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Advanced Landing Gear Sensing and Monitoring

Periodic Reporting for period 4 - ALGeSMo (Advanced Landing Gear Sensing and Monitoring)

Reporting period: 2019-09-01 to 2020-10-31

The primary objective of the Advanced Landing Gear Sensing and Monitoring (ALGeSMo) project is to bring together a world-class team from across Europe to deliver a state-of-the-art optically-based load monitoring system for aircraft landing gear. This will yield a step change in the way landing gear is utilized and managed leading to a new paradigm in aircraft availability and operability. This will assist the development of European capability within this domain such that Europe will be well-positioned to provide state-of-the-art equipment to future aircraft development.

The goal of the project is to develop and deliver the following:

- A Fibre Bragg Grating (FBG) optical sensor system using Wavelength Division Multiplexing (WDM) techniques to accurately detect and translate landing gear strain and torque to the aircraft control systems.

- A state-of-the-art and highly accurate integrated photonics fibre-optic interrogator that will result in the smallest system currently available anywhere in the world.

- Detailed design of system architecture and integration to ensure that the appropriate Design Assurance Level (DAL) requirement can be achieved.

- Definition and build of loading and calibration rigs in order to characterise sub-system and system components using a full landing gear slider tube assembly.

- Completion of system testing and partial qualification testing to de-risk the technology and demonstrate TRL5.
Optical interrogators are traditionally very delicate systems. The Fibre Optic Processing Unit (FOPU) has been subjected to High temperature, Low temperature, Vibration, Shock and Humidity. The FOPU showed satisfactory results in all of these tests, and in many others and is considered suitable for aerospace use.

The entire optical load measurement system (a pair of axle sensors, an optical harness and a FOPU) was fitted to a landing gear axle and installed into the test rig. A comprehensive test campaign has been performed. The axle sensors were instrumented with strain gauges and temperature sensors for comparison. It started with calibration tests and moved on to high-load tests, dynamic tests, tests at high and low temperatures, shock and vibration tests. The outputs of the ALGeSMo systems were continuously recorded and compared to the loads actually applied and the measurements of the strain gauges and temperature sensors. Three successive iterations of axle sensors have been tested, each of them improving on the issues identified on the previous one. The loads applied during testing are representative of aircraft operation. Loads along all three axis of the landing gear and the torque about the wheel axles were measured and showed a very good correlation with the applied loads. The load measurements were generally within the required accuracy, with the exception of a few data points.

The research activities have advanced the state of the art in several key areas for the deployment of optical sensing systems for safety-critical applications (e.g. integration of optical fibres into composite material, robust optical connections, avionic-compliant optical interrogator and landing gear load measurement). The technology building blocks developed during this project can also be used in other aerospace applications, not only for landing gear load measurement. There is a general consensus in the aerospace industry that optical sensing can bring significant benefits. Thanks to ALGeSMo, the project partners and more generally the European industry are better prepared for the “more optical aircraft”

Dissemination included the general public, European industries and organisations offering engineering training. The dissemination activities of the project were of course impacted by the COVID-19 crisis. The original plan was to increase dissemination activities in conferences,expos and fairs towards the end of the project once the final system test results were available. These events could not take place but dissemination and communication activities are still planned after the end of the project.

Exploitation is based on the anticipation that the technology will be adopted in the next evolution of the Airbus A320 family of aircraft and into the ZEROe concepts. The ALGeSMo advanced load sensing capability forms part of an integrated solution across landing gear systems. This integrated solution is an enabler for a step change in availability and operability for Landing Gear which ultimately aims to reduce the operational costs and increase the value of the aircraft.
1) Load measurement on landing gears

Since the 1950’s many companies have tried to design and put on the market load-monitoring for aircraft landing gears. Few systems reached the production state and none of them were successful. Some system have been used in service for short periods of times but have always been discarded due to a lack of reliability and convenience. Unlike the previous systems, ALGeSMo relies on optical sensing rather than strain gauges. ALGeSMo offers significantly higher reliability than previoes strain gauge systems and It measures the loads along all three axis and the torque and therefore offers more benefits that previous systems.

2) Integration of optical fibres in a composite material

It is a common practice to embed optical fibres in a stiff composite panel designed for a narrow temperature range. However embedding optical fibres in a thin, flexible composite panel designed to withstand high temperature is a novelty.The ALGeSMo team successfully tackled these challenges, and went beyond the previous state of the art.

3) Integrated optical interrogator in avionics

Optical interrogators are typically bulky, fragile devices with high power consumption. This is not compatible with use on an aircraft. The FOPU was made possible by the technology developed by the ALGeSMo consortium. The Application-Specific Photonic Integrated Chip (ASPIC) technology developed by Technobis allowed the system to have a small and robust interrogator. The combination of the optical interrogator and the processing and communication electronics makes the FOPU a novel product.

4) Optical connections for analogue signals through single-mode fibres

The few optical avionic equipment currently in-service are communication equipment (data transfer). They transfer digital signals into multi-mode optical fibres. The ALGeSMo system is a sensing system as opposed to a data-transfer system and uses FBG as sensing elements. Therefore it transfers analogue signals into single-mode optical fibres. Analogue signals are much more sensitive to perturbations and power loss, and single-mode fibres have significantly smaller cores than multi-mode ones. This makes the connections a lot more challenging than on existing systems, especially under varying temperatures and vibrations. The connectors have been carefully selected for their performance, whilst still complying with the weight and dimension constraints. A fruitful collaboration with a connector supplier led to the development of a new reliable and robust connection for this type of environment. This is a clear example of how the ALGeSMo project went beyond the previous state of the art.
ALGeSMo Fibre Optics Processing Unit Prototype
ALGeSMo Test Rig
ALGeSMo Axle Sensor
ALGeSMo high level system schematics
ALGeSMo Fibre Optic Interrogator