Date: 201506
Local time of day: 0601-1200
Location: EGLL.Airport, FO
Flight conditions: VMC
Light: Daylight
Aircraft: B767 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Final Approach
ATC / advisory: TRACON EGLL
Second aircraft: B747 Undifferentiated or Other Model
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Wake Vortex Encounter
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Procedure; Aircraft; Environment - Non Weather Related
Primary problem: Ambiguous

Narrative: Cleared to EGLL via OCK 3E. First Officer (FO) was pilot flying; aircraft configuration clean at 220 KIAS with the center autopilot engaged. Flight conditions were VFR and clear with no turbulence. On initial call to Heathrow Director over OCK at FL090; given heading for radar vectors to 27L. Subsequent vectors effectively 'S-turned' us back and forth across the downwind leg; presumably for spacing. Progressive descents took us down to FL070. Final Director cleared us to 5000 feet MSL. Descent was initiated using Vertical Speed; at 800 fpm down. At approximately 6000 feet MSL the aircraft abruptly encountered moderate turbulence; with a lateral rocking motion followed by a rapid roll to the right. The FO disengaged the autopilot and applied yoke to counter the roll. The event lasted 5-7 seconds; and ended as abruptly as it began; with a sensation of the aircraft being 'released' from the rolling force. Approximately 20 seconds later; the sequence repeated. The FO was still hand-flying; and had to use both yoke and rudder to counter the roll. On enquiry; Director stated that preceding traffic was a B747; and that we were five miles in trail.All crewmembers were unanimous in assessing the events as encounters with a wake vortices. There were no further repeats; but the events took on some significance very shortly after when we were slowed to 180 KIAS and began deploying flaps. With flaps 5 selected; the LE ASYM EICAS message and LE light came on. During the subsequent events; there was some concern that the turbulence/roll events might have been some form of asymmetric lift/buffet; caused by the slat problem. However; it soon became apparent that this was not the case. Control inputs throughout the LE event were relatively normal; with no difficulty in controlling the aircraft's flight path. No flight control annunciations were observed during the rolls; these did not occur until flap extension was initiated. I can find nothing to link the events; and based on the characteristics of the turbulence/roll events; remain certain that they were wake turbulence.


Second narrative: We all agreed that the wake turbulence encounters did not seem to correlate to the slat issue; but they may have caused us to lose a bit more altitude during the arrival phase and thus we may have briefly leveled at 4000 feet when Heathrow prefers that you keep a constant rate of descent until landing.


Synopsis: B767 flight crew reported encountering wake turbulence in trail of a B747 on approach to EGLL.