What happened
On February 9, 2010, an Agusta A109A air ambulance was performing a medical transport mission from Rostock to Bad Oeynhausen. After completing the patient transfer, the crew flew to the Porta Westfalica airfield to refuel for the return leg.
At approximately 15:00 local time, the pilot began the takeoff roll from the fuel station area. As the pilot applied collective pitch to initiate the takeoff, a violent jolt shook the entire aircraft. Upon inspection, the pilot discovered that components of the tail rotor and the tail rotor gearbox had been torn from the airframe and were lying on the ground approximately five meters behind the helicopter. There were no fatalities or injuries resulting from the incident, though the aircraft sustained heavy damage.
The investigation
The BFU investigation focused on the structural failure of the tail rotor assembly. Metallurgical analysis conducted by the Technical University of Braunschweig determined that the failure was caused by a fatigue fracture (Schwingbruch) in one of the tail rotor blades. Two opposing fatigue cracks, measuring 9-mm and 37-mm, originated from a structural weakness at the end of a reinforcement plate.
The investigation also scrutinized the maintenance records of the operator, an approved maintenance organization. It was discovered that the tail rotor assembly had been rotated between different aircraft in the fleet. Crucially, the maintenance software used by the operator failed to include the specific Airworthiness Directives (AD) regarding crack inspections for these particular blades. Furthermore, the investigation found that the required 100-hour inspections and the necessary documentation for the tail rotor blades were incomplete or missing.
Findings
Several factors contributed to the failure of the Agusta A109A:
- The primary cause was the detachment of a tail rotor blade due to an undetected fatigue crack.
- A structural design weakness in the tail rotor blade created a stress concentration point that promoted crack development.
- The maintenance program was superficial and failed to adequately incorporate specific Airworthiness Directives into the tracking software.
- The operator's practice of transferring components between aircraft without issuing an EASA Form 1 meant that the required inspections for the newly installed tail rotor were not properly triggered.
- The pilot's pre-flight inspection was ineffective, as the necessary tools (magnifying glass and solvent) for detecting cracks were not on board, and the cracks were located in a difficult-to-inspect area near the paint transition.
Safety action
Following the accident, the manufacturer issued an information bulletin stating that the affected tail rotor blade part numbers would no longer be manufactured or repaired, and offered a discount to replace old blades with newer, improved versions. The BFU issued safety recommendations to the German Federal Aviation Office (LBA) and the operator to ensure that aircraft maintenance, documentation, and approved manuals strictly adhere to legal requirements.