Colorado Aviation Accidents & Incidents

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 fu