On July 12, 2001, about 0745 eastern daylight time, a Taylorcraft F21B, N4418U, was substantially damaged while landing at the Simsbury Airport, Simsbury, Connecticut. The certificated flight instructor (CFI) and private pilot were not injured. Visual meteorological conditions prevailed and no flight plan was filed for the local instructional flight conducted under 14 CFR Part 91.According to the CFI, the private pilot was executing touch and go's on runway 03, a 2,205-foot long, 50-foot wide, asphalt runway. During the third landing, after touchdown, the airplane's tailwheel began to "chatter." The airplane started to veer to the right and did not respond to left rudder or brake inputs by the CFI, who had taken over the controls. The airplane then departed the right side of the runway, where the left main landing gear wheel dug into the ground and the airplane nosed over. Examination of the airplane by a Federal Aviation Administration inspector revealed that the high and low frequency anti-shimmy compression springs on the tailwheel assembly "had slack." The "slack" allowed for about 2 inches of rudder movement before the compression springs became effective. The inspector also observed that the rudder horn had "play," and could be rotated left or right. The two attach bolts for the rudder horn were in place; however, the lower bolt was loose, and the bolt holes were elongated. Review of the airplane's maintenance records revealed that it had seven annual inspections performed after the installation of the tailwheel assembly. The most recent annual inspection was conducted on March 4, 2001. The airplane had accumulated about 15 hours of operation since the annual inspection. According to FAA regulations, Appendix D of Part 43, Scope And Detail Of Items To Be Included In Annual And 100-Hour Inspections, each person performing an annual or 100-hour inspection shall inspect all units of the landing gear group for poor condition and insecurity of attachment. The winds reported at a nearby airport, about the time of the accident, were from 320 degrees at 10 knots.