B737-500 encounters likely wake turbulence in descent following B747.

Attributes
ACN	803487
Time
Date	200809
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl single value : 28500
Aircraft 1
Controlling Facilities	artcc : zzz.artcc
Operator	common carrier : air carrier
Make Model Name	B737-500
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 180 flight time total : 16000 flight time type : 4000
ASRS Report	803487
Events
Anomaly	inflight encounter : wake turbulence other anomaly
Independent Detector	other flight crewa
Resolutory Action	controller : separated traffic flight crew : exited adverse environment flight crew : regained aircraft control
Supplementary
Problem Areas	Environmental Factor
Primary Problem	Environmental Factor

Narrative:

Mod turbulence was occurring at cruise at 37000 ft through 32000 ft during the descent. Below 30000 ft it smoothed out as we were below the clouds. I turned the seat belt sign off. At approximately 28500 ft; we hit a severe bump and the aircraft autoplt and autothrottle kicked off with the aircraft's wing dropping 35-45 degree turn to the right. We were in a descent so I'm not sure what the loss of altitude was. I regained control and notified ATC of the severe turbulence to warn other descending behind us. I called the back to make sure everyone was ok; which everyone was. The first officer then sent a message to maintenance. I asked ATC if we were following anyone. ATC said only a B747; but he is 20 miles ahead and 4000 ft below you. At about that time we started hitting his wake turbulence again. I requested an off set for the descent. The rest of the flight was normal.

Original NASA ASRS Text

Title: B737-500 ENCOUNTERS LIKELY WAKE TURBULENCE IN DSCNT FOLLOWING B747.

Narrative: MOD TURBULENCE WAS OCCURRING AT CRUISE AT 37000 FT THROUGH 32000 FT DURING THE DSCNT. BELOW 30000 FT IT SMOOTHED OUT AS WE WERE BELOW THE CLOUDS. I TURNED THE SEAT BELT SIGN OFF. AT APPROX 28500 FT; WE HIT A SEVERE BUMP AND THE ACFT AUTOPLT AND AUTOTHROTTLE KICKED OFF WITH THE ACFT'S WING DROPPING 35-45 DEGREE TURN TO THE R. WE WERE IN A DSCNT SO I'M NOT SURE WHAT THE LOSS OF ALTITUDE WAS. I REGAINED CTL AND NOTIFIED ATC OF THE SEVERE TURB TO WARN OTHER DSNDING BEHIND US. I CALLED THE BACK TO MAKE SURE EVERYONE WAS OK; WHICH EVERYONE WAS. THE FO THEN SENT A MESSAGE TO MAINT. I ASKED ATC IF WE WERE FOLLOWING ANYONE. ATC SAID ONLY A B747; BUT HE IS 20 MILES AHEAD AND 4000 FT BELOW YOU. AT ABOUT THAT TIME WE STARTED HITTING HIS WAKE TURB AGAIN. I REQUESTED AN OFF SET FOR THE DSCNT. THE REST OF THE FLT WAS NORMAL.

Data retrieved from NASA's ASRS site as of May 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.