Static Electrification of Electrically Heated Aircraft Windscreens: An investigation into the causes of electrical failures experienced by different aircraft and possible solution!

TWO years ago, the Aircraft Equipment Division of The English Electric Co. was confronted by unexpected damage to a number of windscreen temperature controllers. The damage could only have been caused by certain terminals being subjected to extremely high voltages. Incorrect installation of the system was originally suspected. Further failures then occurred on one particular flight, coincident with a windscreen heater element failure during an electric storm, and associated with an unusually severe electrical flash across the outer surface of the windscreen in question. This particular aircraft, of the modern jet executive type, and another of similar design having windscreens of almost identical construction, had already been prone to continual tripping of the alternators during cloud or storm flying. Alternator control regulators had also been damaged, again apparently due to severe voltages being applied where none should exist. Something unexpectedly severe was happening and an intensive investigation of the problem was initiated.