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LIGHTNING
Lightning Protection for Structures and Systems on Light Aircraft Using Lightweight Composites

Background

Lightweight composite materials, especially carbon fibre, are being increasingly used in light and large aircraft construction, but composite airframes also give less intrinsic electromagnetic shielding to structures and systems compared to aluminium, particularly for light aircraft. This proposal addresses the need to investigate and optimise lightning protection systems for aircraft with lightweight composite structures.

Project objectives

The programme will consider where lightning protection has been found difficult to incorporate in the design or certify, because of the lack of available test data, and will provide proven design approaches. Impulse loads will be measured, their effects modelled, and validated against test sample results. Avionics protection/power wiring systems protection will be investigated, along with canopy protection. The ultimate aim will be to provide general guidelines for certification. Both high current and high voltage-testing facilities will be applied, in order to:

  • Identify lightning protection solutions for lightweight composite structures, flight control surfaces, fuel systems and avionics systems, and provide design guidelines.
  • Carry out lightning strike testing and modelling of materials, structures and components to provide data to support the early design phase and type certification. Materials to be investigated will include lightweight composite skins with carbon, glass, foam or honeycomb.
  • Investigate the use of fibre optic strain gauging to monitor the structural health of airframes after lightning strike.
  • Investigate avionics systems and power bus protection, including testing of mock-up systems.

Description of the work

Review and Plan: Review of protection materials available, including woven mesh, expanded foil, and woven Al/carbon fabric, and some possible novel materials.

Flat Panel Testing: Lightning arc attachment tests will be carried out on 160 flat panel samples, in a matrix of different protection and manufacturing methods. The lightning arc attachment tests will be carried out at different test levels. Damage effects and impulsive loads at the panel surface will be recorded.

Larger Structure: Tests will be carried out on components such as elevators, rudders and wing structures, manufactured using a selection of the optimised techniques identified in the flat panel tests. These tests will check that the flat panel tests give the same typical results as the larger structure tests, so that they can in future be used with confidence as a certification method.

Fuel Tank Skins: This will include the protection of the structure against sparking. Typical installations of components such as fuel filler caps will also be checked, in order to identify and test generic design approaches, which have inherent protection against sparking.

Structural Strength Monitoring: The main spar of the test wing to be provided by AEL will be instrumented with built-in fibre optic strain gauges. Prior to the lightning strike test, the ‘healthy’ state can be recorded by simple loading at the wing tip and, after the lightning strike, can be checked for changes in strain exhibited under the same loading.

Structural Modelling: carried out in conjunction with the flat panel tests:

  • to understand the impulsive nature of the forces during lightning attachments, which will allow modelling to predict the lightning levels at which different structures will fail
  • to develop an equivalent simple mechanical impulse test that can simulate the impulsive loads from lightning attachments.

Insulating Structure Protection: New testing standards for lightning include tests to determine whether a swept lightning arc can puncture aircraft canopies. High voltage tests to a standard acrylic canopy will be carried out to provide supporting test data to certificate all similar designs of canopy installations on general aviation aircraft.

Power Systems Protection: To investigate installation and protection techniques (appropriate cable routing, bonding and grounding), investigated on a power bus mock-up, including forward fuselage, firewall and engine support truss.

Avionics Systems Signal Wiring: The test bed would include cable bundles, avionic racks, instrument panel, and various screening and shielding methods. The objective is to define an optimised avionics installation in which the transient levels can be kept below those defined as level 3 in RTCA DO-160D.

Expected results

The main deliverable of the programme will be validated lightning protection design guidelines for small aircraft, as well as experimental data to help support aircraft certification for lightning. The scope of the work will include lightning protection for conducting or glass composite structures, fuel systems, avionics and electrical power systems. The project will aim to reduce the weight penalty of lightning protection by 50%. In order to achieve these general objectives, lightning protection of different critical functions of the aircraft will be devised and demonstrated.

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