On August 16, 2015, at 0906 eastern daylight time, a Piper PA-28-161, N82587, was substantially damaged when it impacted trees and terrain in Bristow, Virginia. The student pilot was seriously injured. Visual meteorological conditions prevailed and no flight plan was filed. The local solo instructional flight was operating in the airport traffic pattern at Manassas Regional Airport (HEF), Manassas, Virginia and was conducted under the provisions of Title 14 Code of Federal Regulations Part 91.During an interview with a Federal Aviation Administration (FAA) inspector, the student pilot stated that she had just reduced engine power and air traffic control asked her to extend the downwind leg of the airport traffic pattern due to other traffic. She became distracted looking for the other traffic and did not increase engine power to maintain altitude. Air traffic control then instructed her to turn base leg of the airport traffic pattern and she complied; however, the airplane continued to descend and collided with trees, before coming to rest upright on the ground. Examination of the wreckage by the FAA inspector did not reveal any preimpact mechanical malfunctions, nor did the student pilot report any. The inspector noted damage to both wings and the fuselage. The recorded weather at HEF, at 0855, included wind from 180 at 3 knots, 10 statute miles visibility, sky clear, temperature 25 degrees C, dew point 20 degrees C, and an altimeter setting of 30.17 inches of mercury.