What happened
On June 15, 2013, a Robinson R44 II helicopter, registration PR-RGM, was performing a repositioning flight from Carlos Prates Aerodrome to the Helicentro Helipad in Belo Horizonte, Brazil. While overflying the Vale da Mutuca, the pilot experienced a sudden drop in rotor RPM. Upon hearing the low RPM horn and seeing the warning lights, the pilot immediately lowered the collective to enter autorotation.
Due to the perceived loss of engine power, the pilot executed a running landing on a viaduct along the BR-040 highway. The aircraft sustained no damage, and the pilot, along with the passenger, emerged uninjured. Following the landing, the aircraft was transported by truck to a maintenance facility.
The investigation
CENIPA's investigation revealed that the incident was triggered by a failure in the right magneto, which provides the electrical signal to the aircraft's governor. This magneto had undergone recent maintenance 98 flight hours prior to the event. The failure caused the governor to cease functioning correctly, leading to a drop in RPM to approximately 70% during hover conditions before the system could recover.
Investigators also noted that the aircraft's warning system lacks an indicator to alert the pilot of a governor malfunction when the RPM remains above 80%. Furthermore, the investigation found that the maintenance company performed unauthorized post-accident testing on the aircraft. The investigation also highlighted a breakdown in communication, as the air traffic control center (APP-BH) did not notify the accident investigation agency (SERIPA III) of the landing in an unplanned location.