Circular maintenance of aircraft

Sustainability Snapshots: Circular maintenance, repair & reuse

Extending the lifetime of an aircraft can happen through different approaches. One of these approaches are circular maintenance, repair and reuse technologies – also abbreviated as MRO (maintenance, repair and overhaul) in the aviation and aerospace sectors. The fourth edition of our Sustainability Snapshots series explores circular maintenance, repair and reuse of aircraft, and how SUSTAINair will contribute to this crucial solution for circular aviation.

Current situation in the aviation & aerospace industries

Presently, aircraft are usually maintained according to fixed intervals or in a reactive manner when something fails for non-critical systems. These are generally human-based decisions in so-called adaptive, condition-based interventions. Historically, MRO in aviation and aerospace industries has mainly followed planned/scheduled or preventive maintenance approaches, as part of an integrated, long-term strategy to keep aircraft structures safe and fit for flying for as long as possible. However, nowadays data-driven maintenance approaches, such as predictive or condition-based maintenance (CBM), are progressively gaining traction in the industry. Such approaches are not only implemented by MRO companies like e.g. Lufthansa Technik, but also by Original Equipment Manufacturers (OEMs) themselves.    

Circular maintenance, repair and reuse of aircraft
An aircraft pulled into a hangar for maintenance (© Shutterstock)

Circular maintenance, repair and reuse of aircraft

In the public conversation on applying circular economy principles to the maintenance, repair and reuse of aircraft, the focus mainly lies on effectively leveraging data coming from sensors and algorithms to support efficient damage diagnosis and prognosisThe Future Sky Research Initiative of the Association of European Research Establishments in Aeronautics (EREA) notes in its White Paper on Circular Aviation that current practices are actually not that far off from effectively marrying MRO with circularity principles: 

"Current aviation practices connected to MRO fit perfectly with circularity principles. Aircraft structures are designed to last longer than most commercial products on the market; in order to achieve such long product life, MRO solutions are of paramount importance. In addition to build on the already significant experience in the field of MRO to support life extension programs, further research for integrated MRO solutions and predictive and prescriptive maintenance concepts, to optimize usage of aircraft and fleet shall be supported, in particular towards broadly implementing structural health monitoring systems and developing advanced non-destructive inspection and repair methods compliant with circularity principles."
Excerpt from EREA White Paper on Circular Aviation
Action lines for circular aviation across the aircraft lifecycle

Structural health monitoring, or SHM in short, is thus the key element in making circularity in maintenance, repair and reuse a reality. During aircraft operations, the full operational status could be maintained by integrating the information from the SHM system with the repair solutions designed synergistically with the aircraft itself. When it comes to repair/refurbishment of aircraft, circularity is favoured by modular design of structural components and interiors.

This brings us to the final piece in the MRO circularity puzzle, which is reuse as the number one circular economy strategy for MRO in aircraft. Reuse is essentially using a component again for the same purpose that it was originally completing. Reuse occurs when parts are removed from an aircraft to be used for the same function on another aircraft. Clever reuse of parts can thus prolong the function of an aircraft, extending its service life and reducing demand for new aircraft and components, with associated benefits of reduced energy and raw material requirements. The aviation and aerospace sectors currently successfully reuses some components from in service or retired aircraft in cases where the cost of new parts is high, such as e.g. the landing gear. Yet, in many cases replacement parts are still relatively inexpensive and easily sourced, meaning that new components are just fitted by default when part replacement is necessary. To increase the reuse of parts, more value must be attributed to the second-hand component, such as through:

Aircraft repair in hangar
Engineer repairing the wing of a passenger plane (© Shutterstock)

How SUSTAINair advances circular maintenance, repair & reuse

SUSTAINair will focus in its research on circular maintenance, repair and reuse of aircraft on the concept of Structural Health Monitoring (SHM). The project’s main objective is to develop a real time and onboard damage diagnosis and prognosis system – aka an SHM system – to deliver next generation condition-based maintenance of hybrid structural joints. SUSTAINair aims to achieve through:

  • Development of representative structural models validated by mechanical testing
  • Development and structural integration of novel sensors for SHM applications
  • Evaluation, selection and development of SHM methods for damage diagnostics
  • Experimental validation of SHM methods

The outcomes should provide strategies for SHM and repair of metal-to-metal, metal-to-composite and (thermoplastic) composite-to-composite material combinations to enhance cost-competitive joints in line with circular economy.

SUSTAINair will focus in particular on incorporating two key enabling technologies for circular maintenance. Firstly, it will deliver design for improved MRO operations of the morphing wing structure. This technology enables cost-effective future application of morphing structures in aircraft models, thereby filling the gap between innovative wing design and reliable repair solutions. SUSTAINair’s envisaged technology will allow for selective removal of only the damaged flexible skin, by debonding its interface and re-bonding a new skin without effect on the surrounding parts. Such a repair concept could save 25-50% in repair costs for morphing devices.

Secondly, the project will implement piezoelectric Zinc Oxide (ZnO) nanowires for SHM sensors in metallic and polymer parts. This novel technology used on all structural materials (i.e. metals, fiber-reinforced plastics – FRPs) enables the integration of sensors into the structural component and its manufacturing process. Compared to the current typical bonding of sensors on a structure’s surface after the actual manufacturing process, the sensor integration has the potential to reduce manufacturing costs and increase the robustness of the SHM system. Consequently, maintenance downtimes are expected to be decreased and lifetime of individual parts could be increased by up to 100% of calculated lifetime due to the SUSTAINair condition-based maintenance approach.