What happened
On November 8, 2016, an HB-350B helicopter, registration PT-HYE, was performing a vertical takeoff from an inn in Alto Paraíso de Goiás, Brazil. The flight was intended to transport a pilot and one passenger to Brasília.
As the aircraft reached an altitude of approximately eight meters, the pilot applied forward cyclic command. At that moment, the low rotor RPM warning horn activated, and the helicopter simultaneously lost lift. The pilot attempted an emergency recovery by lowering the collective and applying forward cyclic to clear a swimming pool located directly below the aircraft. However, the maneuver failed, and the aircraft struck the edge of the pool, slid along the margin, and swerved 90 degrees clockwise before becoming partially submerged. The impact caused substantial structural damage to the airframe, including breaks in the main and tail rotor blades and the stabilizers. The pilot and passenger sustained minor injuries.
The investigation
CENIPA investigators examined the wreckage and the engine, an Arriel 1B. Analysis of the engine components revealed that the engine was producing power at the moment of impact, evidenced by the high torque required to cause a 1mm displacement of the input pinion nut.
Investigators focused on the tachymeter box, which regulates the bleed valve to prevent engine stalls. The box's safety seal had been breached, and it bore an annotation reading "intermittent." The pilot reported that a similar low RPM warning had occurred during a flight approximately one month prior, but no maintenance was performed at that time. Furthermore, the investigation found that the tachymeter box had reportedly been sent to a maintenance organization (NAVTEC) that was not certified by the Brazilian civil aviation authority to perform such services. Additionally, while the aircraft had a valid airworthiness certificate, the logbook records were outdated, and there was no evidence of the required 100-hour inspection.
Findings
- The intermittent failure of the tachymeter box led to a temporary loss of engine power and a fluctuation in rotor rotation speed.
- Inadequate maintenance interventions likely resulted in the aircraft operating with equipment that was not in a state of perfect airworthiness.
- The takeoff location was highly restricted and contained several obstacles, failing to meet the safety requirements for non-homologated landing/takeoff sites established by RBHA 91 regulations.