flake and expose the underlying metal. The flare system uses a pair of snake robots, the diameter of a pen, one of which is accrued with cameras and the other with a flame-spray nozzle that melts powdered ceramic at around 3,000°C and fires it at a damaged area. “When we find a bit of coating that’s flaked off, we can go inside the engine and put a patch back on; rather like a high-tech sticking plaster, to make sure that the engine gets back to base when it’s scheduled to,” Heason explained. This system is being developed in partnership with Nottingham University and Dudley-based company Metallisation, which specialises in flame-spray technology.

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Cobra, seen here outside the engine, can make cracks in turbine blades less likely to propagate. Image: Rolls-Royce

Another system, Cobra, is a 5m-long, 8mm diameter, highly manoeuvrable snake robot that would be used in conjunction with augmented reality technology to inspect the interior of a jet engine to diagnose any problems it might have. Cobra can also carry tools to melt, for example, cracked turbine blades so that the crack is replaced by a scallop shape that cannot propagate and break off the blade while the engine is in service. This system is being developed in association with RACE (Remote Applications in Challenging Environments), the UK Atomic Energy Authority robot development organisation, as it may also have applications in inspecting the interiors of nuclear reactors, said Heason.

The third system is called Swarm and, as the name implies, it consists of very small bug-like robots. These are based on a chassis developed by Harvard University, which would explore the interior of an engine combustion chamber. “A range of these beetles would be introduced into the engine using a snake-like robot. Each of them is looking at a feature within the engine and it is sending its position, as well as what it is seeing, back to a centralised location. That enables you to complete a scan of what you’re seeing. And once you’ve finished, all of those little devices are then collected back by the same snake robot and taken out of the engine.” Although the swarm beetles would not themselves carry out any repairs, their data would help a system such as Cobra or Flare to be deployed more quickly and effectively, Heason explained. Currently, the beetles use electrostatic forces around their feet to enable them to walk upside down inside the complex shaped spaces of the combustion chamber.

Unique repairs

Another system that Rolls-Royce is working on, although not this time in the on-wing paradigm, is the automated repair of blisks: bladed discs that make up part of the compressor section of a gas turbine engine. Blisks are lighter and stronger than discs onto which blades have been welded, but they are high-value components that are difficult to repair, Heason said. The company has been working with Swansea and Birmingham universities on a system that uses relative layer manufacturing to repair damaged blisks.

It’s a multistage process. First, the blisk is sprayed with a thin coating of a white paint so that a visual scanning system is not confused by reflections from the highly polished metal of the blisk. Then, a camera system mounted on a robot arm is used to inspect blisks from all angles to build up a detailed three-dimensional scan of its geometry, including any damaged or snapped blades. “They may have picked up a lot of damage. They might have bent a little bit slightly. The point is they don’t look like the CAD model that we sent out the door. They have to be digitised first. Every repair that we make in that cell is highly adaptive and unique to that damage feature. We need to have some kind of digital twin to make sure that that adaptive repair takes place.”

The scanned blade is then put into a laser additive layer manufacturing cell under a carefully controlled argon atmosphere, where a metal powder jet is used to repair any defects. “There aren’t any other laser cells like that in the world that enable you to have such a control of the atmospheric conditions, so the resulting repaired blade is as good as it would be when it leaves the factory floor. So, we don’t have any kind of fatigue life loss within this component after its repair.” The final stage of the process is re-machining the repaired blisks on a six-axis machine-tool so they conform precisely to the original design specifications.

Business paradigm

The MRO business is currently undergoing significant changes, according to Cranfield’s Ian Jennions. This, he explained, comes from the airlines realising that the profit margins in MRO are considerably higher than those for operators: 25 to 30 per cent compared with 10 to 15 per cent. This has led to companies such as Delta Air Lines    moving aggressively into the MRO market, he said. “They overhaul engines; they overhaul everything. They recognised very early that the money was in the parts, not just in attached labour. So, they reverse-engineered the entire structure of the blades, for example, got them approved, and then they do their own blade repairs.”

But this is not the case everywhere, he added. In Europe, KLM, Air France and Lufthansa also offer similar services, but Middle East carriers such as Etihad and Emirates prefer the OEMs to carry out their own MRO activities, Jennions explained “It’s a fascinating space because on one side, you’ve got the MRO organisations of airlines like Delta saying, ‘we have all this operational data on how to fly Boeings. Would you like it and how much are you going to pay for it?’. On the other hand, you’ve got equally large airlines like Emirates saying, ‘okay, Mr Boeing or Mr Airbus, you arrange the maintenance for us. Just take care of it.”