Printed headline: The Paradoxical Component
Nacelles and thrust reversers look simple but are in fact complex, due to multiple design drivers. They do not require scheduled maintenance but are highly exposed to damage. Now, the evolution of their design may be making the maintenance technician’s life harder. Maintenance, repair and overhaul service providers are striving to find more efficient tools to meet new needs and streamline repair processes.
Some engine designers think the nacelle is just an envelope. But a nacelle, including its thrust reverser, has to play several roles. One of them is structural, as the pylon that attaches the nacelle to the wing has to transmit engine thrust.
The acoustic part is increasingly important—the nacelle is the ideal place to suppress engine noise. That is especially true for the thrust reverser, due to its location when inactive. It also acts as a nozzle for bypass air (the cold air flow around it that goes through the engine core). The geometry of the thrust reverser helps smooth the air flow, limiting drag. In other words, nacelle design affects fuel burn. For the thrust reverser, assisting the brakes is important when the runway is slippery.
Moreover, the nacelle also has an aesthetic role, as it often carries the airline’s logo. The representative of a customer airline once arrived at an Airbus delivery facility with a magnifying glass. He was checking the golden edging on his company’s logo.
Finally, the nacelle operates in a violent environment of high temperatures and reverse thrust blast and is designed to withstand 175C (347F). In certain places, thermal insulation blankets are installed to protect the composite structure from higher temperatures. Those blankets are made of metal foils filled with ceramic fiber, says Henrik Radzanowski, Lufthansa Technik’s head of product engineering and planning.
A Tailored Suit
The nacelle is a fitted suit to the engine. In the old days, a common-type nacelle could be put on different engines, says Dana Stephenson, vice president of aftermarket services for United Technologies Aerospace Systems’ (UTAS) aerostructures business. But those uniform nacelles led to a loss of efficiency. Now, in the geared turbofan age, it is even more of a necessity to tailor the nacelle to the engine. A geared turbofan has a larger fan diameter, which poses a greater weight challenge.
“A recent evolution has been, on our A320neo geared turbofan system, interchangeability among nacelle components between engines No. 1 and 2; it limits the number of assets an operator has to procure,” Stephenson says.
Another new feature on UTAS’ recent nacelles is a detection system installed on fan cowls that verifies they are latched correctly.
A nacelle is designed for on-condition maintenance, but access to the engine is frequently needed, so the access door location and type of fastening system are design factors. “Let’s minimize downtime and improve access; carriers operate with tighter times at the gate, so access time is money,” he points out.
“Maintainability is increasingly important for operators, and we take it into account right from the design phase; one benefit of being active in MRO, for us, is gathering operational feedback,” says Jean-Hugues Cousin, head of Safran Nacelles MRO activities.
Dings, Damage and Burns
However, recently designed nacelles—such as those found on the Boeing 787, Airbus A350 and A320neo—have lower limits for allowable damage, according to Lufthansa Technik’s Radzanowski. “This is the result of a weight-saving focus, and extensive shop repairs are required more often,” he says.
So what kinds of damage are nacelles and thrust reversers exposed to? Typically, a baggage cart or a boarding bridge hits them and causes a dent. Or a bird strikes the engine air inlet. “The inlet is the part of the nacelle that suffers the most from bird strikes and foreign object damage,” he explains.
Damage also can come from ice thrown by a fan blade, or a lightning strike may cause a small hole and a delamination.
Similar to other composite parts, nacelles suffer from environmental influences such as UV radiation, temperature changes and contamination that degrades bonding or surface protective coatings such as paint. If that occurs, water, oil and kerosene ingress may cause delamination, says Radzanowski.
Overall, the main MRO cost drivers for nacelles are delamination, as well as dents and cracks due to foreign-object damage or aging, says Philippe Servant, head of the aerostructures engineering department at Air France Industries-KLM Engineering & Maintenance.
“Repairs on composite materials are always different, so we have to adapt to the damage we see,” he says.
In Servant’s view, there has been little evolution in nacelle design since the Boeing 777’s composite nacelle. What has changed are the limits for allowable damage, depending on the precise location on the nacelle.
Nondestructive testing (NDT) is used extensively on nacelles. Detecting damage starts with a walkaround—for visual inspection—followed up with NDT methods such as thermography, ultrasound or X-rays to detect delamination and other abnormalities, says Radzanowski.
“To perform a repair, the technician detects and removes the damaged area. That is followed by scarfing the surface and reapplying composite material,” he explains. A local repair can be cured with a heat blanket, which provides a temperature of 125-175C. Sometimes an external doubler is installed, which is faster but negatively affects aerodynamics. Larger composite-assembly repairs have to be cured in an autoclave.
Technically, most parts are repairable—but the effort needs to be worthwhile in terms of cost. A ratio between the cost of the repair and the value of the component of 60-70% is the tipping point, says Stephenson.
What if a nacelle is not repairable? That is very unlikely, at least at the entire-nacelle level. Events like a landing gear breaking loose during landing are rare.
The inlet’s metallic lip is often not repaired because the aluminum is too thin. Generally, in case of damage, one lip sector is replaced.
“We manage to repair increasingly large nacelle components; we used to replace the A330’s thrust reverser door, if damaged, and now we can repair it,” Cousin says.
Another trend is a need for quicker turnarounds. “Market pressure forces us to devise fast repair processes,” says Cousin. An A320neo can fly 8-9 legs per day, so availability is even more critical. “Our mobile teams use an ultrasonic tool for damage detection.” It can help avoid component disassembly, but few technicians know how to use the tool because of the required training to analyze graphic results.
NDT is improving, especially to detect delamination, according to Air France Industries’ Servant. Thermography will keep evolving to find lower levels of water ingress.
A 3D scanner AFI-KLM E&M recently has developed in-house helps measure damage. As it precisely depicts a component’s shape, it also helps with creating tooling for repair.
The company is developing a robot that could machine plies of composite material at the edge of a cut-out. Hand-scarfing or hand-stepping is difficult, and a machining robot would save time. Such robots already can repair a 5-in.2 impact on a fuselage. “We are trying to create one for a greater curvature,” says Servant. Development is ongoing, in partnership with another company and a university.
A broader source of partnership for the industry overall is the Commercial Aircraft Composite Repair Committee (CACRC), which gathers airframers, airlines and MRO service providers to improve the maintenance, inspection and repair of composites. “We share repair methods, and we try to improve their performance,” says Servant. The CACRC meets once every 10 months.
Get It Scheduled?
A mantra for MRO providers seems to be persuading operators that they need a maintenance program for their nacelles. A nacelle or a thrust reverser is an on-condition item; it does not have to undergo scheduled shop maintenance at fixed intervals. Nevertheless, Lufthansa Technik offers a nacelle maintenance program. It starts with on-wing inspections “to get a good idea of the condition,” says Radzanowski. For an operator, signing up for the program results in lower maintenance costs over the years, Lufthansa Technik promises. “For example, an early detection of delamination involves limited repair work. Such a preventive maintenance concept translates into better in-service reliability,” he says. A team of technicians travels worldwide to perform inspections and on-wing repairs. Extensive repairs are performed at Lufthansa Technik’s workshops in Hamburg, Dubai and Shenzhen, China.
“No scheduled maintenance is required, but we demonstrate there is a real economic interest to have a contract for regular maintenance,” adds Cousin. Safran Nacelles offers tailored maintenance programs to operators, aiming at reducing their nacelle-maintenance spending over the life of the aircraft. “Thanks to our Fleet Data Center, we can determine the optimum flight hours/flight cycle ratio for a scheduled shop visit. For example, we suggest our customers operating the Airbus A330 with Trent 700 engines organize a first shop visit seven years after entry into service,” he adds.
Carriers operate nacelle systems very differently, UTAS’ Stephenson notes. Some perform on-condition maintenance, using their nacelles until they discover a problem. “We recommend soft-time programs,” he says. UTAS’ Prime Solutions customizable suite of nacelle maintenance services can include rotable assets as well as MRO services for an operator’s fleet on a power-by-the-hour basis to help avoid surprises. “It is based on a sampling program at a given number of hours, and then we work out a customized removal schedule,” Stephenson says. It depends on how the operator uses a nacelle and engines and in what environment [such as in hot and heavy air]. Engine temperature and the use of brakes, for instance, also affect the lives of nacelles and thrust reversers.