On July 20, 2003, about 0830 central daylight time, an Enstrom F-28C helicopter, N51699, piloted by a commercial pilot, sustained substantial damage when the main rotor blade impacted the tail boom during a forced landing near Waverly, Iowa. The helicopter experienced in-flight vibrations prior to the forced landing. The sightseeing flight was operating under 14 CFR Part 91. Visual meteorological conditions prevailed at the time of the accident. No flight plan was on file. The pilot and two passengers were uninjured. The local flight originated from Waverly Municipal Airport, near Waverly, Iowa. The pilot stated: Flying with two passengers. Notice small [vibration] and about one minute later, extreme vibration 200 ft [above ground level] over trees and houses. [Initiated] emergency landing to parking lot. Vibration very extreme, if seat belts were not on it would have thrown occupants from aircraft. Control 10% to nil. Vibration was so bad that visibility was poor to none. Touchdown was not hard but #1 blade was flopping about and struck tail, damage to blade and tail cone. If we would have been over 200 [feet above ground level] this would have certainly been more serious. The Federal Aviation Administration and helicopter manufacturer performed an examination of the accident helicopter. The examination revealed that one of three Push-Pull Rod Assemblies, part number 28-16253-1, was found with a separation. The internal surface of that rod assembly was found corroded. Another rod assembly was disassembled and was found to contain a liquid. The helicopter's third rod assembly was disassembled. No anomalies were found with that third rod assembly. The operator's fleet of aircraft was examined and four other control rods were found with corrosion. The two rods from the accident helicopter were sent to the National Transportation Safety Board's Materials Laboratory for examination. The laboratory produced Materials Laboratory Factual Report number 03-085. Excerpts from the report stated: Visual examination of the fractured rod portion showed the presence of substantial rust-colored corrosion deposits on the inside of the rod adjacent to the fracture. These deposits extended over a distance of about 2 inches above the fracture location. The corrosion damage had thinned the wall of the rod adjacent to the fracture, and in some areas the corrosion appeared to penetrate nearly through the wall thickness to the exterior surface of the rod. ... Examination of the fracture areas on the larger portion of the broken rod after ultrasonic cleaning in acetone revealed that most of the fracture areas were on a plane that was nearly perpendicular to the longitudinal axis of the rod, indicative of a brittle fracture mechanism such as fatigue cracking. Concerning the pitch control push-pull rod assemblies, the pilot stated: Pitch control rod has no inspection procedure at this time. It rusted from inside and not visible from outside. Must remove both pitch control ends and visually check inside tube. Subsequent to the accident, Enstrom issued Service Directive Bulletin (SDB) No. 0096. The bulletin directs visual inspection of control rods with part number 28-16253-1 and 28-16253-101 according to their time in service on F-28A, F-28C, F-28F, 280, 280C, 280F, and 280FX helicopters. Excerpts from the bulletin compliance section stated: Within ten (10) hours time in service or at the next annual inspection, which ever occurs first, review the aircraft maintenance records to determine the date "new" main rotor push-pull rods were installed in the aircraft. If the installation date for "new" main rotor push-pull rods can not be determined from the maintenance records, use the aircraft "DATE MFD." found on the aircraft data plate. For main rotor push-pull rods (P/N 28-16253-1 or -101) in service for more than twenty (20) years, inspect the push-pull rods in accordance with (IAW) paragraph 5.1 of this SDB within ten (10) hours time in service or at the next annual inspection, which ever occurs first. For main rotor push-pull rods (P/N 28-16253-1 or -101) in service between ten (10) years and twenty (20) years, inspect the push-pull rods IAW with paragraph 5.1 of this SDB within fifty (50) hours time in service or at the next annual inspection, which ever occurs first. For main rotor push-pull rods (28-16253-1 or -101) in service less than ten (10) years, inspect the push-pull rods IAW paragraph 5.1 of this SDB before the push-pull rods reach ten (10) years time in service.