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A350 XWB — Maintenance Efficiency By Design

The reduction of maintenance costs was a key consideration in the design of the A350 XWB. Here, Airbus gives an overview of the benefits this has brought about, as well as the maintenance programme itself.

The A350 XWB is the answer to maintenance cost reduction, thus we can say that it is a “Maintenance Efficiency by Design” aircraft. The A350 XWB is indeed designed to meet economy, flexibility and efficiency - this comes from our 'Make it Simple' design concept, leading to a simpler architecture compared to current generation aircraft. At system level, examples include the hydraulic system (ATA 29), the new pneumatic system (ATA 36) with the EBAS technology already in use and proven on the A380, and the simplified design of the main landing gear with no shortening mechanism and the extended use of titanium for a higher robustness and corrosion resistant materials.

Our 'Make it Simple' design concept extends to the A350 XWB's avionics system design which is based on the well proven (A380) Integrated Modular Avionics (IMA) technology. On the A380, 23 functions are covered by IMA, but the A350 XWB IMA will manage more than 40 functions such as the air conditioning system, the landing gear and brake systems, the fuel, the pneumatics, the fire detection, door and slide control system etc.

The major benefits are maintenance costs reduction due to integration and simplification of systems (fewer systems mean less potential for fault-finding errors), but also in terms of weight savings. At component level, the use of IMA technology will reduce the number of components and its complexity (reduced wiring connections, Ethernet connectivity with higher speed etc). At the same time the reliability will increase, and so will the redundancy.

The IMA concept is based on standard computing modules (CPIOMs, Core Processing Input/Output Modules) running several applications in a partitioned environment providing full segregation between functions. These modules, which replace the standard Line Replaceable Units (LRUs), are onboard loadable and onboard replaceable (Line Replaceable Modules), and provide standard services as data loading and Built-In Test Equipment (BITE) for onboard systems test. In the event of failure, in most of the cases the affected module can be easily replaced onboard and the new module reloaded with the latest standard software. This will save time for troubleshooting and fault rectification.

It is also worth mentioning in the cockpit itself, the pilots share six large identical LCD displays. These screens are fully interchangeable and have the same part number for reduced spares requirements, while their advanced design and mature technology cuts maintenance costs by up to 80 per cent.

Design features to improve component reliability

Many design features have been put in place to reduce the maintenance cost and to improve the A350 XWB component reliability. One such example is the hydraulic system. The A350 XWB (as with the A380) has only two hydraulic systems (each running at 5,000 psi) compared to the current aircraft and to the 787. As one can image, this will reduce the number of hydraulic lines, connections and valves by one third, reducing at the same time the potential for leaks and weight. Additionally, the accumulators and heat-exchangers are maintenance-free.

Airbus also decided to design the A350 XWB with a hot air-bleed system to de-ice wings leading edges instead of using an electrical system (bleed-less). The state-of-the-art pneumatic system (EBAS) - proven on the A380 - benefits from electrically commanded and pneumatically actuated valves and of its "localised" leak detection system. It has proven to be so efficient and leak-proof that the number of in-service operation interruptions decreased by 70 per cent compared to the older pneumatic system.

The choice to use a bleed system is also justified by the fact that the electrical systems (ATA 24) require much less power (550kVA) compared with a bleed-less aircraft that will need three times more power (around 1,500kVA). Another advantage of a bleed system versus a bleed-less one is related to the air conditioning system (ATA 21), in that a bleed-less choice will need a heavy-duty compressor to compress the air.

Maintenance benefits of intelligent airframe / advanced material choices

Taking into consideration the intelligent airframe and advanced material choices made on the A350 XWB, with 53 per cent Carbon Fibre Reinforced Plastic (CFRP) and 14 per cent titanium, the number of tasks of the former six-year check will be drastically reduced - the CFRP being corrosion-free and fatigue-free and the titanium being corrosion-free. It is important to note that the Airbus aircraft have a solid experience of composites, starting from the A310 movable surfaces as spoilers, rudder, airbrakes, up to the A380 that is built with 25 per cent of composites. We have performed an intelligent material selection to push the heavy maintenance tasks (tasks with heavy access requirements) to at least 12 years (former D check).

Onboard Maintenance System and advanced diagnostics

The A350 XWB will build on the advanced monitoring and diagnostics systems already developed for Airbus aircraft such as the A380, which has been in-service for some years now. The A350 will be fitted with an advanced Onboard Maintenance System (OMS), instead of a stand-alone Centralised Maintenance System (CMS). The A350 XWB's OMS has increased the monitoring of aircraft systems and allows the maintenance staff to isolate any kind of fault and to get in a smart way to the final troubleshooting solution with interactive links to the main maintenance manuals, the AMM, IPC and TSM. This allows (as is the case today with the A380) a reduction in the time taken for fault-finding and fault-isolation, with less potential for non-routine work and with reduction in terms of No-Fault Found (NFF) removal rates. We estimate that the A350's NFF removal rate will be reduced by 50 per cent compared to current-generation aircraft. Obviously the consequences will become visible in terms of reduced spares requirement and reduced manhour requirements for line maintenance work.

As part of its new advanced OMS, the system on the A350 XWB adds another feature which allows reduction of fault-finding and fault-isolation time, the so-called Dispatch Advisory (DA) function. The DA function consists of cockpit messages which are created only for Cockpit Effect having an impact on aircraft dispatch (linked to a Master Minimum Equipment List, MMEL). A DA message indicates that the MMEL must be consulted for dispatch, and each DA message is used as the entry point into the MMEL (one DA message is related to a single MMEL item). The advantage is to have a clear and single link between root cause and associated dispatch consequences, thus reducing unnecessary messages and troubleshooting time. Operational information/warnings during the flight itself do not generate a DA. This means that the crew will see a DA message during the flight only if they wish to see it in the dedicated page.

Moreover, as per other Airbus aircraft, the A350 XWB will benefit fully from the world famous AIRMAN solution. The parameters of all the principal onboard systems will be available in real time at the operator's preferred location, such as its Maintenance Control Center (MCC). This allows the engineering and maintenance people to access at any time aircraft data such as failure and/or warning messages while the aircraft is still flying. This will assist on-time troubleshooting but also proactive maintenance and failure anticipation, making the maintenance staff ready for any component replacement well before the aircraft lands at its final destination.

Overall the A350's innovative design will raise maintenance efficiency and lower costs, while simultaneously supporting high levels of utilisation and reliability. This results in less maintenance at longer intervals. For example, the innovative technology and system simplification and integration have much to do with our overall target to reduce the airframe direct maintenance costs of the A350-900 by 40 per cent compared with the 777-200ER, and by 10 per cent compared with the 787-9.

A350 XWB maintenance programme overview

First draft of the A350 XWB Maintenance Review Board Report (MRBR) delivered

During the Industry Steering Committee (ISC) meeting held on January 15-17, 2013, in Toulouse, Airbus Customer Services delivered the first draft of the Maintenance Review Board Report to the ISC Members and Maintenance Review Board (MRB).

The MRBR provides the initial minimum scheduled maintenance tasks and the corresponding intervals for the aircraft systems, powerplant, structure and zones. It is one of the compliance documents to CS25.1529 Appendix-H and it is developed in accordance with FAA Advisory Circular AC121-22 “Maintenance Review Board Procedures” and EASA Work Instruction WI.MRB.00002-001 (MRB Team).

As it is a draft, the document is subject to approval by the MRB, although it nevertheless represents a significant achievement - which was highly appreciated by Airbus' customers. The document has been produced from extensive works involving mainly Airbus Maintenance Program Engineering teams, supported by A350 Program Engineering teams and about 270 persons from 24 A350 XWB operators and MROs, the MRB (including: EASA, FAA, TCCA, CAAS and CAD) and a few selected suppliers.

During the 54 one-week Maintenance Working Group Meetings organised, more than 350 MSG-3 Dossiers had been reviewed in a collaborative working environment, allowing the establishment of a set of more than 700 scheduled maintenance tasks which were validated during the 11 ISC meetings. Airbus will be the first "customer" of this document as it will be used as the basis to develop the Flight Test Aircraft Maintenance programme to be jointly developed within various Airbus domains (SEME, SEMSV, QEV and A350 MAP).

Overall, the completion of the draft MRBR was a significant achievement toward the objective of document approval by EASA, FAA and TTCA by the time of A350 XWB type certification next year.

Flexible maintenance programme concept

The A350 XWB maintenance programme is based on a "usage parameter" concept which is now adopted on all Airbus programmes. The most appropriate usage parameter and its associated interval are defined for each maintenance task. This prevents the development of a too conservative approved maintenance programme, driven by task planning considerations (that may differ from one airline to another).

With this concept the operator has the flexibility to choose the timing of the tasks to be performed depending on the aircraft utilisation, having the possibility to split the work during different night shifts for example, or keep the tasks package concept if preferred. This allows maintenance to be performed only when necessary, to package tasks according to aircraft actual utilisation, airline maintenance policy and practices, rather than having to adhere to pre-defined block-checks.

Airbus' target for the A350 is to have a reduction of around 40 per cent MPD Maintenance Manhour (MMH) compared with current-generation aircraft (e.g. the 767). Today, based on the latest assessment Airbus is confident that this objective will be met and even exceeded.

If we consider a yearly utilisation of 4,700 flight hours (FH), the A350 XWB will have almost 20 fewer light checks (former A checks) compared with a current-generation aircraft over a 12-year period. In the same period of time, the target is to reduce the number of base checks (former C checks) from eight to four, based on a typical block-check concept.

Last but not least, another milestone that the A350 XWB can reach is to remove the intermediate check (the so-called six-year heavy check) and to perform the first heavy check at 12 years only, together with the first landing gear overhaul.

Line maintenance

The A350 XWB is designed to have no scheduled maintenance below 10 days, apart from some visual checks at transit or daily check. The target is to have a first interval check at 48 hours for visual inspection of wheels, tyres and brakes that can be easily and quickly performed. The next group of tasks concerns the oil monitoring level. These tasks can be performed with just one man in about one hour, including access time. (The A350 will have the capability to monitor, to some extent, the hydraulics liquid and rotable components through the OMS and its associated pages.)

A checks and base maintenance

The target interval is at 1,200 FH; this can be compared to a current A check (today typically at 800 FH for the A330), with a more flexible check content based on operator utilisation.

There will be multiple such tasks at 2,400 and 3,600 FH which will include some Variable Frequency Generator (VFG, ATA 24) oil filters and some light operational check which can be easily performed onboard the aircraft through the OMS. All this will reduce the grounding time, because these tasks can be easily performed overnight.

Base interval target for base check inspections

The A350's base check interval (former C check) will be at 36 months. So, it will be intervals at 72, 108 and then 144 months respectively. The 144-month check, the so-called 12-year heavy check, will be the largest check comprising the 1C, 2C and 4C checks. Within a base check, our target is not only to reduce the number of tasks, but also to shorten the elapsed time to perform similar tasks. Based on the intelligent airframe and advanced material choices made on the A350 XWB, the previous intermediate six-year heavy check will be split into different base checks at 36-month intervals, with a short grounding time of around four to five days.

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