Date: 201502
Local time of day: 0601-1200
Location: ZAN.ARTCC, AK
Flight conditions: VMC
Light: Daylight
Aircraft: MD-11
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Airspace: Class A ZAN
ATC / advisory: Center ZAN
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Wake Vortex Encounter
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Procedure; Environment - Non Weather Related
Primary problem: Ambiguous

Narrative: Approaching point PINTT the Pilot Flying (PF) took out the 2NM right SLOP in anticipation of entering radar controlled airspace. As we approached centerline of the track we started experience moderate wake turbulence from the aircraft approximated 15NM ahead of us and 1000 feet above us. I called to return to the offset; recognizing the turbulence was caused by the preceding aircraft's wake. The PF was slow to get back to the offset (seconds) as he attempted to return to the 2R offset by entering it in the FMS. The turbulence was great enough that as pilot monitoring I aided the PF by selecting a 15-20 degree heading change back to the offset the PF entered in the FMS. The wake turbulence caused the plane to start to climb. The plane may have corrected back on its own if left alone; but the PF initiated a descent with the vertical speed wheel passing about 100 feet high. The plane continued to climb as the autopilot was slow (milliseconds) to the vertical speed input. The PF then hit an altitude hold followed by another vertical speed descent input; which arrested the climb. We topped out approximate 410 feet high from our assigned altitude of FL290. The time from altitude deviation due to wake turbulence and back down to FL290 was less than 10 seconds. We could not have reacted quicker. The preceding aircraft was approximately 15 miles ahead of us. No TCAS traffic advisories or resolution advisories. The autopilot was never disengaged.If one keeps the SLOP offset until radar contact facilitates 2;000 feet vertical separation; one might have a better chance of avoiding wake turbulence and any altitude deviations caused by it.


Second narrative: At position PINTT; FL290; approximately 15-18 miles in trail of an aircraft at FL300. Anticipating radar contact near PTZGR I took out the 2nm offset from the ocean crossing. Heading approximately 060 with quartering tailwinds of approximately 220/70 knots. When we got back on airway centerline we encountered the wake turbulence of the preceding aircraft. We immediately turned right and re-inserted our 2 nm offset. While accomplishing this; we heard the 'ALTITUDE' callout and saw that we were about 400 feet high. I hit Altitude Hold; the autopilot leveled off at about 500 feet high; and I immediately used vertical speed to return to assigned altitude of FL290. We were off altitude for less than 10 seconds. The autopilot was ON for the entire time of the incident.Approaching radar contact took out SLOP as prescribed. With the winds the way they were I didn't think that the wake turbulence would be a factor; but it was. I don't know why the autopilot would initiate an uncommanded climb.With an aircraft that close (15-18 nm and 1000 feet above us) keep the SLOP in and tell ATC when in radar coverage that you are offset for wake turbulence separation.


Synopsis: MD-11 flight crew reported wake vortex encounter in cruise flight that resulted in an altitude deviation.