C172SP pilot reported encountering wake turbulence at 20 ft AGL in trail of a B787.

Attributes
ACN	1359886
Time
Date	201605
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	Landing
Route In Use	Direct
Flight Plan	VFR
Aircraft 2
Make Model Name	B787-800
Flight Phase	Taxi
Flight Plan	IFR
Person 1
Function	Single Pilot Pilot Flying
Qualification	Flight Crew Student
Experience	Flight Crew Last 90 Days 16 Flight Crew Type 47
Events
Anomaly	Inflight Event / Encounter Wake Vortex Encounter

Narrative:

I was cleared to land behind a boeing 787. I was warned about wake turbulence and substantially extended my downwind leg for what felt at the time like a sufficient distance in order to avoid the wake turbulence. When I was approximately 20 ft AGL my plane made an uncommanded 30-45 degree roll to the right. I was able to recover using the go around procedure.at the time I thought that the high level of turbulence I was experiencing at traffic pattern altitude had been to blame but on further reflection the air near the surface was otherwise very calm. I believe that what I experienced was wake turbulence from the landing 787. I knew that I needed to give the 787 ~8 miles of separation but a review of my GPS data indicates that at best I had ~6 miles of separation. My lack of awareness about exactly how far I was from the landing AC made me rely on estimates that turned out to be incorrect. It would be helpful if ATC could give advisories about how far or long I needed to circle or extend to avoid the wake turbulence.

Original NASA ASRS Text

Title: C172SP pilot reported encountering wake turbulence at 20 ft AGL in trail of a B787.

Narrative: I was cleared to land behind a Boeing 787. I was warned about wake turbulence and substantially extended my downwind leg for what felt at the time like a sufficient distance in order to avoid the wake turbulence. When I was approximately 20 ft AGL my plane made an uncommanded 30-45 degree roll to the right. I was able to recover using the go around procedure.At the time I thought that the high level of turbulence I was experiencing at traffic pattern altitude had been to blame but on further reflection the air near the surface was otherwise very calm. I believe that what I experienced was wake turbulence from the landing 787. I knew that I needed to give the 787 ~8 miles of separation but a review of my GPS data indicates that at best I had ~6 miles of separation. My lack of awareness about exactly how far I was from the landing AC made me rely on estimates that turned out to be incorrect. It would be helpful if ATC could give advisories about how far or long I needed to circle or extend to avoid the wake turbulence.

Data retrieved from NASA's ASRS site 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.