Saturday, April 1, 2006
Coming Soon: The Innovative Airbus A380
Airbus Promises easy maintenance on innovative A380 super jumbo
INNOVATIVE WEIGHT-SAVING DESIGN FEATURES WILL ENABLE THIS GIGANTIC AIRCRAFT TO FLY. BUT HOW WILL THESE FEATURES IMPACT THE MAINTAINABILITY OF THE AIRBUS A380?
Airbus is promising the unprecedented use on the A380 super jumbo of composite materials, will not change the maintenance technician's habits. Similarly, the first use of 5,000 psi hydraulics--a major weight saving--on a commercial airplane should have little impact on checks and repairs. Another weight saver is the use, for the first time, of aluminum instead of copper in the wiring. The challenge has been proper installation.
Airbus anticipates the A380 will get its type certification in October, after some delays mainly due to cabin interior and electrical harness development. The 550-seater should then enter into service by the end of 2006. Singapore Airlines is the launch customer.
The A380 is the first aircraft to use Glare, a hybrid material (fiber metal laminates), built-up from alternating layers of aluminum foils and unidirectional glass fibers, impregnated with an adhesive. Dutch-based Stork developed this new fiber laminate with Airbus. Its qualities are a compromise between conventional aluminum alloys and carbon-fiber reinforced plastic (CFRP).
"Glare is lighter than aluminum and has very good mechanical properties," Eric Grosjean, materials and processes manager, A380 program, emphasized. The density of conventional aluminum alloys is close to 2.8, whereas that of Glare is approximately 2.1 and CFRPs have a density of about 1.6.
Fatigue and damage resistance are superior to those of aluminum. The energy required to create a dent (of a given depth) is much higher than it is for aluminum. Cracks initiate earlier but are stopped at each glass fiber layer. As a result, the propagation is so slow that a crack cannot reach a critical size in the entire aircraft's life in service. Due to its high resistance to impacts, Glare was chosen for the leading edges of the empennage. Corrosion, too, is stopped by the sandwich structure. Tests have shown that corrosion is stopped at the first glass fiber layer, whereas, in the same conditions, it goes through the aluminum's thickness.
Several types of Glare have been defined. They depend on the layout of the layers of glass fiber. For example, those with layers oriented at 45 degrees in each direction are suitable for door area reinforcement.
Glare was chosen for the upper fuselage in the forward and aft sections because of its fatigue performance and damage tolerance. Aluminum was chosen for the center section because it is submitted to higher static loads, due to the presence of the wing. A butt strap, however, is made of Glare in the center upper fuselage. A German military A310 transport was fitted in 1999 with a Glare fuselage panel. Since then, it has accumulated 5,056 flight hours and 1,339 cycles. "There was nothing special to report, compared to conventional aluminum panels," Grosjean said. For example, two small circular detachments of topcoat were found on the surface but were not caused by the Glare material. A non-destructive test (NDT) crack inspection was performed, without any finding.
Grosjean clarified that choosing a material for a given portion of the aircraft involved the customers. Since very early in the program (in fact, even before it was launched in 2000), they have been asked for their input through customer focus groups and technical progress reviews. "When we had to choose between two technologies that were very close, experience feedback was an important criterion," he added.
Where Glare is used, CFRP was also an option. It is lighter and its performance, especially in terms of fatigue resistance, is very close. However, its production cost is higher. And, most importantly, it was not seen as mature enough for use on fuselage panels. Grosjean cited the material's impact on maintenance. "Although airlines have developed CFRP repair techniques, their know-how is mainly in metallic repairs," he told Aviation Maintenance.
Glare Inspection and Repair
As Glare is a new material, an inspection program has been set up. It affects the first years of operation of the A380. These inspections will be added to the existing scheduled maintenance program. In the mid-term, Airbus is confident that it will require fewer inspections than conventional aluminum alloys. After a repair on a Glare panel, no specific inspection will be required.
After accidental damage on a Glare panel, a visual inspection is supposed to allow a first assessment of the damage extent. Therefore, this does not require special skills in addition to those required for visual inspections of aluminum panels. "If you see some damage, then the damage should not extend farther than what you see on the surface," added Thierry Jordanet, senior engineer of A380 maintainability. If necessary, in case of very large impact damage, ultrasonic inspection is possible as it is on aluminum panels.
"Repair principles are the same on aluminum and Glare," Grosjean stated. A riveted aluminum patch can be used to permanently repair Glare. "Nothing special is required in terms of repair know-how or tooling," he stated. This choice was done in accordance with customer-expressed needs, Jordanet pointed out. "Aluminum is more easily available anywhere in the world and less costly, too," he explained.
Repair principles for new A380 technologies were presented during a dedicated workshop held in Memphis in May 2005, including Singapore Airlines representatives.
Technicians use conventional tooling, for example, carbide tools for manual trimming and drilling. The steps they have to follow are exactly identical to those of an aluminum panel repair procedure. Of course, the repaired Glare panel's properties are at least as good as they were before the damage.
"We have taken into account in-service experience from other Airbus aircraft as well as customer feedback to establish our structure allowable damage limit (ADL) requirement," Jordanet told AM. Most frequently affected areas are where servicing trucks can hit the aircraft on the ground--namely the lower fuselage and the doors' surroundings. In general, after structural repair manual (SRM) consultation and as soon as damage is found above the ADL limit, maintenance engineers have to report to Airbus for repair advice before any further flight.
Regarding CFRP repair concepts, Jordanet said either flush or external patches can be used. "Flush repairs are aesthetically and aerodynamically better, however, they are more sophisticated and therefore not suitable to some areas that are difficult to access," he told AM.
For a structural repair on a solid laminate part, the patch is either made of metal, mainly aluminum alloy, or pre-cured carbon fiber. For a sandwich structure, a CFRP patch can be used. Riveted repair standards are applicable to CFRP as they are to welded and Glare structures. Cosmetic, bonded repairs are still possible in some instances on CFRP. Curing temperatures goes from room temperature to 80 degrees Celsius.
"In short, a maintenance technician who knows how to repair another Airbus aircraft's airframe will know how to repair an A380," he said.
Generally speaking, on the scheduled maintenance program, Jordanet emphasized the enhanced flexibility. "When applicable, operators can choose intervals either in flight hours, flight cycles or calendar time, according to what suits them best," he said. A "flexible plan" can be worked out, allowing tasks to be allocated to visits according to limitations, resources and downtime. On the Glare and CFRP panels, only visual inspections are required during scheduled maintenance. In calendar time, base maintenance should be performed every month and a half or 750 flight hours, light maintenance every 24 months, intermediate every six years and major maintenance every 12 years. The latter two are an improvement over the previous five and ten years, respectively.
June will be an important milestone for the A380's Maintenance Program Proposal (MPP): the final version of the A380 MPP will be submitted to the aviation authorities (4 months prior to type certification). The A380 will get a joint certification from the Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA).
The maintainability of the A380's vertical tailplane has been considered. After the A300-600 crash in Belle Harbor, NY, on November 12, 2001 (flight AA587), tailplane inspections of those aircraft proved difficult. "On the A380, we have taken into account flutter vibrations in the design of the flight control surface attachments and have developed new materials for the flight control bearings," he said. Access panels have been added to ease inspection and to facilitate potential removal/installation of the movable surfaces' fittings. Airbus is currently optimizing replacement procedures and time for these components.
High Pressure Hydraulics
The use of a higher pressure in the hydraulic system, 5,000 psi instead of the conventional 3,000 psi, translates into significant weight savings. The hydraulic power equals the pressure multiplied by the fluid flow. Therefore, a higher-pressure circuit needs less flow and therefore smaller-section pipes. The pumps, too, can be made smaller. The order of magnitude of the direct weight saving is one metric ton (2,200 pounds). "But the indirect savings are significant, too," said Fares Mahjoub, A380 hydraulic system senior manager. For example, even though the hydraulic circuitry is smaller, the rear part of the wing is already cramped with systems. A conventional hydraulics circuit would have been more bulky than the high-pressure one. So the designers would have had either to relocate some fuel tanks or increase the wing's thickness.
According to Mahjoub, maintenance technicians will find little difference in servicing the higher-pressure system. "Some components do change but without any major difference," he said. The metal of the pipes is thicker, he admitted, so they can take longer to cut. Repairs can be performed with swaged fittings, a technology already used on current airliners.
In terms of safety, he asserted that the threat posed by a micro leak is no bigger at 5,000 psi. A bigger fluid jet is deemed almost impossible. Flammability standards are the usual ones, with fluid self-igniting temperature above 400 degrees Celsius. "On the landing gear, adequate protections have been taken to isolate the fluid from the hot surfaces of the brakes disks," Mahjoub added.
With respect to hydraulic system reliability, Mahjoub said that it was in fact more mature than it was at entry into service on previous Airbus aircraft. More stringent rules were set for qualifying any hydraulic distribution component, like fittings. All components had to undergo 300,000 pressure cycles. The aircraft is designed for 19,500 cycles and, even though there are several cycles per flight, the tests give a fair margin. "We ran quite intensive qualification tests on the various components, including long endurance tests," he explained. To design the hydraulic fluid collectors Messier Bugatti has built a demonstrator that included several concepts. Several were rejected before the final one was chosen.
Another example was a pump derived from the F/A-18 fighter. It ran 2,000 hours, feeding actuators and other hydraulic components. "We took a close look at wear and corrosion before validating the pump," Mahjoub stressed. The iron bird was very useful as well to test system reliability. Fewer leaks and failures were recorded than expected.
A major innovation is the number of hydraulic systems: two instead of the usual three. On the A380, the so-called green circuit is connected to engines number 1 and 2 (those under the left wing) and the yellow circuit is connected to engines number 3 and 4 (those under the right wing). There are two electric circuits, too, hence the "2H2E" name of the architecture. Electric power is used as a back up power source for the flight controls and landing gear. It is locally converted into hydraulic power at the actuator level, though.
The A380 is the first civil aircraft to feature 5,000-psi hydraulics. Boeing is following with a 5,000 psi system on the 787. Concorde, the Anglo-French supersonic airliner, had a 4,000-psi system. "Concorde used a different fluid, which was more difficult to handle because of its reactivity with air," Mahjoub stressed. The A380 sticks to the conventional phosphate ester family. One of the latest variants will be used, Type 4 or Type 5. Durability is extended and viscosity is reduced, which is better for power transmission performance.
So what have been the main challenges? "Finding suppliers, developing new components and, generally speaking, pioneering," Mahjoub said. Those who have the largest experience with the higher-pressure systems are the military--with the F/A-18, the V-22 Osprey and the Rafale, among others. But it is a different world, Mahjoub insisted. For example, a hydraulic pump lives 2,000 flight hours on a fighter. "We request 50,000 hours," Mahjoub said. And an airliner operation does not imply frequent changes in rotation speed. "At the end of the day, we drew more experience from civil 3,000-psi systems than we did from military 5,000-psi ones," Mahjoub said.
Some 300 of the 500 kilometers of wiring use aluminum instead of the conventional copper as the current conductor. Thus, 20 percent of the conventional weight has been saved. Aluminum itself is 50 percent lighter than copper.
The aluminum wiring technology has been used on all types of Airbus aircraft for big sections, that is wiring or cable where the cross-sectional dimension is 5 square millimeters (sq mm) or more. The A380 is the first aircraft to use the technology for small sections, under 5 sq mm. and for the connections. There are about 100,000 electrical links on the A380 compared with 60,000 on the A340.
The challenge was protection against corrosion. The new wires are made of nickel-plated, copper-clad aluminum strands. A special protection, based on composites material, has been developed against corrosion. A new specification was worked out for the aluminum connectors. The inside of the insulation material is made of hydrolysis improved polyimide tape. The outer insulaton is made of improved PTFE tape. Airbus also highlights the insulator's resistance to arcing.
These features are intended to make the A380 more maintainable; so much so that Direct Maintenance Cost (DMC) per seat is hoped to be 24 percent lower than that of the Boeing 747-400.
Virtual reality tools aid Airbus maintenance training
Last September Airbus's Maintenance Training Center in Hamburg introduced two virtual reality devices expressly for the purpose of maintenance training, becoming, as Airbus maintains, the first such organisation to receive approval for the task.
Designed to aid training courses on the A320 Family initially, with the system to be extended to the A330/340 and then A380 aircraft in 2006, the virtual reality aids are said to represent a leap forward in instilling the latest maintenance practices in trainee technicians. The virtual reality system is the outcome of past months of collaborative work between Airbus and CAE and was approved by the French Direction G�n�rale de l'Aviation Civile (DGAC). Airbus is calling this the MFTD (Maintenance/Flight Training Device).
The first of the two devices provides replication of the flight deck of the A320 series using computer screens to simulate the aircraft systems and maintenance functions. The Virtual Aircraft simulates the complete airplane and enables trainees to walk around the aircraft, visit different work areas, or work on components, including removing and replacing them in a three-dimensional, on-screen environment.
Together the two simulation-based tools cover an entire aircraft, both interior and exterior, allowing trainees to consolidate their theoretical knowledge and gain practical skills before working on the actual aircraft. The programs do not negate the need for hands-on training but enhance the whole maintenance training program.
Thorsten Behrendt, head of the maintenance training center in Hamburg, explained that the introduction of virtual reality was a feature of the new concept called `active learning and competence-focused training' (ALCFT) which gave enhanced training but also provided trouble-shooting insights. Up to 12 people can be trained at one time in a classroom, using two screens, with the flight deck screen providing a 3D environment with a touch-screen facility. On the simulated flight deck the centralised fault display system can be accessed in order for the trainees to understand the procedures for picking up maintenance problems as reported. Prior to the introduction of ALCFT the school used cockpit simulators which offered relatively basic skills training with the trainee having to greatly refer to the maintenance manuals, Behrendt told Aviation Maintenance.
Some 50 percent less time has to be spent on the actual aircraft as a result of the virtual reality system, Behrendt said, and feedback has revealed that trainees themselves are happier with the clearer understanding they have from the new program. "We already have a good feedback from trainees that they are well prepared, for afterwards they realise they are exercising trouble-shooting practices as in a real environment. As an example," Behrendt continued, "we carried out an early trial on component locations using a virtual aircraft and then when we went to the actual aircraft from the Airbus production line, which is part of the theoretical and practical training, students said they had seen the equipment before in a virtual reality environment and were already familiar with it. The overall course time had not been shortened, but the time approved by the authorities for instruction on the aircraft had been reduced from ten days to five, producing a 50 percent benefit to the Airbus MTC."
In regard to the larger, twin-aisle Airbuses, the ALCFT concept will be applied near the end of 2006. The timescale for this provides for the A330/340 series to be subject matter first and then the A380. A380 maintenance training will be performed at Toulouse. A350 maintenance training will follow later. -- By Roy Allen
Jointly with ANAC (Akzo Nobel Aerospace Coatings) and PPG, Airbus has developed a new paint process that should simplify subsequent stripping and re-painting. An intermediate layer is the secret. It is located between the primer layer, the main role of which is to protect the airframe, and the upper or external layer, which features the airline's livery. The upper layer is porous enough to allow a special chemical fluid (called a benzyl alcohol stripper) to reach the intermediate layer. The intermediate layer reacts with the benzyl alcohol stripper and morphs into a foam that "inflates" rapidly. The increase in volume breaks the upper layer. PPG calls this a selectively strippable process.
According to PPG, the stripping process is thus made 40 percent shorter. Mechanical stripping (blasting compressed air and sand onto the painted surfaces) is more laborious for the workers. In addition, the primer does not have to be replaced. Moreover, the new paint is the only one to allow chemical stripping on composite materials, which are increasingly used on today's aircraft.