A B747-400 captain reported encountering what he felt was likely a wake vortex from a preceding B747...

Attributes
ACN	911392
Time
Date	201009
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	B747-400
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	Visual Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	B747-400
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	Pilot Flying Captain Check Pilot
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 200 Flight Crew Total 16000 Flight Crew Type 11200
Events
Anomaly	Aircraft Equipment Problem Less Severe Inflight Event / Encounter Wake Vortex Encounter

Narrative:

Aircraft upset while on autopilot. We were turning right base. The autopilot was engaged. While in a 25 degree bank; the aircraft suddenly and somewhat rapidly increased bank. I grabbed the wheel; disconnected the autopilot; pushed in left rudder and turned the airplane back toward wings level. I believe the bank angle may have exceeded 45 degrees and because of the quick and steady increase in bank; I believe the aircraft would have continued over on its back if we hadn't responded quickly enough. My assumption at the time was that it was the result of flying into the wake of the aircraft in front of us; [which was] also a 747. However; [later]; I wondered if it was from another cause. There was no turbulence at all coincident with the upset. It was very smooth. Second; we were very high and I believe [the other aircraft] likely was lower than us; but I'm not sure. (We had to use 30 flaps and gear down to get down because of the early base turn). I called maintenance from my cell phone and explained what happened. I noticed from the [maintenance history] later in the day that they found the autopilot checked normal. So I'm at a loss to explain why this happened.

Original NASA ASRS Text

Title: A B747-400 Captain reported encountering what he felt was likely a wake vortex from a preceding B747 on approach; but also could be an autopilot fault.

Narrative: Aircraft upset while on autopilot. We were turning right base. The autopilot was engaged. While in a 25 degree bank; the aircraft suddenly and somewhat rapidly increased bank. I grabbed the wheel; disconnected the autopilot; pushed in left rudder and turned the airplane back toward wings level. I believe the bank angle may have exceeded 45 degrees and because of the quick and steady increase in bank; I believe the aircraft would have continued over on its back if we hadn't responded quickly enough. My assumption at the time was that it was the result of flying into the wake of the aircraft in front of us; [which was] also a 747. However; [later]; I wondered if it was from another cause. There was no turbulence at all coincident with the upset. It was very smooth. Second; we were very high and I believe [the other aircraft] likely was lower than us; but I'm not sure. (We had to use 30 flaps and gear down to get down because of the early base turn). I called Maintenance from my cell phone and explained what happened. I noticed from the [maintenance history] later in the day that they found the autopilot checked normal. So I'm at a loss to explain why this happened.

Data retrieved from NASA's ASRS site as of April 2012 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.

A B747-400 captain reported encountering what he felt was likely a wake vortex from a preceding B747 on approach; but also could be an autopilot fault.