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Maintenance

Contained, comprehensive care

Bernie Baldwin reports on the range of nacelle support service from OEMs and third party MRO providers
 

The nacelle system is a unique technical proposition. It is designed to withstand the extreme environments of thermal cycle, vibration and operation in close proximity to the engine while being sufficiently lightweight to minimise fuel burn, durable enough to sustain repeated exposure to cyclic loads and able to offer noise reduction through acoustic engineering technology.”
   
If you ever wanted to know the level of challenge in maintaining the nacelles of modern aircraft engines, then that description by Steve Callan, Senior Director MRO at UTC Aerospace Systems, of a nacelle’s task is as good a place to start as any.
   
There are many different elements to a nacelle – engine cowling, inlet cowl, fan cowl, thrust reverser, core cowl, the exhaust system and more – and the amount of work required for each can vary. Moreover, as Callan notes, there are no prescribed shop maintenance intervals for nacelles.
   
“While some airlines base their nacelle maintenance on reactive events such as FOD [foreign object damage] incidents, our experience shows that this methodology increases long-term maintenance costs and impacts on nacelle reliability,” says Callan. “Taking a preventative approach and selecting the right maintenance partner can benefit airlines both operationally and financially.” 
   
He adds that, by tailoring repair workscopes to the appropriate phase of a customer’s aircraft lifecycle, UTAS  can deliver OEM quality service at market  competitive prices. „According to Roy Campbell, MRO Programme Manager, Europe & Asia for Bombardier Aerostructures & Engineering Services, of all the elements in the nacelle, the thrust reverser (TR) systems traditionally demand more attention from operators because of the nature of the component in terms of use and design. “The TR operates in a harsh environment. Actuation systems (moving parts), deployed engine bypass loads and temperature combine to challenge the component,” he explains.
   
“The TR is an on-condition maintenance component; however, many operators elect to follow a proactive, preventative maintenance approach. Operators will typically review TRs during scheduled maintenance activities, which contributes to a greater removal rate,” Campbell adds.
   
The next component requiring attention is the inlet cowl. “While static, its position at the front of the nacelle means that it is susceptible to FOD (bird-strike, debris, ground support damage),” he continues. “This component does not have any scheduled maintenance other than general visual checks. However, its anti-icing system creates a harsh environment which can have detrimental effects on the component.”
   
The Bombardier executive emphasises that the scope of any nacelle repair “is dictated by the level of disassembly and damage to be addressed and it is not uncommon at tear-down stage to find more damage than that reported”. A visual inspection can fail to pick  up hidden damage under the surface.
   
As an OEM, Safran Nacelles’ MRO capabilities bring together its experience as the nacelle designer, manufacturer and MRO services supplier. “Applying this expertise to MRO activities is highly valuable, especially with the growing use of composites in engine nacelles, making diagnostics of thermal degradation increasingly important,” comments Olivier Savin, the company’s Vice President of Customer Support and Services.
   
“An example is the inner fixed structure (IFS) element of a nacelle’s thrust reverser, which encases the engine’s portions located between the fan case and the nozzle. One side of the IFS faces the engine, while the other faces the bypass air duct and creates an aerodynamically smooth path for the cooler air,” Savin elucidates.
   
Vladimir Cervera, Vice President and General Manager of HEICO Component Repair Group – Structures Division, does not believe that any one element of a nacelle inherently needs more attention and/or replacement than the others. “The amount of work required is determined mostly by the condition of the unit received for service,” he observes. “However, for standard wear and tear, thrust reversers generally require the most work because they have a greater number of attaching components and are subject to higher aerodynamic and static loads during operation. Additionally, the thrust reverser also has a large number of moving parts that tend to develop wear more frequently than non-moving parts such as those belonging to the fan and inlet cowls.”
   
Expanding on Cervera’s points, Hastings Siegfried, Vice Chairman of NORDAM Repair Division, remarks: “Nacelle and reverser repairs are similar to those on other structures; however, the range of material types, design features, operating temperatures and service environments add complexity. Each nacelle component has its own specific requirements which greatly affect what we typically see when evaluating components for repair.
   
“The reverser requires more attention and is the more complex assembly of the engine nacelle system due to its dynamic nature, with inlet cowls being second,” Siegfried adds. “While component maintenance and structural repair manuals provide repair instructions for most types of minor damage, some components suffer more damage that requires the design of customised repairs.” >>

 


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