On March 6, 2007, approximately 0900 mountain standard time, a Diamond Aircraft Industries DA-20-C1, N921DA, owned and operated by Doss Aviation Incorporated, sustained a partial forward horizontal stabilizer separation while in flight near Pueblo Memorial Airport (PUB), Pueblo, Colorado. Visual meteorological conditions prevailed at the time of the occurrence. The instructional flight was being conducted under the provisions of Title 14 Code of Federal Regulations Part 91 without a flight plan. The student pilot and commercial certificated flight instructor were not injured. The local flight originated at PUB at 0800. According to the flight school, the training flight included maneuvers such as steep turns, power on stalls, slow flight, and forward slips to a landing. The airplane returned to PUB, at which time the flight instructor took control of the airplane and noted "abnormal nose-up trim was required." The training flight continued with an additional 5 landings, as "there were no excessive flight control inputs required to safely fly" the airplane. The flight concluded without additional or further incident. After the training flight, a 2 to 3 inch gap between the horizontal and vertical stabilizer was noted by the operator. Examination of the airplane revealed that the two bolts securing the front mounting bracket of the horizontal stabilizer to the vertical stabilizer had separated. The fractured bolts including the nut side of one bolt and the washers were sent to the National Transportation Safety Board Materials Laboratory, in Washington, D.C., for further examination. In addition, the forward bracket bolts from two other similar airplanes owned by Doss Aviation (N920DA and N922DA) were also forwarded to the NTSB lab for examination. According to the Materials Laboratory Factual Report, both bolts had fractured through the threaded portion of the shank. The left bolt contained multiple facture surfaces that intersected the first and second full thread roots from the head side of the bolt. Examination of the fracture intersections indicated the transverse fracture at the second thread was the primary fracture with the slanted fracture from the first thread being the secondary fracture. The nut side of the bolt exhibited a smooth, finely textured fracture surface that contained ratchet marks and curved crack arrest lines indicative of fatigue cracking initiating in the thread root radius with propagation across the bolt section. The fatigue propagated completely through the bolt with no discernable overstress region. Assembly (torque) paint was noted on the nut and adjacent bolt threads. Visual examination of the washers revealed that one of the washers exhibited fretting and wear patterns that matched those on the washer face of the nut from the left bolt. On the washer face of the nut, the fretting was predominately adjacent to the fatigue region on the bolt. The right bolt had a single fracture plane intersecting the second full thread root from the bolt head. The right bolt fracture was very similar to the transverse fracture of the left bolt showing features consistent with fatigue initiation in the thread root. Propagation was directly across the cross section with a small overstress region opposite the fatigue initiation region. No damage or abnormalities were noted in the thread roots at the fatigue initiation regions of either fractured bolt. Examination of the forward bracket bolts from N920DA and N922DA revealed no anomalies. According to the Diamond Aircraft parts manual for the horizontal stabilizer, part number AN3-11A identifies the two hex bolts. The bolts are marked with a raised "X" indicating non-corrosion resistant steel bolts and raised letters "AFC" denoting the bolt manufacturer. According to manufacturing procedures given by Diamond Aircraft, the installation of these bolts occurs prior to the post cure of the composite structure. Following the post cure, a quality insurance inspector checks the torque and applies the witness paint. These bolts are to be torqued to a value of 20 to 25 inch pounds. Diamond reported that there is no torque for field maintenance, as this connection is not normally serviced in the field. According to Doss Aviation, this airplane had 189 hours total time. A review of the maintenance logs revealed no discrepancies or maintenance on the horizontal stabilizer or brackets. The Transportation Safety Board (TSB) of Canada provided an accredited representative for the investigation. Diamond Aircraft Industries and Transport Canada were the technical advisors to TSB Canada.