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Under wraps

Through composite material repairs, LRU equipment testing and rectification, and through-life maintenance programmes, engine nacelle MRO requires competencies across multiple domains, involving complex systems that contribute to the efficiency of the aircraft and engine. Paul E Eden assesses the state of the market

Once upon a time, airliners flew with uncowled engines, before the practicalities of reaching higher aircraft speeds led to the introduction of cowlings as a drag-reducing necessity. With vents and variable flaps added, cowlings became more complex until, with the advent of turbine propulsion, they became nacelle systems with important dynamic and safety functions.


Olivier Savin, Vice President – Customer Support & Services, Safran Nacelles, explains:  “The nacelle is an integral, structural part of the aircraft, at the nexus of the airplane and engine. But it’s more complex than that, since the nacelle has other key functions: acting as a containment shield protecting the aircraft and passengers from kinetic damage, should an engine fan blade separate after a bird strike, for example; providing the smoothest airflow possible from intake to exhaust, helping to optimise the engine’s specific fuel consumption; providing the thrust reverse function upon landing; and acting as an acoustic damper to reduce noise for passengers and those on the ground.”


Tim Huppler, President of AeroConsult, has many years of engine and nacelle experience behind him. He agrees with Savin, saying: “The nacelle is a complex, highly engineered piece of machinery. It’s all too easy to underestimate the effort required to design, develop and certify a nacelle while meeting reliability and maintenance requirements.”


Efficient, easily-maintained nacelles are crucial  to aircraft performance, and their configuration is considered early on in the design process. “The aerodynamic design around the nacelle and, in particular, the area where the nacelle blends into the pylon and wing is critical – an efficient, low drag design can give the airframer a competitive edge,” Huppler explains. “The nacelle cross-sectional area must be minimised to reduce drag. To do this, compact packaging of the nacelle equipment is essential, but easy access to the line replaceable units for ease of maintenance must also be ensured.


“The nacelle must have doors with foolproof latches and removable panels allowing access for routine maintenance, including oil checks and replenishment, and permitting easy removal of engine accessories. Today, the target removal and replacement time for the majority of accessories is 20 minutes.


“The nacelle is a combination of structures, including the nacelle skin, engine mounts, thrust reverser structure and a variety of other systems of varying complexity. These typically include inlet anti-ice, fire detection and suppression, health monitoring systems, and a variety of air ducts, hydraulic tubes and electrical harnesses.”


Aerodynamically clean on the outside, on the inside the nacelle is more complex, with multiple brackets, electrical and hydraulic lines. “There are bleed air, electrical and hydraulic connections to the aircraft system,” Huppler says. “And while the inlet is generally part of the engine supply, MRO organisations must ensure its structural and geometric parameters meet the operational limits found in its maintenance manuals.”


Yet on the face of it, the nacelle still looks a lot like an aerodynamic cover with access panels. Thrust reverser mechanisms aside, what are the MRO requirements? Safran Nacelles manufactures nacelles as well as maintaining and repairing them and, Savin says:  “For Safran products there is programmed maintenance and regular inspection, except for life-limited parts that are not included in the nacelle business. I’d say the nacelle is perceived as an on-condition product, while a critical element is the ‘soft life’ refurbishment of the thrust reverser, which enables airlines to optimise their operations, reduce lifecycle maintenance costs and lower disruption in flight operations.


“Another aspect of aircraft operations that mobilises the customer, nacelle designer, and support and MRO organisations, is ground-related events, including FOD, ground service vehicle impacts, and so on, which can damage nacelles because of their close proximity to the ground.” Most damage therefore occurs on or in the vicinity of the airport, with bird strikes shortly after take-off or during the landing approach another risk.


While wing-mounted nacelles are more vulnerable to vehicle impacts and perhaps FOD, Huppler reckons their MRO requirements are otherwise little different to those mounted on an aircraft’s rear fuselage. “They include similar structures – mounts, cowls, thrust reverser systems. Wing-mounted nacelles can provide easier access for ground maintenance crews, since ladders and so on may not be required, but this depends on the location of access panels and aircraft type.”


And if damage should occur? “Most nacelle structures, while repairable and, if required, replaceable, are not considered rotable, having been designed for the life of the aircraft. Conversely, most line replaceable units are rotable.”


Savin says: “We’ll always look first to repair a component rather than scrapping it entirely, while still providing a timely and effective response. In addition to offering repairs through shop visits, we also dispatch specialists, including MRO experts, for on-site actions. 


“They can be activated immediately and have a worldwide reach that helps them accelerate an aircraft’s return-to-service. Our dedicated Le Havre customer centre responds to logistical, technical, engineering, documentation, on-site support, and other questions 24/7, while MRO network proximity is key to reducing turn-around times, avoiding the need to transport hardware and facilitating repairs.”


As an OEM, Safran Nacelles designs, produces and sub-contracts parts and nacelle sub-modules, operating three primary manufacturing facilities at Le Havre, France, Burnley, UK and Casablanca, Morocco. Subsequently, Savin reports, “We stock the main parts in France, along with three other AOG and generic stock sites in Singapore, the US and Dubai.

We also promote access to spare parts pool exchanges, helping our customers manage their working capital and spares provision.” >>


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