On November 23, 2002, about 1240 eastern standard time, a Cessna 172G, N1152F, was substantially damaged when it struck trees while on approach to Minute Man Air Field (6B6), Stow, Massachusetts. The certificated flight instructor (CFI) and passenger were not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the personal flight conducted under 14 CFR Part 91. The CFI reported he was conducting a demonstration flight in the airplane for the passenger, who was interested in purchasing the airplane and receiving flight instruction. They departed 6B6, and landed at the Fitchburg Municipal Airport (FIT), Fitchburg, Massachusetts. They then departed FIT to return to 6B6. The CFI reported the airplane was on final approach to runway 30, a 1,600-foot long, gravel runway, when he experienced a sudden downdraft. The airplane struck trees and came to rest short of the runway threshold. The CFI further stated that he did not experience any pre-impact mechanical malfunctions. The CFI reported 8,000 hours of total flight experience, which included about 5,000 hours in the make and model of the accident airplane. Winds reported at an airport about 12 miles east of the accident site, at 1256, were from 300 degrees at 19 knots, with 31 knot gusts.