What happened
On August 27, 1998, an HB-350B operated by Viganó Táxi Aéreo Ltda. was performing a flight from Belo Horizonte to Inhapim, Minas Gerais. Approximately 20 minutes into the flight, while overflying São Gonçalo, the pilot observed the main rotor speed (NR) dropping to between 370 and 375 RPM. The pilot responded by lowering the collective to restore normal RPM.
Shortly after recovering power, the NR dropped again, reaching levels between 350 and 360 RPM, which triggered the low-speed alarm. Faced with mountainous terrain and dense eucalyptus forests, the pilot identified a river valley as a suitable landing site. The pilot executed an autorotative descent and landed the aircraft in the river. During the landing, the right skid struck a submerged rock, causing the helicopter to rotate and strike another rock with the tail. The aircraft sustained severe damage, but the pilot and four passengers escaped uninjured.
The investigation
CENIPA investigators examined the engine, transmission, and various electrical components. Analysis of the engine and accessories revealed no mechanical failures, fuel contamination, or engine surges that could have caused the observed NR drop. However, investigators found that pin No. 3 of the female connector of the magnetic pickup wiring was obstructed by a small piece of metal. This obstruction could have prevented a proper connection between the magnetic pickup and the transmission wiring, potentially causing intermittent NR indications and triggering the alarm.
It was noted that the electrical connectors were not protected after being removed from the aircraft, which may have led to the loss of critical evidence regarding the origin of the metal fragment. The investigation also noted that the operator had recently undergone significant personnel changes, hiring new pilots during a peak period without specific company adaptation training.
Findings
- Deficient Judgment: The pilot did not cross-check other engine instruments (such as gas generator speed or temperature) to confirm whether the NR drop was a genuine mechanical failure or merely an instrumentation error.
- Instrumentation Anomaly: An unidentified metal fragment obstructed a pin in the magnetic pickup connector, which likely caused intermittent main rotor speed indications and the activation of the low-speed alarm.
Safety action
- The Civil Aviation Department (DAC) was directed to reinforce the necessity of preserving all aircraft systems and components during initial investigations to avoid compromising future analysis.
- The regional aviation authority (SERAC-3) was tasked with conducting a safety inspection of the operator's maintenance procedures.
- The aircraft operator was instructed to ensure pilots undergo recurrent training on emergency procedures related to main rotor speed fluctuations.