Lightning Hazards to Aircraft Fuel Tanks

The hazards of lightning strokes to aircraft fuel tanks have been investigated in artificial-lightning-generation facilities specifically constructed to duplicate closely the natural lightning discharges to air­ craft determined through flight research programs and analysis of lightning-damaged aircraft over a period of many years. Explosion studies were made in an environmental explosion chamber using small fuel tanks under various simulated flight conditions. The results showed that there is a primary hazard whenever there is direct puncture of the fuel-tank wall, whereas the ignition of fuel by hot spots on tank walls due to lightning strikes is unlikely. Punctures of fuel-tank walls by artificial-lightning discharges produced explosions of the fuel in the mixture range from excessively lean to rich mixtures. None of the aluminum alloys, 0.081 inch thick or over, were punctured by the laboratory discharges representative of natural-lightning discharges to aircraft; however, reliance on this wall thickness for complete pro­tection would not be justified, because occasional strokes are known to be of greater magnitude and because statistics reveal variations in the damage pattern. Data gathered by the Lightning and Transients Research Institute on lightning strokes to aircraft show that 90 percent of the strokes recorded have occurred in the temperature range of -10° to +10° C, where many of the jet fuels are flammable but where aviation gasoline is overrich. Also, 10 percent of the strokes recorded have been to the wings, which are the principal fuel-storage areas for modern aircraft. Thus, there is a hazard, particularly for jet fuels. Certain protective measures are indicated by the studies to date, such as the use of lightning diverter rods, thickening of the wing skin in areas near the most probable stroke paths, and the use of fuel-tank liners in critical areas.