Nacelles and thrust reversers: Taking care of your engine’s ‘tailored suit’

We examine the latest trends and innovations in nacelles and thrust reversers, looking at the latest solutions and challenges in developing, producing and maintaining these essential parts.

[This article first appeared in MRO Management September 2021, which you can read in full here.]

Engine nacelles are perhaps one of the most unassuming elements of aircraft. While most people think of them only as an external protection for the engine, nacelles are in fact key contributors to an engine’s performance, efficiency and flight safety by directing and optimising high-speed airflow through the powerplant.

Nacelles operate in severe environments – with temperature extremes from -60°C on the exterior to as much as 600°C on the interior – and are subjected to significant loads, including engine thrust and the fluctuations caused by forces and pressures, both external and internal.

The nacelle systems also play a significant role in reducing engine noise, including the integration of complex acoustic treatment that has been micro-drilled with millions of small holes on the inner composite surfaces, acting as sound traps.

With the thrust reverser, they also have a vital role in flight safety as they contribute to the aircraft braking during runway landings, and even add to an aircraft’s aesthetics, especially since the nacelle is often placed at eye-level and can include an airline or operator’s branding.

With all of this in mind, there are a number of challenges in developing, producing and maintaining engine nacelles.

Rising to the challenge

Middle River Aerostructure Systems (MRAS), an ST Engineering company, explains that first and foremost maintenance can be a challenge, something that every company spoken to for this feature agrees with.

The nacelle systems specialist says that there is a specific type of science involved because the airflow through and around needs to be optimised. “It includes minimising what are called ‘steps’ and ‘gaps’,” the company explains. “A ‘step’ occurs when the panels and components on a nacelle are not precisely aligned, which can cause aerodynamic drag during flight. ‘Gaps’ refer to the leaks of air from the engine’s stages that occur when an assembled nacelle is not optimally ‘tight’, which reduces the powerplant’s performance.”

Collins Aerospace’s vice President, customer support Dana Stephenson notes that any nacelle damage “necessitates a deeper level of analysis and manufacturing capability” to return the structure to its certified condition, compared to some other parts. This is because the most current generation of nacelle structure components, which are made with composite material, are “highly optimised” to complement a specific engine.

“They have been developed to achieve maximum propulsion-system performance at the lowest weight while maintaining lifestyle product durability,” he explains. “Previously, these structure components were made with metallic material. The nacelle represents a ‘tailored suit’ for the engine to achieve those propulsion goals. From a maintenance perspective, this means highly engineered load cases in nacelle structures with tighter tolerances than even a generation ago with their metallic predecessors.”

In addition, due to the high by-pass engines, nacelle components are also significantly larger than their processors and are increasing in size with every new generation of aircraft. “The physical size poses logistical challenges and can be expensive to transport,” points out Stephenson. As a result, Collins Aerospace has “significantly invested” in its eight MRO sites around the world, and introduced a license network. “When nacelle components need repairs due to ground mishaps, for instance, we can often minimise aircraft downtime by immediately forward-deploying teams or assets from those regional locations.”

Lufthansa Technik says that the use of materials, such as carbon-fibre reinforced plastics (CFRP) is constantly increasing, and as such, repair procedures have remained the same: conducted manually, resulting in labour-intensive processes, time and subsequently cost.

“Moreover, accuracy and repeatability of manual repair processes are limited. As a result, bonded repairs of aircraft structures are limited in size. Furthermore, the manual processes limit the repair strategies leading to technically unnecessary high material use,” explains Lufthansa Technik’s senior director of airframe related components Sven Duve.

Material changes

As materials change over the years, so are the ways in which they are used. Nacelle and thrust reverser systems specialist NORDAM believes that the market has embraced composites but perhaps out of necessity more than anything else, explains CEO Meredith Siegfried Madden. “Composite content grew considerably in the current cohort of products and will continue to grow in successive generations,” she says.

“The more innovative repairs we develop to restore a component or sub-assembly within that component, the more money we save our customers by avoiding replacement costs, whether that be from replacing an assembly in its entirety or replacing piece parts within the repair.”

One of the biggest differences the company has made is in developing repairs and tailoring work-scopes to support its customers’ preferences, perhaps because composites present a different challenge compared to their metal counterparts.

“Understanding wear conditions, damage tolerances and repair schemes can require more sophisticated scrutiny and evaluation to ensure repair work performed provides a reliable product back to the customer,” adds Siegfried Madden.

Stephenson agrees. “The sophistication of the composite designs, and the load cases to develop such designs, make the analytics of the repairs more difficult than previous generations,” he explains. “Those in the market that develop the capability to properly analyse and carry out significant composite repairs have more of a challenge than previous metallic structures.

“This technical complexity is compounded by the capital investment in bond tooling, autoclaves and the other ancillary capital investment required to perform these composite repairs.”

Despite this, the change is considered a positive. The company believes that composites have allowed designers to do things in aircraft design that were not able to be done with metals. “They have enabled a highly efficient and optimised nacelle and engine propulsion system, lowering both weight and fuel consumption.”

Lufthansa Technik’s Duve notes that in recent years, research and development into composites means that MROs now have the ability to perform large-scale repairs whereas in the past major composite repairs often resulted in a whole component replacement.

In fact, even during the last year, the MRO has witnessed some of the first approvals for automated processes in maintenance manuals, and the development of autoclave repairs to control and monitor bonding cycles “has taken a large step forward” allowing them to control the heat behaviour of large-area bonding repairs in a defined manner, “which was previously difficult, or even impossible”.

Automating manual processes has been a challenge for the MRO, but together with individual process automation specialists iSAM, Lufthansa Technik has developed the first automated adaptive robot system that it claims can be applied to various airframe related component repair processes.

“Just recently, it has commenced daily operations in our Hamburg base,” Duve adds. “With the automation of the milling and drilling processes, we managed to significantly increase the process efficiency and to reduce the turnaround time in repair shops. The accuracies allow us fast processing with little to no manual re-work. As a result, we require less spare parts and can reduce the amount of repair materials, such as pre-pregs, with the automated process.”

Time for integration

Along with composites, integrated propulsions systems are another element of the nacelle and thrust reverser systems making changes, an element which MRAS has focused on, and which it says can also contribute to improving maintenance and repairs.

“Prior to the integrated propulsions systems, a typical process involved the design of an engine by one company, while others would develop the nacelle and pylon,” explains MRAS. “Requirements on performance, interface points etc, would be passed from one company to the other – with each entity factoring in its own margins. Obviously, this is not an optimum procedure, and clearly not a process that is conducive to a truly integrated system.”

With the integrated propulsions systems, the idea is that the nacelle producer works hand-in-hand with the engine maker in a more seamless approach that removes the ‘perimeters’ between engine, nacelle and pylon. The result is a “new level of propulsion system integration that results in better aerodynamic performance and lower weight”, MRAS says, leading to reduced fuel burn, along with improved maintainability and higher reliability, which all contribute to lower direct operating costs.

MRAS adds that it is involved with two nacelles that have been developed for integrated propulsion systems, working through the company’s joint venture with Safran Nacelles. One is utilised on CFM International’s LEAP-1C engine for the Comac C919 jetliner, while the other is for GE Aviation’s Passport engine on the Bombardier Global 7500 business jet.

“A key feature of the LEAP-1C’s integrated propulsions system is its O-Duct thrust reverser configuration, which replaces the two-piece D-shaped doors on a traditional thrust reverser,” MRAS explains. “The O-Duct concept eliminates bifurcation in the airflow path caused by the D-doors, increasing the thrust reverser’s efficiency while also reducing complexity for maintenance. Another feature is how the entire O-Duct is designed to transition aftward on pylon-mounted tracks and sliders, opening up access to the reverser’s inner fixed structure for maintenance.”

NORDAM has also found positives with integrated systems, which Siegfried Madden says have proven to be more aerodynamic and efficient due to new lightweight materials and improved manufacturing processes.

But from an MRO perspective, it can create another set of challenges. “The variations in materials used can create challenges within the supply chain to maintain more diversified inventories of different types of adhesives and pre-pregs which depend on the requirements of the component being repaired,” she says. “These materials often have long lead-times and short useable lives, in some cases driving excessive inventory and increasing material scrap rates as materials expire.”

As a result, the CEO says that one of NORDAM’s goals is continuing to search for opportunities to standardise materials across platforms to accelerate the repair process and reduce costs for its customers.

An uphill struggle

As with the rest of the industry, the nacelle and thrust reverser market has certainly felt the impact of the Covid-19 crisis, but things are starting to look more positive.

NORDAM found that as nacelles were often treated as discretionary spending, this removed “many time constraints that drive MRO activity in airframes and engines,” says Siegfried Madden. Lufthansa Technik agrees, noting that as markets begin to re-establish themselves there has been a marked uptake in demand.

“Nacelle components are unique in that maintenance is ‘on condition’,” Collins’ Stephenson says. “We observed that elective/scheduled nacelle restorations have been deferred by some operators, while in in other cases the airline community is returning more aircraft to operations. For example, long-term storage in various climate conditions around the world has provided environmental challenges to all parts of these aircraft.”

On the production side, MRAS says initially airframers asked supply chains for reductions in the manufacturing cadence on hardware systems for new production aircraft. However, since passengers began to take to the skies again, “the situation has dramatically turned around.”

This is particularly noted with the A320 neo as MRAS supplies the thrust reverser’s transcowl, the engine build-up unit (EBU) and thrust reverser actuation system (TRAS) on the aircraft’s LEAP-1A engines.

The MRAS spokesperson adds: “On the maintenance, repair and overhaul side of the nacelle market, the demand for services is expected to swing back as grounded aircraft are brought into service, and as new jetliners join airlines’ fleets.”

In line with demand, the company is expanding its repair station capacity with a multi-million dollar investment, including the acquisition and installation of new equipment ranging from paint and power wash booths, bonding ovens, component lift stands and vertical part carousels to a 15-ft. diameter autoclave for bonding, as well as a hydraulics test stand for thrust reverser deployment functional testing.

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