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 British Airways 787-9, operating from London 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.
US investigators had previously identified deficiencies in cockpit-voice recordings on the 787, following a probe into a battery fire on a Japan Airlines 787-8.
The UK inquiry also points out that testing of installed cockpit-voice recorder systems is "largely subjective" and led to "variation" in performance. It is recommending that a "repeatable and objective" analysis technique be introduced into minimum performance specifications for recorders.
BA has already acted on another aspect of the incident, when a pre-recorded announcement to passengers, after the loss of pressure, was rendered inaudible.
Passengers appeared "unaware" of the emergency, says the inquiry. "Some remained asleep and many did not fit their oxygen masks," it adds. "Of those that did fit the masks, some fitted them incorrectly."
Cabin crew were left confused by the apparent absence of the announcement, believing the oxygen masks had deployed inadvertently from turbulence. Instead of following depressurisation actions, they returned service carts to galleys and took their seats.
The cabin service director took a portable oxygen system and went to the cockpit in a bid to understand the situation.
BA has been updating the audio system software installed on its 787 fleet to prevent the volume of the cabin depressurisation announcement from being attenuated.
Investigators also found that safety cards on BA 787s incorrectly indicated that the privacy screens on business-class seats would automatically retract when oxygen masks deployed. They are not required to do so, but cabin crew training did not reflect this difference with other aircraft in the BA fleet.