1987-10-01

Power Quality of Aircraft Electric Systems 871885

Approximately 40 years have passed since ac electric power systems have been applied to aircraft. For the first 35 of these 40 years, numerous specifications and standards were written to specify the ac power quality. The requirements were based almost entirely on existing characteristics of power generating systems with very little regard to the actual needs of the utilization equipment. Over the past ten years, with the rapidly increasing state of the art of electronic componets, both military and commercial aircraft have become increasingly dependent on proper operation of complex avionic equipment. With the introduction of modern commercial transports using microprocessor-based avionics subsystems in the early 1980s, it became apparent that a communication gap existed between the suppliers of avionic systems and suppliers of power generation systems.At the general session of the Airlines Electronic Engineering Committee (AEEC) of Aeronautical Radio, Inc. (ARINC) held in Brussels, Belgium in October of 1982, it was stated that transients on the electric power supply to avionics equipment were causing microprocessor-based systems to shut down. This required time-consuming, complex reset procedures and resulted in numerous scheduling delays in commercial aircraft operations. This was an intolerable condition and caused the formation of a subcommittee in August, 1984 to begin studying the effects of power quality on modern microprocessor-based avionics equipment. As a result, Project Paper 609, “Design Guidance for Aircraft Electric Power Systems”, was adopted by ARINC at its general session in October, 1986.Concurrent with the preparation of Project Paper 609, another subcommittee of AEEC was producing a guidance paper for design of avionic equipment. This paper, Project Paper 607, “Design Guidance for Avionic Equipment,” was adopted at the ARINC general session in October, 1985 and contains the following statement under a section entitled “Electric Power.”“Aircraft electrical power systems are documented in ARINC Report 609. However, since equipment acceptance on the flight line can be seriously impaired by deficient power transient survivability, a few remarks on this issue are appropriate. Although it has been proven that well-designed systems can tolerate power interrupts, voltage drops, spikes, ripples, it seems to come as an ever-new surprise that ship's power is not as clean as expected. As a result, erratic operation, nuisance failure indications and disconnects have still been common experiences. Manufacturers should realize that the real world is often much worse than the airframe manufacturer's specification suggests. Accordingly, an avionic unit should never be designed to barely meet the specification. In specific, a computer should never fall into an uncontrolled status simply because aircraft's power is not behaving according to expectations.

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