A330
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Situational Awareness
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
General Declared Emergency
Manuals; Procedure; Weather
A company flight 30 min ahead of us at FL 390 reported descending to lower altitude on Guard Frequency due to severe turbulence and advised us to do so also. We were unable to contact Control in a timely manner [although we were already] entering moderate turbulence; so we [advised our intentions in the blind]; checked TCAS for traffic; turned slightly off course and descended to FL300 to avoid severe turbulence; injury; and possible damage to the aircraft. We then advised Control of our descent on CPDLC and were approved to cruise at lower altitude (no SIGMET had been issued). We advised Dispatch; checked fuel; and climbed back to flight plan altitude later upon coast in with Control.  We then continued without incident. (Note: As Captain; I was in the cabin on scheduled rest break during above incident and was not in cockpit to witness above events but I would have done the same thing the relief crew did using RVSM contingency plans as far as practical to avoid severe turbulence.)
I was the Relief Pilot and pilot monitoring. I was occupying the left seat while the Captain was on a scheduled rest break. The First Officer was the pilot flying in the right seat. We contacted them for further info and found they had to descend to FL300 to get out of the severe turbulence. We...turned on all the external lights and began a descent.
When they received reports of severe turbulence ahead at their altitude an A330 flight crew exercised emergency authority to descend without clearance when they were unable to obtain clearance in a timely manner from oceanic control.