BA incident revisits 787 cockpit-recorder clarity

UK investigators are seeking modifications to the Boeing 787 audio system to improve clarity of cockpit-voice recorder information. The advisory follows a loss of cabin pressurisation on a BA 787-9, operating from Heathrow to New Delhi on 29 April last year, which resulted in deployment of oxygen masks. While the pressurisation problem was traced to a detached air conditioning fan duct, the Air Accidents Investigation Branch inquiry turned up several other safety-related issues associated with the flight. The 787's audio system combines crew headset and oxygen mask microphone signals with the 'sidetone' signal - the audio reproduced through headset speakers, consisting of the pilot's voice, radio transmissions and interphone communications. "When the oxygen masks are used, the sidetone signal can be easily obscured due to the much higher signal level of the oxygen mask microphone," says the BA incident inquiry. This could mask air traffic communications and quieter background sounds, it adds, and potentially significant information could be lost. The inquiry has recommended that the US FAA requires Boeing to modify the 787's audio system so that sidetone signals on the cockpit-voice recorder are not cloaked when crew oxygen mask microphones are in use.<br/>