Event Summary
On 26 August 2002, a Hiller UH-12E helicopter (serial number 5148) was operating in the Rautenkranz area, conducting liming operations with an external bucket. At approximately 16:00 local time, about 30 seconds after starting a climb with a freshly filled bucket, a bang occurred, followed by mast bumping and oscillations. The pilot jettisoned the external load and attempted an emergency landing. During descent, the helicopter became uncontrollable with severe pitch and roll oscillations, especially when collective pitch was reduced. The pilot increased collective to maintain rotor RPM, causing the helicopter to climb. After flying about 3.5 km, at approximately 700 ft, the pilot initiated an autorotation, fearing structural failure of the mast. Immediately, a loud bang was heard as the main rotor severed the tail rotor, causing the helicopter to spin. It then crashed into the bank of a stream in a forest clearing.
Injuries and Damage
The pilot sustained severe injuries. The helicopter was substantially damaged. Additionally, ground damage (property damage) occurred.
Aircraft Information
The Hiller UH-12E, manufactured in 1981, was equipped with a two-blade main rotor with paddles, a two-blade tail rotor, and skid landing gear. It was powered by an Allison 250 C-20B turboshaft engine. At the time of the accident, the helicopter had accumulated 8,002 total flight hours, with the engine at 9,142 hours. The main rotor blades had 3,284 and 5,186 hours respectively. The helicopter held a certificate of airworthiness in the "Non-commercial and Aerial Work" category and was owned by a private aviation company.
Investigation Findings
Examination of the wreckage revealed that one of the paddles from the rotor control system had detached at the junction where the paddle blade spar tube fits into the paddle blade cuff. This part was later found between the loading site and the jettisoned lime bucket. The broken components showed severe corrosion. Laboratory analysis at the Technical University of Braunschweig determined that the unlacquered inner portion of the spar tube was heavily corroded despite sealing. The fracture was caused by two extensive fatigue cracks originating from one of the two outer bolt holes. These cracks propagated circumferentially in both halves of the tube until they met from opposite sides, leaving only a small web that sheared off under axial offset.
Probable Cause
The accident resulted from the fact that at the time of the accident, the cross-section of the blade spar tube was already 95% severed by two fatigue cracks, and the tube was held only by the existing surface pressure between the blade spar tube and the blade cuff. It cannot be ruled out that a corrosion or crack inspection of the rotor control system components had been incorrectly performed by a maintenance organization prior to the accident.