What happened
On January 29, 2008, two flight instructors were conducting training maneuvers at Bonn-Hangelar airfield to gain required type experience for the Eurocopter EC120 B. The flight involved simulated engine failures. During the first autorotation exercise performed by the pilot in command, the pilot attempted to initiate the flare at approximately 1,200 ft MSL.
As the aircraft approached the landing target in a grassy area, the pilot attempted to flare the helicopter to reduce forward speed. While the crew attempted to maintain a nose-up attitude, the aircraft's right skid dug into the soft, moist soil. This caused an asymmetric deceleration, resulting in a roll and a sudden pitch-forward movement. The nose of the fuselage struck the ground, and the aircraft subsequently tipped onto its left side, causing the main rotor blades to strike the ground. The pilots were able to exit the aircraft without injury.
The investigation
The BFU examined the aircraft's landing gear design, the ground conditions, and the flight profiles used during the maneuver. The investigation noted that the aircraft's skid shoes protrude slightly, which can act as a hook on soft surfaces. The BFU also surveyed 13 certified examiners for the EC120 B, most of whom reported that the aircraft has a natural tendency to pitch forward during the flare phase of an autorotation.
Evidence from the crash site showed that the right skid had dug into the ground, breaking the turf, while the left skid had merely slid. The investigation also reviewed a previous incident involving the same aircraft six weeks earlier, where a similar skid-digging event had occurred during training.
Findings
- The primary cause of the accident was that the pilot likely overcorrected the aircraft during the flare, causing the right skid to catch the turf.
- The design of the Eurocopter EC120 B landing gear, specifically the protruding skid shoes, contributed to the skid digging into the soft ground.
- The limited ground clearance provided by the tail stinger restricted the usable range of pitch attitudes during the flare.
- The soft and moist nature of the landing site increased the risk of the skid digging in.
- The pilots had low experience with this specific aircraft type, with only 21 and 12 hours respectively.