What happened
On August 2, 2016, a two-person crew was conducting a training flight in an Agusta A 109E helicopter. The flight began at the Ottobrunn helipad, proceeding around Munich to the Oberschleißheim special landing site. The crew was performing maneuvers simulating single-engine operations, specifically practicing CAT-A approaches with simulated engine failures.
During the final approach, the crew executed a landing at a designated point in front of runway 26. The pilot landed with low forward speed, quickly reduced the collective lever, and decreased the main rotor RPM from 1-02% to 100%. Once the helicopter was stationary on the ground, the instructor pilot deactivated the single-engine training mode. Immediately following this, the helicopter began experiencing vertical vibrations, which the crew identified as ground resonance.
In an attempt to stop the vibration, the instructor pilot pulled the collective lever. However, this action intensified the vertical oscillations and introduced horizontal shaking. The helicopter bucked violently between its left and right main landing gear. The resulting tilt caused the main rotor to strike the ground, leading the fuselage to rotate around its vertical axis. The aircraft sustained significant damage, including a broken left main landing gear and damage to the main rotor blades and gearbox. The crew successfully evacuated the aircraft; the instructor sustained a minor head injury.
The investigation
The BFU examined the aircraft and the landing site. Technical inspections of the main rotor head, landing gear dampers, and the nose-wheel lock mechanism revealed no mechanical failures or defects. X-ray analysis of the dampers showed no internal damage or loose connections. The damage found on the rotor head components was classified as secondary damage resulting from the accident itself.
The investigation also looked into the landing surface, which consisted of uneven ground covered with loose stones, weeds, and moss. The BFU also considered whether a foreign object, such as a stone kicked up by the tail rotor, could have caused a rotor blade imbalance. While a small indentation was found on a tail rotor blade, no definitive cause for a primary blade imbalance could be confirmed.
Findings
- The primary cause of the accident was ground resonance following a simulated single-engine landing.
- The landing was performed on unsuitable, uneven terrain consisting of loose stones and vegetation.
- The transition from 102% to 100% rotor RPM, combined with the aircraft's movement over the uneven surface, likely provided the input necessary to trigger the unstable oscillations.
- The aircraft's weight was at the upper limit for a ground-based helipad landing in single-engine training mode.
- The crew's attempt to use the collective to stop the resonance may have inadvertently contributed to the instability.