History of Flight
On June 24, 2010, about 1530 Pacific daylight time, a Bell UH-1H helicopter, registration N205KS, experienced a structural failure in the tailboom while hovering near a cliff at Lake Cachuma in Santa Barbara, California. The helicopter was operated by the Santa Barbara County Fire Department as a public-use training flight. The two commercial pilots and three passengers were not injured. The flight originated from Santa Ynez, California, about 1000, and was staged at a campground adjacent to the lake throughout the day. Visual meteorological conditions prevailed, and no flight plan was filed.
The pilot-in-command (PIC) reported that while hovering about 200 feet above ground level near a cliff at the north area of the lake for rescue personnel training, he heard a loud sound and felt the helicopter yaw slightly. He made a precautionary landing without incident. A post-landing examination revealed that the left tail boom upper attachment fitting (part number 205-031-821-1) was fractured.
Aircraft Information
The aircraft was a Bell UH-1H, serial number 9526/67-17328, with a total time in service of 13,609.1 hours. It was maintained on an Approved Airworthiness Inspection Program (AAIP), with the most recent inspection completed on June 16, 2010. The helicopter was modified with numerous supplemental type certificates (STCs), including an upgraded Honeywell T58-L-703 engine, FastFin system, strakes, and composite tail rotor blades. The tailboom's upper skin was replaced on January 17, 2007, during the FastFin installation. Two previous accidents were recorded: one in 1971 in Vietnam resulting in major damage to stress panels and airframe, and another in 1973 involving a hard landing that required replacement of the skid cross tubes.
Examination Findings
The failed fitting was sent to the National Transportation Safety Board Materials Laboratory for examination. Examination of the aft face of the upper left longeron attach fitting revealed a fracture intersecting two rivet holes. Crack arrest features typical of fatigue cracking were observed, emanating from the upper rivet hole. The longeron was attached to the fitting by rivets and an adhesive layer containing scrim cloth. The rivet fracture faces showed mechanical damage but no corrosion.
Bell Helicopter had not performed a fatigue analysis on the fitting or surrounding structure, and no other fatigue analysis data were located. Additionally, Bell had no design load data related to the fitting. A Safety Board structures engineer confirmed that the static strength was congruent with the manufacturer's data.
Additional Information
A representative from Select Aviation Services, Inc., which overhauls and refurbishes UH-1H helicopters, reported numerous failures of left longeron fittings, including a complete crack-through during maintenance due to improper repair and preloading. Other cracks were found aft of the fitting. The representative cited several causes for such cracks: incorrect shimming of longeron mounting pads, misalignment or unequal torquing during tailboom attachment, improper maintenance torquing procedures, difficulty aligning the fuselage and tailboom during attachment, incorrect use of shims, and placing the helicopter on a trailer, which could induce loads the helicopter was not designed for.
Regarding the STCs, no engineering data existed on how their combination affected structural integrity. The Federal Aviation Administration does not require STC holders to perform such analysis; instead, the installer must determine the interrelationship between multiple STCs.