What happened
On January 23, 2007, a Bell 206B helicopter was conducting flight tests at Karlsruhe-Baden Airport to calibrate two navigation systems for the HELLAS obstacle warning system. The flight program required the pilot to perform a 360-degree rotation on the ground, pausing every 30 degrees to verify the systems. To facilitate this, the pilot intended to keep the rear of the skids on the ground while lifting only the front portion.
During the rotation, specifically between 210 and 240 degrees, the aircraft encountered a significant wind shift. The pilot felt the helicopter lift entirely off the ground and tilt forward. In an attempt to correct the pitch and land the aircraft, the pilot moved the cyclic control to its rear mechanical limit and reduced the collective pitch. However, the aircraft'ed forward momentum caused the front skids to strike the grass, which hooked into the ground and caused the aircraft to tip forward. This motion resulted in the main rotor striking the ground with enough force to tear the main gearbox and rotor assembly from the airframe, causing the destruction of the Bell 206B.
The investigation
The BFU examined the aircraft, the flight data from the HELLAS and DOSAR INS test equipment, and the flight recorder from a Boeing 737-300 that was taxiing on a nearby runway at the time. The investigation also reviewed meteorological data, which indicated a wind warning was in effect for the area, noting strong winds from the northeast with gusts up to 30 knots.
Technical inspections of the helicopter revealed no mechanical failures or pre-existing defects. The aircraft's weight and center of gravity were within limits, and the pilot was fully qualified and experienced. The investigation also ruled out the nearby Boeing 737 as a contributing factor, noting that the jet's engine thrust was directed away from the helicopter due to the wind direction.
Findings
- The primary cause of the accident was the uncontrollable flight attitude caused by wind speeds exceeding the tested limits.
- At the time of the accident, the wind was blowing from 050 degrees at 22 knots, which exceeded the 17-knot limit previously verified during the HELLAS system testing.
- The wind direction (nearly from behind during the 210-240 degree rotation) created a loss of control authority, as the wind pushed the aircraft forward and lifted the tail.
- The pilot's rapid attempt to land the aircraft caused the skids to dig into the grass, leading to the subsequent rotor strike.