Aircraft lightning testing is concerned with two aspects of the direct attachment of lightning to the aircraft:
- Direct damage, including fuel explosion
- Induced electrical transients causing upset or damage to critical electronic systems, known as indirect effects.
Direct damage tests take place on individual items of the airframe and are not discussed further here as FOLs are not normally used. Unlike HIRF, currents are directly injected into the airframe either at the threat level (200 kA) or at a sub threat level and the resulting induced currents and voltages extrapolated to the threat and reinjected at equipment level. The injection is achieved by making the aircraft the centre conductor of a coaxial return conductor system with return conductors placed around the airframe. Various configurations are used to simulate the various attachment points and current path of the lightning to the aircraft e.g. nose to tail, nose to wing etc. The image below shows a coaxial return conductor rig fitted around a typical aircraft (Jaguar T2).
A lightning return conductor rig simulating nose to tail lighting strike
In this type of test, FOL sensitivity is not so much of an issue, unless testing at low level and extrapolating to high threat levels. The main challenge, however, is achieving adequate shielding of the FOL transmitter against the high level low frequency magnetic H-fields surrounding the aircraft to prevent breakthrough swamping the wanted signals within the FOL transmitter. This is where good H-field shielding is required, utilising high permeability materials, such as µmetal shields.