ACE 2001 highlights
More 1
2
3
4
BAE Systems and EADS explore lightning strike protection
The external structure of fast jet aircraft are now often constructed largely from carbon-fiber composites (CFC). Much of the external skins form part of the fuel tanks. A lightning strike to a tank skin is likely to cause arcing and sparking at structural joints. This can lead to particles of molten metal or resin being injected into the fuel tanks along with hot gasses, presenting the risk for fuel explosion. Therefore, BAE Systems and EADS have begun studying integral lightning strike protection for CFC fuel tanks. This integral protection is based on a network of copper strips co-bonded into the composite tank shell. The protection details, such as foil thickness and fastener installation, may be varied to suit the lightning strike threat.
Ideally, fuel tanks should be located in zone 3, but new zoning rules and aircraft performance requirements make this impractical. It is often necessary to position the fuel tanks in lightning strike threat zones up to 1C or even to 1A. The difficulties are compounded by the requirement to inspect and maintain equipment within the fuel tanks. This means that access panels, which are even more difficult to protect against lightning strike than fixed structures, may have to be located in these high-threat zones. CFC fuel tanks, including all the detailed features, must be capable of safely tolerating lightning strikes up to zone 1A levels without a risk of sparking.
To protect against sparking, a protection system based on secondary bonded aluminum foil was developed. In this system, all external fasteners in the fuel tanks are covered by an interconnected network of aluminum foil strips, which are separated from the fastener heads by a thin insulating film. The foil is applied after the structure has been assembled and before final painting. Post-assembly aluminum foil protection is very effective and has been qualified to zone 1A levels.
A disadvantage with this form of protection is that the integrity of the foil network must be maintained throughout the life of the aircraft. This places a significant maintenance requirement on the operator. If the foil covers an entire panel, accessing it requires removal and replacement of the foil and restoration of the paint. Although foil repairs are straightforward, the removal of damaged foil, preparation of the structure, bonding new foil, and restoring the paint finish can become a lengthy process.
To account for the disadvantages of aluminum foil protection, lightning strike protection based on the integration of co-bonded copper foil strips in the CFC laminate was developed. Copper foil is co-bonded into fuel tank skins during manufacture as part of the laminate lay-up process. The copper foil runs along all joint lines on the outer surface of the skins.
The fasteners are then installed through the copper foil, which unlike the post-bonded foil protection, leaves the fastener head exposed. Although the foil will dissipate much of the lightning energy, significant lightning currents could still be injected into the fasteners. The fastener installation must therefore be capable of carrying the remaining lightning currents safely. This must be addressed at the design stage of the joints, with the appropriate fasteners selected to ensure that the remaining currents are insufficient to cause sparking.
The role of the co-bonded copper foil is to transfer and diffuse the lightning currents into the CFC laminate as quickly and safely as possible, whereas the role of the post-assembly aluminum foil is to maintain the currents on the outside of the laminate for as long as possible. The aluminum foil protection allows more freedom in joint design, whereas the copper foil protection demands greater emphasis on lightning strike performance in the basic joint design.
By combining several protection measures with the copper foil system, sparking at fuel tank access panels can be prevented at lightning strike threat levels up to zone 1C. CFC fuel tanks can be designed to withstand lightning strikes up to zone 1A levels without sparking in the fuel area. The protection includes both fixed structures, which have been tested to zone 1A level, and access panels, which have been tested to zone 1C levels.
Information was provided by Barry Pridham, BAE Systems, and Dieter Jaeger and Manfred Schreiner, EADS.
More 1
2
3
4
|
| Sign Up!
