The European Aviation Industry has been quick to use lightweight composite materials, especially Carbon Fibre, for primary structures and flight control surfaces. Composites are lightweight, stiff, and provide smooth, aerodynamically efficient surfaces. Carbon Fibre is particularly attractive because it has good corrosion and fatigue properties. This results in enhanced efficiency and performance and a significant weight reduction. However, lightning protection is difficult to incorporate into a composite aircraft compared to a conventional aluminium one. Composite airframes also give less electromagnetic shielding compared to aluminium, making it more difficult to protect avionic systems. Both aspects are worse for light aircraft because of their small size and low weight.This project addresses the need to optimise lightning protection systems for aircraft with light weight composite structure. It will allow manufacturers to make safe use of the performance advantage provided by such materials.
The LIGHTNING project focussed on areas where lightning protection was difficult to design or certify on general aviation aircraft because of the lack of available test data and guidelines. Its consortium formulated and helped to develop designs for lightning protection which were tested with high currents and voltages.
More specifically, the project targets were to:
- identify, through testing supported by modelling, lighting protection solutions for lightweight composite structures;
- determine how effective dielectric materials such as plexiglass and fibreglass could resist lightning swept stroke puncture or initial arc attachments;
- identify installation methods to protect avionics systems and power bus, demonstrating them by testing of mock up systems.
Work included the following tasks:
- Task 1 was to review the available lightning protection methodologies for both glass and carbon composite structure, as well as reviewing the available methods of manufacture;
- Task 2 was the major task of manufacturing the samples;
- Task 3 was the lightning testing and measurement of the panels, which was carried out in several phases throughout the programme
- Task 4 - modelling of the impulse effects;
- Task 5 studied lightning effects on insulating surfaces such as windscreens and fibreglass unprotected structure;
- Task 6 was to improve lightning protection for avionic systems and the electrical power bus;
- Task 7 - project management.
The programme outputs included test reports which incorporated guidelines on implementing lighting protection. These guidelines were also composed in a standalone report which aimed to assist European aircraft manufacturers in the design of lightning protecting aircraft. Moreover, many of the involved partners developed and manufactured test samples which were not submitted as itemised deliverables.
Immediate benefits to be provided to manufacturers of light aircraft:
- Test data on panels provided a valuable starting point for design of lightning protected structures. This included not only appropriate metallisations and their expected performance, but also effective bonding techniques to such metallised skins which retain smoothness of aerodynamic surfaces. The data also indicated acceptable use, and limits of use, of glass fibre and large windscreens/canopies;
- Avionics test bed results and simple analytical techniques could be read across to aircraft installations to help identify protection levels for avionics equipment. This is based on use of interbonded aluminium tubes or U-channel raceways, with mixed bundles of shielded/unshielded wires. Tests showed that high currents (typically 10kA) could be expected onto battery/power bus from alternator cables, and it is necessary to protect the power bus from transients of this level. Protection from these effects was discussed.