What happened
On December 15, 2007, a Hiller UH-12B helicopter departed from Aachen-Merzbrück, Germany, bound for Bonn-Hangelar. After a warm-up period on the runway, the pilot took off into the wind with the carburetor heat applied and a slight engine overspeed. During the initial climb, the pilot noted an airspeed of 60 knots and attempted to reduce this to the optimal climb speed of 4-5 knots. To achieve this, the pilot pulled back on the cyclic, creating a flare effect that increased rotor aerodynamic loading. Simultaneously, the pilot reduced engine power using the throttle.
As the forward airspeed decreased and the flare effect subsided, the aircraft was left in a state of low engine and rotor RPM. The pilot attempted to recover the rotor speed by increasing engine power and reducing blade pitch, but was unable to regain sufficient RPM. Consequently, the helicopter lost altitude, rolled along its longitudinal axis, and drifted to the right with the wind. The aircraft struck a field with its right skid first and at a forward tilt, resulting in heavy damage to the aircraft but no injuries to the pilot.
The investigation
The BFU examined the aircraft, which was in good maintenance condition and well within its weight limits. The investigation focused on the pilot's maneuvers and the specific handling characteristics of the Hiller UH-12B. The investigation noted that the pilot had relatively limited flight experience, with only 111 total hours, of which only 6.5 hours were in this specific model. The investigation also considered the environmental conditions, noting that while visibility was good, wind gusts of up to 22 knots were present.
Findings
- The primary cause of the accident was pilot error during a simultaneous maneuver involving the reduction of airspeed and engine power.
- The pilot's attempt to flare the aircraft while reducing throttle led to a critical drop in both engine and main rotor RPM.
- The pilot's limited experience with the specific handling requirements of the Haggis UH-12B, particularly the manual management of rotor RPM compared to the Robinson R22, contributed to the error.
- The low power-to-weight ratio of the Hiller UH-12B required more precise power management, which was not maintained during the maneuver.