Date: 201307
Local time of day: 0601-1200
Location: ZAN.ARTCC, AK
Flight conditions: VMC
Weather elements / visibility: Turbulence
Light: Daylight
Aircraft: MD-11
Operator: Air Carrier
Operating under: Part 121
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Airspace: Class A ZAN
ATC / advisory: Center ZAN
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Wake Vortex Encounter
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Environment - Non Weather Related; Human Factors
Primary problem: Ambiguous

Narrative: I was pilot flying; Captain was pilot not flying. After takeoff we climbed to FL310 and we heard [a B747-400] on frequency at FL320 about 20 miles ahead of us. Flying over S. Korea; we were getting a little turbulence from [the B747-400] so we asked and received direct ANDOL to give us a little different track to get out of its wake. After check-in with Tokyo we were given direct PUTER. This apparently closed the gap a bit with [the B747-400]. We asked for higher with no success due to traffic. As we approached POXED I programmed in a 2 mile right offset and that seemed to solve the wake issue. I then retreated to the back for my rest period. After my rest period was over I came back up front to relieve the Captain. I was briefed that everything was status quo. I noticed; however; the 2 mile offset had been removed. I sit; Captain goes back; and the Relief First Officer (RFO) takes his seat. All of a sudden we encountered the wake of the 747 again. I immediately reached up; turned and pulled the heading bug to initiate a right offset. The RFO was attempting to program the FMS amid the turbulence. We both looked up at the PFD and ND to make sure it was programmed correctly; when we noticed that the wake was pushing the airplane down. I then disengaged the autopilot and corrected the descending trend. I didn't think that we lost more that 300 FT. The RFO said he thought it looked we lost 500 FT. The weird thing was that the autopilot didn't disengage; nor did we get an altitude deviation alert. After I leveled off at FL310; I re-engaged the autopilot; and everything was stabilized and smooth with the offset programmed. The RFO then contacted ANC ARTCC and informed them what had happened. His response was something like; 'ahhhh.....okay.' Apparently he didn't notice and there was never even a threat of a conflict. I then went back to notify the Captain of what had happened and everything that was done. We discussed the issue upon his arrival back to the flight deck after his rest period; and we decided to write-up the autopilot in case there was an internal malfunction. And even though ATC didn't notice the event; we agreed that is was prudent to each write this report. I feel that if initial offset that was programmed at coast out was still in place; this would not have happened. I [am] not sure why it was removed. Maybe the turnover brief could have been better. And I could have asked more direct questions as to why the offset was no longer programmed.




Synopsis: MD-11 First Officer reported wake vortex encounter in trail of a B747-400 at FL320 that caused a 300 feet altitude loss.