Analysis
GE90-115B Combustor - Resized

21st-century engine repair

As aircraft engines become more fuel efficient, they also become more complex and high-tech in terms of the materials and manufacturing processes they use. Repairs for parts in such engines require parallel investment in the research and development of new technologies. Chris Kjelgaard reports on today’s and tomorrow’s engine repair business.

The turbofan and turboprop engines powering today’s commercial aircraft — and those which will enter service within the next decade — are far different from those of yesteryear. Today’s engines are more powerful, more fuel efficient and experience higher core temperatures than do previous-generation powerplants. They employ exotic new materials in their construction and their fan, compressor, and turbine blades feature complex 3D airfoils not found in earlier engines.

This near-revolution in engine design and fabrication is also requiring a similar research and development effort in engine repair technologies and techniques. At the same time, rapid advances in engine technologies are making future prospects challenging for the independent maintenance, repair and overhaul (MRO) shop.

Take the use of composite materials in engine fan sections. Today, some fan blades are largely made of carbon-fibre composite materials but feature thin metal layers blended on to their leading edges. These designs combine the leading-edge thinness required to optimise blade aerodynamics with the strength and light weight of composites, to produce very strong and very efficient 3D airfoil fan blades.

But such structures create repair R&D challenges for MRO companies, even companies as technically proficient as StandardAero. For one thing, composites do not act like isotropic metals, which feature the same properties in all directions, notes Kevin Cusack, director of business development for StandardAero’s airline business. In a repair, blending of metal and composite to reproduce the original airfoil creates issues for an MRO company in maintaining the integrity of the airfoil and of the gas path across it.

“And how do you ensure that a composite can be returned to its original state?” asks Cusack. In modern engines, some materials are complex and repair processes are going to require “high investment", and so "the average MRO isn’t going to be able to handle them”. Larger MRO companies can create and perform such repairs, but can usually only do so in consultation with the original equipment manufacturers (OEMs). Why? In order for MROs to be licensed to operate to Federal Aviation Regulations (FAR) Part 145 or equivalent standards, licensing authorities require MROs to use approved technical and certification data from OEMs.

Patent research and repair R&D

All repairs and materials are patented, so it is hard to develop a new repair, according to Thilo Seitz, director of propulsion system engineering for Lufthansa Technik (LHT), “because you need to do patent research and a lot of R&D”. In addition, licensing authorities require MROs to demonstrate by means of testing and calculation that self-developed repairs produce parts which operate as well as the originals. So MROs usually obtain the knowledge they need to perform a new repair by getting a licence from the OEM, Seitz says. The need for approved data and consultation on the suitability and quality of any new repair means MROs “always look for a good partnership with the OEM”.

For this reason, MROs such as LHT and StandardAero often work with OEMs on jointly developing repairs. According to Brent Ostermann, director of engineering for StandardAero, one area of intense focus for OEMs and MROs alike is laser welding and joining. This is “far superior” to traditional welding processes from a process standpoint”, in that laser welding and joining allows far more accurate measurement and control of distortion, diameter and dimension of the join, according to Ostermann. Another key area of R&D is using robotic tooling, to ensure repeatability of repairs and the ability to make repairs in tight spaces.

Also important is the development of new coatings and materials, which MROs research as well as OEMs. Though the latter do most of the heavy work in this area, large MROs do develop their own materials and coatings for repairs. Continuing development of non-destructive inspection (NDI) and testing techniques using digital X-ray photography, thermography, ultrasonic imaging, and eddy current inspections will remain important for all as machined tolerances and blade-tip gaps within new engines tighten to less than one ten-thousandth of an inch.

With carbon-fibre composites becoming common in modern commercial engines, and ceramic matrix composites soon to make their first appearance, “that will be the growth strategy in repair development”, says Ostermann. “We’re focusing on all the new materials, to be ahead of the game when new technologies come in. Repair technology is going to be significantly different in 15 years.”

Advances in borescoping

Borescoping has long been involved in engine maintenance, but in the past decade the technique has seen big advances in miniaturisation of cameras and instruments which can be attached as sensor heads, says Paul Brooker, chief technical manager for IBA Group. These advances provide several advantages. One is digital recording of imagery within a compressor in an on-wing engine, allowing an airline’s maintenance department to study the imagery at its leisure and let the aircraft return to service more quickly. Second is the new ability to measure highly accurately — and re-measure repeatably at a later date for comparison, allowing predictive maintenance — the lengths and depths of cracks or other flaws in an engine part.

Another plus, says Brooker, is the ability which maintenance engineers now have to record different types of imaging data from different borescope sensor heads and to merge these digitally to create very detailed 3D images of parts inside an engine. Pioneered by General Electric (GE), this technique can allow maintenance engineers to determine, for instance, whether an individual compressor blade is worth repairing or if it should be scrapped. Cusack adds that borescoping has now gone a step further with the development of boroblending, which uses miniaturised tools inserted through an engine’s borescope port to perform blending repairs inside the compressor without needing to take the compressor case off.

Brooker notes that Morgan Technical Ceramics has invented a brazing technique which covers temperature and pressure sensors with a ceramic material which is then brazed to an Inconel nickel-chromium alloy. This assembly is then inserted by means of ceramic servos through a boss in the turbine casing of a hot engine and data can be pulled out. Such an assembly can withstand a higher temperature and pressure than conventional sensors and can extract data more reliably.

While large independent, or airline-affiliated, MROs frequently work with OEMs in developing repairs, they also invest heavily in research and development of their own. As a result, they often develop their own repairs. There are several reasons why, according to Seitz. One, which is particularly applicable to MROs that are integrally linked to airlines, is that an airline’s MRO organisation might need to develop a repair specifically to address a reliability issue unique to that airline.

Some Lufthansa Technik repairs

For instance, notes Seitz, Lufthansa created a repair after experiencing wear in CF6 engines of a small bushing called an igniter ferrule, located around the igniter in the engine’s combustor. Engine vibration eventually caused the worn ferrule to fall off into the combustor, allowing more air to enter the combustor around the igniter. In severe weather conditions, this could prevent the combustor relighting if the flame blew out and Lufthansa had to make two unscheduled removals of CF6 engines.

A second reason for self-developing repairs is to save expensive parts from having to be scrapped, according to Seitz. If a flange in a large turbofan engine casing becomes worn or cracked, it can cost between $200,000 and $500,000 to replace the casing if a repair and re-installation cannot be performed quickly. “So there is a high motivation to develop a very quick and independent repair. When you can use and re-install casings or seals, it saves a lot of money for customers,” he says.

Thirdly, MROs sometimes self-develop repairs to increase the efficiency of an engine. This was LHT’s motivation in developing its advanced recontouring process (ARP) to analyse and repair dust ingestion erosion of the leading edges of compressor blades. The repair first analyses a worn compressor blade electronically and then uses a robotic grinding technique to re-grind the leading edge, thus re-contouring it.

The restored profile of the repaired 2D blade is not exactly the same as that of a new blade. Instead, the analysis calculates the best size and shape for the leading edge to match the reduced chord of the worn blade, to provide the best possible aerodynamic profile. In performance tests run by LHT, ARP improved the manufacturer-recommended exhaust gas temperature (EGT) margin of worn blades by three to four degrees Celsius and extended the blades’ lives by 25 to 40 per cent.

In current- and new-generation engines, 2D airfoils have made way for 3D airfoils in many compressor and turbine stages. Since the airfoil shape and chord in a 3D blade is even more important for optimising gas path flow than in a 2D blade, LHT has developed a repair which uses numerically controlled laser welding and adaptive milling of eroded areas to restore the original chords of 3D high-pressure compressor blades.

This repair extends blade life, increases engine efficiency, and reduces the scrap rate — a key consideration, when scrapping a set of HPC blades can cost $50,000 or more per engine. The company plans to develop the repair further by hard-facing the affected areas with new materials to reduce abrasion stress. This will be more efficient than continuing to use existing erosion coatings, according to LHT.

OEM data highlights what the engine needs to be “serviceable and repairable” but it doesn’t indicate what action is needed to provide best performance, notes Seitz. So LHT has developed performance analysis methods involving extended test cell measurement techniques, computerised cycle models for each engine type and numerical models for specific engine modules. This analysis shows the company which are the most critical modules and parts needing attention in a given engine in order to improve performance.

Performance measurement has provided Lufthansa with some startling results: puzzled by a dramatic loss of performance in a recently rebuilt engine, LHT found that the problem actually lay in the engine’s EGT probe, which was faulty. There was nothing wrong with any of the engine modules.

The two-way street

While OEMs may compete with MRO companies in two ways — in providing airlines with MRO through owned or affiliated shops, and when airlines purchase new OEM parts in the aftermarket rather than have used parts repaired — they do gain benefits from MROs developing their own repairs.

OEMs keep tight control over their own intellectual property when they develop repairs and license those repairs to MROs. But while MROs retain the intellectual rights over their self-developed repairs, since these are for engines developed by OEMs and use data provided by those OEMs, invariably MROs share details of their repairs with the engine manufacturers. While this provides OEMs with new repair knowledge they can then use, the system isn’t unfair, says Cusack: “Typically, when you develop repairs [for an engine] you get the opportunity in [the OEM’s engine] manual to be a substantiated vendor.”

That said, as engines become better-performing, they are also becoming more complex and more expensive, and here the MROs face several uphill battles. As engines increase in price, airlines are increasingly choosing to lease rather than own their engines, according to Seitz. However, increasing engine cost is also making lessors less inclined to allow MROs to use self-developed repairs.

Also, as new composite materials come along, “one of the challenges is that new [repair] technologies might require large amounts of capital,” says Cusack. “It might be cheaper to replace rather than repair parts in new engines.” Another problem for MROs is that, since a big part of engine performance improvement lies in longer on-wing times, there is a longer mean time between shop visits and less opportunity for MROs to perform overhauls and repairs.

Additionally, says Seitz, OEMs want to sell new aftermarket parts and may be more likely to regard a part as irreparable when in a certain condition than might an experienced MRO with a high-tech, self-developed repair. However, airlines always want to save costs and OEMs recognise this, helping to create a balance between the potentially conflicting viewpoints of MROs and OEMs.

An OEM’s repair R&D effort

As the largest manufacturer of commercial turbofans, GE Aviation spends $30m to $40m a year on R&D for new repairs and has developed some 15,000 different repairs for its turbofan engines, according to Anna Nabb, component repair general manager for GE Engine Services. GE’s intensive research has led it recently to double the warranty period on some of its existing repairs.

“It’s one of the most fascinating pieces of what we do — the continuous evolution of technology,” says Nabb. “But if it can’t get in the hands of our customers in a timely manner,” then GE’s repair-development effort isn’t working properly. “Doing it on time and reliably and with the absolute highest quality standard is one of the most important parts of our organisation.” A feature of the R&D effort is that it looks far out in time, says Nabb. This is because it focuses mainly on repairs for what GE calls its “New Product Introduction” engines. If GE’s new engines perform as reliably as expected, the repairs now being researched won’t be required for many years.

Maureen Normoyle, general manager for GE Aviation’s repair technology team, says the company is focusing on seven areas of technology for repair development. These are: cleaning; chemical processing, including coatings; inspection, particularly NDI; advanced machining, particularly robotics; welding techniques, mainly laser and friction welding; materials repairs, using techniques such as cold metal gas spray build-up; and other adjoining and advanced technologies.

“As the complexity of the geometry [within the engine] increases, we have to rely more on robotics for repeatability, the build-up of the weld, identifying [the repair] in space, and imaging, so the repair creates a part as good as new,” says Normoyle. In some cases, in existing engines, the repair may provide a part even better than new, she says.

“And now we have 3D compressor airfoils which are highly sensitive — they’re right on the edge for the frequency of the engine,” adds Normoyle. The location and depth of any weld affects the frequency of the compressor blade, so the weld must be highly accurate. The challenge is even greater in repairing blades on compressor stages which are one-piece, integrally bladed disks (blisks): There is less space within the compressor stage to manipulate the repair tool, “so we have to do more manipulative machining”.

GE’s repairs for composite materials

Bobby Sohi, GE Aviation’s director of marketing for component repair, says that while GE has not introduced strain gauges into its carbon-fibre composite engine parts, it has developed inspection techniques to identify voids and other flaws within such parts. The techniques largely rely on ultrasonic imaging. Additionally, repairs developed for the composite fan blades in the GE90 use co-ordinate measuring machines adapted to allow measurement of the blades’ 3D geometry.

“When we introduce a new-material fan blade, it is important from the cost-of-ownership viewpoint to make sure repairs are available, for the health of the fleet,” says Sohi. As GE innovates repair technologies for new engines, these may find their way into older engines as well. “Adaptability is a key across our older and new engines,” he says. For instance, “developing coatings and applying them in different ways in different places” is an important part of GE’s repair R&D effort. One recent repair saw GE develop a platinum aluminide coating for blades on CFM56-5Bs, to improve engine reliability and durability for airlines operating A320-family aircraft in hot and harsh environments.

CMCs are going to be important in GE’s future commercial engines — the CFM LEAP-1 will be the first commercial engine to feature their use — and Nabb says GE is making a “business-wide effort” to understand the repairs which might be needed, as well as the environmental barrier coatings they will require. Such coatings “will be critical for durability”, she says. “The new engines will operate longer at temperature because of the routes they are designed for.”

Here, GE draws on the experience of its Energy group, which makes industrial turbines for power stations. Industrial versions of large aviation turbofans, these engines run for long periods of time without interruption. “We are currently developing the strategy and deciding what we can and can’t do in terms of what we will see in terms of damage, but this is the reason we need to be investing right now in terms of repair,” says Nabb.

GE has already developed coating-based repairs. In 2011 and 2012, it developed repairs for the shank areas under the platforms of the high-pressure turbine stage 1 and stage 2 blades in CF6-80 engines, which are prone to environmentally driven corrosion attack. The corrosion also breaks down the protective coating on the blades, further accelerating the process. Some customers have had to scrap blades as a result.

GE developed an enriched chromium coating known as R195 to cover the corroded areas. This repair removes the corrosion underneath blade platforms, fills it back with enriched chromium and reconditions the chromium content of the coating to restore blades to a serviceable condition. The repair, which GE has newly industrialised at its Singapore facility, offers a two per cent improvement in scrap rate – a significant change, according to Sohi.

Nabb says GE tries to “look holistically at [both] the component and the system” when developing a repair. For instance, in its chemical strip processing research, the company is mindful of compliance changes which Europe now requires. “Some solvents are going to be outlawed,” she says. “We’re trying to work out if there’s a more effective way to treat the coating rather than just attacking the bare blade.” Similarly, GE is investigating different materials and welding techniques to end the problem of overly elongated bolt holes.

For OEMs and for MROs, research into repair innovation never ends.

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