On 13 January 2015, a Gulfstream G550 approaching Hong Kong on a positioning flight suddenly began rapidly descending without clearance and came within 500 feet of the sea surface before a recovery triggered by an EGPWS ‘PULL UP Warning had been accomplished. The Investigation found that the excursion resulted from an inadvertent and unrecognised elevator trim switch input which caused the autopilot to disconnect and that initiation of a recovery was delayed by the continued failure of all three pilots on the flight deck to determine the control status of the aircraft and was hindered by their ineffective CRM.