Robinson R44 Crashes Following Unexplained Engine RPM Fluctuations

Casualties unknown • Kolding/Vamdrup (EKVD), DK

A private helicopter flight ended in a hard landing and substantial damage after the pilot failed to follow emergency procedures during an engine RPM overspeed event.

What happened

On October 4, 2023, a Robinson R44 Clipper II, registration OY-HFS, was conducting a private VFR flight from a helipad near Varde toward Kolding/Vamdrup (EKVD). Approximately ten minutes into the flight, near Holsted, the pilot experienced a significant drop in both engine and rotor RPM, which fell below 70%. The pilot declared an emergency but successfully recovered the RPM levels, after which the governor resumed automatic control and the flight continued.

During the approach to EKVD, the situation reversed. While turning onto the final leg, the engine and rotor RPM surged to 120%. Rather than attempting to stabilize the engine by switching the governor off and manually controlling the throttle, the pilot initiated an autorotation. The helicopter struck a field with high impact, resulting in substantial damage to the landing gear, tail boom, main rotor blades, and firewall. The pilot escaped the crash uninjured.

The investigation

The Danish Accident Investigation Board (AIB) conducted a technical examination of the aircraft's Engine Monitoring Unit (EMU) and the throttle and collective control systems. Extensive testing by the manufacturer, including flight tests and bench inspections, failed to replicate the RPM underspeed or overspeed conditions. Investigators also ruled out electromagnetic interference (EMI) and moisture ingress as causes for the malfunction.

Data analysis from the EMU, ADS-B, and tablet flight logs confirmed the RPM fluctuations occurred but could not definitively determine if the cause was a mechanical/electronic failure or manual pilot input. However, the investigation did establish that the pilot'1s response to the high RPM event did not align with the manufacturer's established emergency procedures.

Findings

  • The investigation could not identify the specific technical cause for the initial engine RPM drop or the subsequent overspeed.
  • During the initial underspeed event, the pilot's handling was close to a catastrophic rotor stall but managed to recover.
  • During the overspeed event, the pilot failed to deactivate the governor and manually override the throttle as required by the Pilot Operating Handbook (POH).
  • The pilot was unaware of a specific manufacturer safety alert regarding governor failures.

Probable cause

The pilot's failure to follow established emergency procedures—specifically the failure to switch off the governor and manually control the throttle during an RPM overspeed—prevented the stabilization of the engine and led to the decision to perform an unnecessary autorotation.

Frequently asked questions

What happened in the 2023-10-04 aircraft accident near Kolding/Vamdrup (EKVD), DK?

A private helicopter flight ended in a hard landing and substantial damage after the pilot failed to follow emergency procedures during an engine RPM overspeed event.

What aircraft was involved and where did it happen?

The accident on 2023-10-04 involved a aircraft, registration OY-HFS, at Kolding/Vamdrup (EKVD), DK.

What was the probable cause of the accident?

The pilot's failure to follow established emergency procedures—specifically the failure to switch off the governor and manually control the throttle during an RPM overspeed—prevented the stabilization of the engine and led to the decision to perform an unnecessary autorotation.

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