Narrative:

Anc approach offered a runway 07L visual approach to follow a heavy cargo 747. We had both the B747 and the airport in sight and accepted the visual approach. At the time we accepted the approach; the crosswind component from the right was less than 5 knots. The clearance set us up a little low on the runway 07L GS so we shallowed out our rate of descent to less than 500 FPM and we were making a nice correction back to the GS. ATC did not provide any information regarding our rate of overtake with the B747 ahead but it was substantial and we were slowing and approaching our minimum maneuver speed for selection of flaps 1 degree. As we reached our minimum maneuver speed and prior to flaps extension; we encountered a sudden and significant wake turbulence that rolled the aircraft rapidly to the right approximately 50 degrees. The 'bank angle' warning sounded simultaneously with the stick shaker. The first officer; who was the PF; added power and executed a stall recovery technique. I noticed that at the time of the encounter the crosswind component had increased to 18 knots from the 4 knots earlier observed. The combination of our being placed low on the GS by ATC; the increase in the crosswind component and our overtake of the B747 conspired to place us in a very poor position relative to wake separation. In the future; I would modify my judgement to avoid accepting a closely space parallel approach behind a heavy jet. The variables that may be encountered are simply too unknown. If this event were to have occurred with close proximity to the ground the potential for an aircraft mishap/incident/accident would be significant. This has instructed me to just take the 5 miles separation between our aircraft and a heavy jet as a necessary and important safety precaution despite the availability of other (closer spaced) options. Similar circumstances could occur in sea; sfo or lax to name just a few. Modifying my judgement and behavior as I have stated above.

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Original NASA ASRS Text

Title: B737 FLT CREW ON VISUAL APCH TO RWY 7L AT ANC ENCOUNTERS WAKE TURB FROM PRECEDING B747 LNDG RWY 7R.

Narrative: ANC APCH OFFERED A RWY 07L VISUAL APCH TO FOLLOW A HEAVY CARGO 747. WE HAD BOTH THE B747 AND THE ARPT IN SIGHT AND ACCEPTED THE VISUAL APCH. AT THE TIME WE ACCEPTED THE APCH; THE CROSSWIND COMPONENT FROM THE RIGHT WAS LESS THAN 5 KNOTS. THE CLRNC SET US UP A LITTLE LOW ON THE RWY 07L GS SO WE SHALLOWED OUT OUR RATE OF DSCNT TO LESS THAN 500 FPM AND WE WERE MAKING A NICE CORRECTION BACK TO THE GS. ATC DID NOT PROVIDE ANY INFORMATION REGARDING OUR RATE OF OVERTAKE WITH THE B747 AHEAD BUT IT WAS SUBSTANTIAL AND WE WERE SLOWING AND APCHING OUR MINIMUM MANEUVER SPEED FOR SELECTION OF FLAPS 1 DEGREE. AS WE REACHED OUR MINIMUM MANEUVER SPEED AND PRIOR TO FLAPS EXTENSION; WE ENCOUNTERED A SUDDEN AND SIGNIFICANT WAKE TURBULENCE THAT ROLLED THE ACFT RAPIDLY TO THE RIGHT APPROX 50 DEGS. THE 'BANK ANGLE' WARNING SOUNDED SIMULTANEOUSLY WITH THE STICK SHAKER. THE FO; WHO WAS THE PF; ADDED POWER AND EXECUTED A STALL RECOVERY TECHNIQUE. I NOTICED THAT AT THE TIME OF THE ENCOUNTER THE XWIND COMPONENT HAD INCREASED TO 18 KNOTS FROM THE 4 KNOTS EARLIER OBSERVED. THE COMBINATION OF OUR BEING PLACED LOW ON THE GS BY ATC; THE INCREASE IN THE XWIND COMPONENT AND OUR OVERTAKE OF THE B747 CONSPIRED TO PLACE US IN A VERY POOR POSITION RELATIVE TO WAKE SEPARATION. IN THE FUTURE; I WOULD MODIFY MY JUDGEMENT TO AVOID ACCEPTING A CLOSELY SPACE PARALLEL APCH BEHIND A HEAVY JET. THE VARIABLES THAT MAY BE ENCOUNTERED ARE SIMPLY TOO UNKNOWN. IF THIS EVENT WERE TO HAVE OCCURRED WITH CLOSE PROXIMITY TO THE GROUND THE POTENTIAL FOR AN ACFT MISHAP/INCIDENT/ACCIDENT WOULD BE SIGNIFICANT. THIS HAS INSTRUCTED ME TO JUST TAKE THE 5 MILES SEPARATION BETWEEN OUR ACFT AND A HEAVY JET AS A NECESSARY AND IMPORTANT SAFETY PRECAUTION DESPITE THE AVAILABILITY OF OTHER (CLOSER SPACED) OPTIONS. SIMILAR CIRCUMSTANCES COULD OCCUR IN SEA; SFO OR LAX TO NAME JUST A FEW. MODIFYING MY JUDGEMENT AND BEHAVIOR AS I HAVE STATED ABOVE.

Data retrieved from NASA's ASRS site as of January 2009 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.