Md-82 captain reported wake vortex encounter six miles in trail of a B747-800 on arrival to ord that...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1113077
Time
Date	201308
Local Time Of Day	1801-2400
Place
Locale Reference	ORD.Airport
State Reference	IL
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	MD-82
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	B747-800 Advanced
Operating Under FAR Part	Part 121
Flight Plan	IFR
Person 1
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Inflight Event / Encounter Wake Vortex Encounter

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.