Sacramento, CA, USA
N916PD
BELL OH 58A
The pilot reported that during the takeoff, flight, and landing for a local routine helicopter patrol flight, he did not experience any turbulence nor did any of the flight controls reveal an indication that something was wrong. The pilot reported that during the postflight inspection, the vertical stabilizer was found bent downward away from the tail rotor. A postaccident examination of the vertical stabilizer revealed a fatigue crack that initiated at the aft end of the inboard skin at about the middle of the span. A stop-drill hole was present above the upper aft attachment hole, indicating that the vertical stabilizer was allowed to continue in service after a crack had been detected that extended about 1 1/4 inches from the aft edge of the inboard edge. The crack intersected the lower side of the stop-drill hole, and a fatigue crack reinitiated from the forward side of the stop-drill and continued to propagate towards the leading edge. The operator acquired the helicopter as US Army surplus. Since 2003, the US Army’s daily inspection checklist included inspection for cracks originating from the four attachment inserts on the inboard of the vertical stabilizer. The operator’s maintenance vendor was not aware of the US Army aviation safety action message for the daily inspection for cracks inboard of the vertical stabilizer. Had those daily inspections of the vertical fin attachment area been accomplished, these cracks would likely have been identified.
On December 4, 2016, about 2100 Pacific standard time, a Bell OH-58A helicopter, N916PD, sustained substantial damage to the vertical stabilizer during a flight near, Sacramento, California. The pilot and observer were not injured. The helicopter was operated as a public flight. The pilot and observer reported noticing the vertical stabilizer damage after the completion of a routine patrol flight, during the post-flight inspection. The vertical stabilizer was bent downwards away from the tail rotor. The pilot further reported that the flight was flown at a cruise altitude of 600-700 ft above ground level, and no turbulence or any other unusual flying conditions were encountered. The helicopter's damaged vertical stabilizer was removed and shipped to the National Transportation Safety Board (NTSB) Materials Laboratory for examination. The data plate of the vertical stabilizer showed part number (P/N) 206-022-113-3. The examination revealed that the inboard side of the vertical stabilizer, inboard skin, was fractured above the attachment holes. The upper portion of the fin remained partially attached by the outboard skin. A drilled hole was located about 1 ¼ inch from the edge of the inboard skin. The skin fracture aft of the drilled hole intersected the lower side of the hole at a tangent and the fracture forward of the hole extended radially outward from the hole. Examination of the lower surface using a scanning electron microscope revealed striations consistent with fatigue. The fatigue features in the skin emanated from a primary origin located at the trailing edge of the skin. A secondary origin was located at the forward side of the drill hole and the fatigue features extended to about 2 ½ inches of the inboard skin. Cracks in the inboard, near the attachment holes of the vertical stabilizers installed on OH-58 Helicopters were a known issue to the U.S. Army. According to U.S. Army Aviation and Missile Command Technical Manual TM 55-1520-228-PMD, the daily inspection checklist included an inspection of the inboard of the vertical stabilized for cracks originating from the four attachment inserts. The inspection was implemented in 2003 via Aviation Safety Action Message (ASAM) OH—58-03-ASAM-6, after a service failure of the vertical stabilizer. The ASAM applied to fin assembly P/N 206-022-113-1 and P/N 5790032-501. Additionally, the ASAM applied to fin assembly P/N 206-022-113-3. Alert Service Bulletin (ASB) 206-26 published in December 18, 1972, addressed the cracks in the inboard skin near the attachment bolts of vertical stabilizers installed on Bell Helicopter model 206 and TH-57A helicopters. The ASB applied to vertical fin P/Ns 206-020-113-5, -7 and -9. According, to a Bell representative, the vertical stabilizer part numbers mentioned in the alert were very similar to the vertical stabilizer installed on the OH-58 helicopter, but the part numbers were different due to minor differences. The ASB 206-26, required visual inspections of the inboard skin for cracks in the attachment area immediately upon receipt and subsequently at 25-hour intervals. ASB 206-26 was superseded by ASB 206-01-731, dated January 9, 1973, which affected the same helicopter but required the first inspection within 25 hours after receipt and the subsequent inspections at 25-hour intervals. Accomplishment of Service Letter (SL) 206-203, terminated the requirement for the repetitive inspections for crack in the inboard skin of vertical stabilizer, required by the ASB. In SL 206-203, dated December 18, 1972, vertical stabilizers were modified or repaired by installing a doubler on the interior skin in the area of the attachment holes. Cracks in the vertical stabilizer longer than 3 ½ inches were considered unrepairable and unairworthy. Since the helicopter was acquired from the United States military as surplus, the operator would inherit the type certificate holder responsibilities. Therefore, the operator would be responsible for the engineering, continued airworthiness, and other associated safety information pertaining to the helicopter. The operator was unaware that the maintenance vendor that provided support to the helicopter, was not aware of a past U. S. Army ASAM, and therefore, was not accomplishing daily inspections of the vertical fin attachment area for cracks.
The failure of the vertical stabilizer due to fatigue.
Source: NTSB Aviation Accident Database
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