A Comparison of the Pulse and Swept-Frequency Transfer Function Lightning Test Responses on an Airplane with Composite Structure

Airplane level lightning testing is used to verify lightning protection effectiveness of airplane systems. Time domain pulse and frequency domain swept-frequency transfer function tests are two acceptable airplane level ground test methods available to the aircraft manufacturer. Although both methods offer an acceptable verification means, both methods pose their own technical challenges.

The intent of this paper is to present data collected from lightning pulse and swept-frequency transfer function tests conducted on a 777 airplane, and to compare these tests results. The data collected for this comparison includes several pulse and transfer function measurements made on flight control wiring routed along the rear spar of the 777 graphite vertical and horizontal stabilizers. The pulse measurement involved injecting 20 kA damped sinusoid pulse into the tip of the stabilizer while measuring the induced voltage on an individual wire within select shielded flight control bundles. The swept-frequency transfer function measurement involved measuring the induced voltage on the flight control wire as a function of the current input at the stabilizer tip over a given frequency range. The time domain induced voltage is then computed by multiplying the transfer function by the spectrum of a particular waveform, in this case the damped sinusoidal waveform of the pulse generator, and then taking the inverse Fourier transform of the product. The results of both airplane tests show that the swept-frequency transfer function responses compares favorably with the pulse test responses.