What happened
On August 26, 2002, a Hiller UH-12E helicopter was engaged in aerial spreading operations near Morgenröte Rautenkranz, Germany. The aircraft was performing routine tasks, involving filling a ground bucket with lime, when approximately 30 seconds after beginning a climb, the pilot experienced a sudden impact.
Following this impact, the helicopter began experiencing severe oscillations, a phenomenon known as mast bumping. As the pilot attempted to stabilize the aircraft, the flight became increasingly uncontrollable, characterized by violent pitch and roll movements. These instabilities were particularly acute when the pilot reduced collective pitch, which aerodynamically unloaded the rotor system. In an attempt to maintain a larger blade angle and stabilize the flight, the pilot increased pitch, which caused the aircraft to climb to an altitude of approximately 700 ft.
Fearing that the continuous mast bumping would lead to a structural failure of the mast, the pilot initiated an autorotation. However, during the initiation of the autorotation, a loud bang occurred as the main rotor severed the tail rotor, causing the aircraft to spin on its vertical axis. The helicopter subsequently crashed into a slope in a forest clearing.
The investigation
The BFU investigation focused on the structural integrity of the rotor blade control system. Investigators recovered a portion of a rotor paddle and its associated spar tube from the ground between the loading site and the discarded external load.
Laboratory analysis of the broken components conducted by the Technical University of Braunschweig revealed that the unpainted, internal part of the spar tube was heavily corroded despite the presence of seals. The investigation established that the failure of the spar tube within the control rotor cuff was caused by two extensive fatigue cracks originating from one of the holes for the outer transverse bolt. These cracks propagated circumferentially through both halves of the tube. By the time of the accident, the tube's cross-section was 95% severed, leaving the component held in place only by surface pressure.
Findings
- The primary cause of the accident was the structural failure of the rotor blade control system, where the spar tube was nearly entirely severed by fatigue cracks.
- Severe corrosion on the internal, unpainted portion of the spar tube significantly contributed to the degradation of the component.
- The investigation noted that the last documented inspection of the mechanical connection between the paddle and the control rotor, which included specialized corrosion and crack testing, had occurred approximately 91 flight hours prior to the accident. It remains unclear how such extensive corrosion and a 95% reduction in the tube's cross-section could have developed in such a short period, raising questions regarding the adequacy of the previous inspection.