Electric-Circuit Burning-Clear and Damage Phenomena on Aircraft Structures

Normal and continuous performance of electric circuits is a prerequisite to the effective use of airplanes. Electric short circuits which result in the loss of power for vital control operations or which cause fires are definite hazards. The extensive use of the 28.5-volt d-c aircraft systems, the proposed 120-volt d-c system, and the consideration being given a three-phase 120-208-volt a-c grounded neutral system for high-altitude planes necessitated an investigation into the burning-clear and damage phenomena on all circuits. Short circuits were applied on an aluminum-alloy section of an airplane, and oscillograms of short-circuit current and voltage taken. In both the a-c and d-c systems it was found that the fault-damage and burning-clear phenomena depended upon the energy level in the short-circuit area. When the contact area between the conductor and aircraft structure was small, the energy level was high, and the fault burned itself clear. In such cases the aircraft structure was perforated. Increase in contact area and a consequent reduction in energy concentration resulted in a sustained fault current. In some of these cases a definite welding of the conductor to the aircraft structure occurred, while in others no burning took place.