STATIC-DISCHARGE PROTECTION FOR TRANSPORT AIRCRAFT 520088
The majority of thunderstorm electrical discharges involving aircraft are in the “Upper-reaches” of a lightning discharge channel where currents tend to be lower than for the lightning we know near the ground: Therefore, the term “Static-discharge”, which many pilots use when the discharge does not seem violent enough to warrant the term “Lightning” may logically be used as a more general term covering all thunderstorm electrical discharges.
In using the term “static-discharge” it is important to avoid confusion with precipitation charging and the associated minute currents normally discharged through “wick-dischargers” to prevent radio interference. It must be emphasized that discharges in the case of precipitation-static involve currents of fractional amperage insufficient to produce any of pitting or burning effects for which protection might be needed.
The discharge currents that are big enough to make any noise, or visible flashes, or pitting or burning marks on the metal skin of the aircraft are external long discharges that an aircraft flying under thunderstorm conditions may intercept. These discharges may be between different cloud charge areas or in a channel progressing towards and terminating at ground; they differ from familiar concepts of lightning only to the degree that they are most generally in lower current regions of lightning channels. However, a low flying aircraft may intercept a main discharge channel, and therefore static - discharge protection must also be adequate for the higher current intensities encountered in lightning strokes to ground.
The transport aircraft is inherently provided nearly complete protection by the metal fuselage which forms a safe path for discharge currents that the aircraft may intercept. External elements such as movable surfaces and antennae can be protected.
Summaries of airline experiences are given from which it is seen that antennas frequently intercept discharges; a current surge conducted along the antenna lead-in frequently damages radio equipment and in some cases has resulted in fires. An effective antenna protection unit was consequently developed which besides protecting the radio equipment also records the bypassed current and charge magnitudes. Analysis of the sphere gap pitting indicates the total charge transfer, and small magnetic links indicate the peak currents by their magnetization. Because of the research importance of the data, these experimental units have been made available to airlines interested in cooperating with the Institute's program of protection researches.
The discharge current records, and supplementary information from questionnaires, are providing data for improved protection and also help resolve statistically such questions as discharge probability variations to be expected with various operating factors of geographical location, altitude, temperature and size and speed of the aircraft.