Hypoxia-induced fatal aircraft accident revealed by voice analysis.

The voice communication was the only clue of the fatal F-104J accident encountered during high-altitude intercept procedures, and it was analysed to prove the presence of hypoxia as a causal factor. A simulated low-pressure chamber flight was undertaken, and the subject's voice, saying the same words as the pilot, was analyzed in the same way. Comparison of these two voices revealed a similarity in characteristic changes of the sound spectrum and time course. The blurred formation of formant, fundamental, and harmonic frequencies, as well as the obscured gap in pre-vocal cord opening time (VOT) of the sound spectrogram, were thought to be the effects of hypoxia. Lowered fundamental frequency of the pilot's voice, even at the stressful period of attack, has strongly suggested decreased vigilance due to hypoxia. Through these findings, it was concluded that the cause of the accident was probably hypoxia in the pilot.