On 29 April 2017, a Boeing 787-9 which had just reached cruise altitude after despatch with only one main ECS available began to lose cabin pressure. A precautionary descent and PAN was upgraded to a rapid descent and MAYDAY as cabin altitude rose above 10,000 feet. The Investigation found that aircraft release to service had not been preceded by a thorough enough validation of the likely reliability of the remaining ECS system. The inaudibility of the automated announcement accompanying the cabin oxygen mask drop and ongoing issues with the quality of CVR readout from 787 crash-protected recorders was also highlighted.