Abstract: Electrical transmission lines are a collision hazard to low-flying aircraft, especially helicopters. Warning of the proximity and bearing of such lines is provided by equipping the aircraft to detect the alternating magnetic flux produced by such lines. The bearing is indicated by intercepting the horizontal flux component by two coils mounted in a vertical plane at right angles to one another. The ratio of the signals induced in the two coils is a measure of the bearing of the flux source. The induced AC signals are amplified and rectified, and their ratio is compared with predetermined ratio values corresponding to given bearing sectors. The output conditions thus obtained, together with a quadrant-identifying signal obtained by multiplying together the two AC signals, are gated to illuminate that element of a directional display corresponding to the sector in which the bearing lies. Early warning is provided by a third coil which intercepts the larger vertical flux component.