What happened
On February 26, 1999, a Bell 407 helicopter, registration PT-YVI, was performing a passenger transport flight from Lagoa Santa to Belo Horizonte, Brazil. While cruising at approximately 500 feet above ground level near Sabará, the pilot reported hearing a loud noise, followed by a sudden leftward yaw and a loss of aircraft control.
During the flight, the pilot had reduced speed from 115 knots to 110 knots to avoid stratus clouds. Following the sudden yaw, the aircraft entered an abnormal attitude. The pilot managed to partially recover control using cyclic and collective inputs, but the aircraft continued to yaw left despite full right pedal input. The pilot performed an emergency landing in an open field.
During the heavy impact, the pilot sustained a spinal fracture. Following the landing, the two passengers exited the aircraft while the main rotor was still rotating. Due to the impact damage to the landing skids, the rotor plane was lower than usual, causing the rotating blades to strike the passengers. Both passengers subsequently died.
The investigation
CENIPA investigators examined the wreckage and found that the tail boom had been severed due to successive impacts from the tail rotor blades. The investigation focused on the tail rotor's stability and the aircraft's compliance with existing Airworthiness Directives (AD).
Investigators discovered that the aircraft had not implemented the required modifications specified in AD CF 98-36R1, which mandated a reduced Vne (never-exceed speed) placard and a new red line at 115 knots. Instead, the aircraft still displayed the original 140-knot limit. Furthermore, laboratory analysis of the hydraulic fluid revealed contamination, which investigators believed could have caused a temporary malfunction of the tail rotor servo actuator, leading to the abrupt change in pitch.
Findings
- Design deficiency: The Bell 407 design did not provide sufficient clearance between the tail rotor and the airframe to prevent contact under all operating conditions.
- Maintenance deficiency: The aircraft failed to comply with the mandatory speed limit placards required by the active Airworthiness Directive.
- Contaminated hydraulic fluid: The presence of contaminants in the hydraulic system likely caused a momentary malfunction of the tail rotor servo.
- Inadequate regulation: Existing advisory circulars (AC 27-1) were insufficient to ensure the necessary separation between the rotor and the structure as required by FAR 27 standards.
- Safety procedures: The passengers exited the aircraft while the main rotor was still in motion, a fatal error likely driven by panic.