MD-82
Flight Crew Air Transport Pilot (ATP)
Inflight Event / Encounter Wake Vortex Encounter
Flight Crew Regained Aircraft Control
Environment - Non Weather Related
We were on vectors for Runway 28 at ORD. Our heading was 360 on the left base leg. We were descending from 5;000 to 4;000 FT; and were informed that we were following a 'Heavy' aircraft. It was a B747-800; and was reported to be approximately 6 miles ahead of us. We encountered what we believe to be wake turbulence; which resulted in a roll to the left of approximately 40 degrees. The autopilot disconnected; but it is unclear if it was a result of the roll itself; or if we may have accidentally done so; as the First Officer and I both grabbed the control wheel when the roll commenced. It only lasted a few seconds; and the aircraft came back under our complete control. Immediately after the event; we notified the Controller; who reiterated the distances between our two aircraft; and the fact that we were above the Heavy's flight path. We continued on the vectors; choosing to fly slightly above the glideslope until just prior to landing. We landed without incident. After landing we were given a contact phone number; and I spoke with [an ATC representative]. She said a quick review of the two flight paths showed that we should have been free of any encounter with wake turbulence.
MD-82 Captain reported wake vortex encounter six miles in trail of a B747-800 on arrival to ORD that resulted in a 40 degree roll. ATC stated there should not have been an encounter based on the flight paths.