Fuel Tank Safety - 3D Computations for Attachment and Conduction Assessment

The evolutions of the Certification Specifications CS 25.954 [1] and CS 25.981 [2] and associated policy [3] [4], change the justification to prevent catastrophic fuel vapour ignition due to lightning. This change results in carrying out many tests for all arrangement and design taking into account manufacturing escape, accident and aging: the “fault tolerant approach”. The new type of computations is now part of the assessment to justify the current distribution in the arrangement of fuel tank and the design of the samples used for conduction and attachment tests. This paper presents the methodology applied by Dassault Aviation to quantify the current distribution for conduction and attachment problematic. Important efforts have been performed - to validate computation by dedicated tests - and to increase technical collaboration with DGA-TA thus making it possible to specify the test set-up and samples with a better understanding of lightning strike distribution produced on the Airplane. The first application concerns 3D computations at aircraft level for conduction aspect, to quantify the current distribution for all arrangements depending on many parameters like: location, type of assemblies, families of fasteners, etc. These computations are more and more reliable to specify the injected current level during the test on samples and to estimate the waveform (rise-time for example). Indeed, for conduction aspect, 3D computations are performed at aircraft level integrated a few thousands of individual fastener joints to quantify the lightning threat levels for each fastener family and each aircraft zones. This work is presented in a dedicated paragraph. For attachment aspect, the same computations are also used to quantify the current on the skin versus the internal current on the stiffener/rib/spar. For metallic, composite or hybrid arrangements, the test set-up could be adjusted to reproduce these distributions. This work requires an important collaboration in terms of computation and measurements with DGA-TA to understand, to optimize and to simplify the methodology if necessary.