Narrative:

Level at 15;000 ft; we encountered severe turbulence. [We] deviated from altitude while recovering from aircraft upset due to what appeared to be wake turbulence. [We] alerted ATC of our deviation [and] resumed arrival to iad at a reduced airspeed. Remainder of flight was normal.

Google
 

Original NASA ASRS Text

Title: An HS-125 First Officer reported encountering wake turbulence at 15;000 FT inbound to IAD.

Narrative: Level at 15;000 FT; we encountered severe turbulence. [We] deviated from altitude while recovering from aircraft upset due to what appeared to be wake turbulence. [We] alerted ATC of our deviation [and] resumed arrival to IAD at a reduced airspeed. Remainder of flight was normal.

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.