The role of HF navigation systems performance in selected aviation accidents

Most aviation accidents are caused by human errors and in poor weather; the effect of technical faults appearing either in the aircraft or ground systems has been negligible. Typically the pilot has ignored or misinterpreted the minimum altitude defined by authorities or performed otherwise hazardous flight operations. Sometimes, however, investigations have revealed, as a by-product, inadequate navigation equipment performance in conjunction with such disasters. The paper discusses previously unpublished examples in which the radiation characteristics of HF navigation beacons have been found unacceptable. Coupling of radiated energy from the beacon antenna to a too close overhead line can cause phantom guidance - often along this power line - when observed by an aircraft ADF receiver. Excessive vegetation when covered by snow may deteriorate the impedance matching whereby the field strength drops even by 10-15 dB. Unwanted frequency modulation close to the carrier, e.g. from the power supply transformer can severely disturb commercial phase locked receivers. Therefore, flight tests do not necessarily reveal false directional guidance caused, e.g., by coupling effects. We emphasize, however, that in none of the cases discussed has there been any evidence related to navigation systems as a cause of accidents.