What happened
On June 26, 2014, an Agusta AB 412 helicopter was parked on the apron at Mannheim City Airport following a refueling operation. The aircraft, operated by a German air rescue company, was preparing for a medical transport flight to the Mannheim University Hospital. The crew included the pilot, a flight mechanic, and a two-person medical team.
During the engine start sequence, the pilot noted that the aircraft sagged after the first engine reached 60–65% N2. Upon starting the second engine and performing the before-hover checks, the helicopter began to drift toward the rear left. The pilot immediately suspected a landing gear issue and notified the control tower, requesting the airport fire service. To prevent the aircraft from tipping over, the pilot used the pitch control to stabilize the helicopter. The aircraft eventually came to rest supported by a broken section of the rear cross-tube and the step. There were no fatalities and no injuries to the crew or medical team, though the aircraft sustained light damage.
The investigation
The BFU examined the aircraft's landing gear, specifically the rear cross-tube. The investigation focused on the fracture site located on the left side of the tube's most curved section. Laboratory analysis, supported by the Fraunhofer Institute for Surface Engineering and Materials (IST), utilized scanning electron microscopy to examine the fracture surfaces. The investigators also inspected the surface of the cross-tube to evaluate the condition of the paint and the underlying metal. The investigation also reviewed the maintenance history and the procedures used by the operator's contracted painting service.
Findings
Technical analysis revealed that the rear cross-tube had failed due to fatigue cracking. The investigation established that the cracks originated from surface irregularities (grooves) on the metal. These grooves were created during maintenance when the painting contractor used mechanical means, such as rotating wire brushes or grinding discs, to remove old paint instead of using the chemical stripping methods recommended in the manufacturer's maintenance manual.
These mechanical grooves acted as stress concentrators (notches) in a highly stressed zone of the tube, where the bending moment is greatest due to the component's curvature. These notches allowed fatigue cracks to develop, which eventually progressed until the remaining cross-section could no longer support the load, resulting in a final ductile fracture. Additionally, the investigation noted that the surface of the cross-tube had undergone various repainting efforts over its service life, leading to inconsistent paint adhesion and thickness.
Safety action
- The maintenance organization of the air operator should notify contractors of the manufacturer's approved procedures when delegating aircraft work and must actively monitor the execution of such tasks.