Md S80 encounters boeing 747 wake at 7 mi in trail and 170 KTS.

Attributes
ACN	315921
Time
Date	199509
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : dfw
State Reference	TX
Altitude	msl bound lower : 3500 msl bound upper : 3700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : dfw
Operator	common carrier : air carrier
Make Model Name	MD-80 Super 80
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach
Route In Use	approach : visual
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	B747 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	approach : visual
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 8000 flight time type : 3500
ASRS Report	315921
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	inflight encounter other
Independent Detector	other flight crewa
Resolutory Action	flight crew : exited adverse environment
Consequence	faa : reviewed incident with flight crew Other
Supplementary
Air Traffic Incident	other

Narrative:

We were cleared for a visual to runway 17L and were 7 mi behind a 747. At approximately 3500 ft we experienced moderate turbulence and were forced to climb 200 ft to get above his wake. Once clear of the wake the rest of the approach was normal. We requested and got runway 17R which also helped. Callback conversation with reporter revealed the following information: the reporter was surprised at how far in trail the wake from a 747 could linger. This analyst feels the wake energy deteriorates as a function of time and not distance. The reporter stated he had flaps 15 degrees and gear up. The distance 7 mi behind the 747 was observed on the TCASII and confirmed by ATC. The reporter was the PNF. He said he was in the wake about 6 seconds and encountered a left roll of about 20-30 degrees back to wings level and a right roll 20- 30 degrees and back to level and then the whole sequence repeated before climbing above and escaping the wake. The reporter felt he was pitched about 100 of the 200 ft needed to escape the wake. He said he experienced less than 1 negative 'G' and more than 1 positive 'G' during the encounter.

Original NASA ASRS Text

Title: MD S80 ENCOUNTERS BOEING 747 WAKE AT 7 MI IN TRAIL AND 170 KTS.

Narrative: WE WERE CLRED FOR A VISUAL TO RWY 17L AND WERE 7 MI BEHIND A 747. AT APPROX 3500 FT WE EXPERIENCED MODERATE TURB AND WERE FORCED TO CLB 200 FT TO GET ABOVE HIS WAKE. ONCE CLR OF THE WAKE THE REST OF THE APCH WAS NORMAL. WE REQUESTED AND GOT RWY 17R WHICH ALSO HELPED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS SURPRISED AT HOW FAR IN TRAIL THE WAKE FROM A 747 COULD LINGER. THIS ANALYST FEELS THE WAKE ENERGY DETERIORATES AS A FUNCTION OF TIME AND NOT DISTANCE. THE RPTR STATED HE HAD FLAPS 15 DEGS AND GEAR UP. THE DISTANCE 7 MI BEHIND THE 747 WAS OBSERVED ON THE TCASII AND CONFIRMED BY ATC. THE RPTR WAS THE PNF. HE SAID HE WAS IN THE WAKE ABOUT 6 SECONDS AND ENCOUNTERED A L ROLL OF ABOUT 20-30 DEGS BACK TO WINGS LEVEL AND A R ROLL 20- 30 DEGS AND BACK TO LEVEL AND THEN THE WHOLE SEQUENCE REPEATED BEFORE CLBING ABOVE AND ESCAPING THE WAKE. THE RPTR FELT HE WAS PITCHED ABOUT 100 OF THE 200 FT NEEDED TO ESCAPE THE WAKE. HE SAID HE EXPERIENCED LESS THAN 1 NEGATIVE 'G' AND MORE THAN 1 POSITIVE 'G' DURING THE ENCOUNTER.

Data retrieved from NASA's ASRS site as of July 2007 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.