What happened
On December 27, 2018, an Mi-8MSB helicopter, registration OB-2133-P, operated by Aero Tecnic E.I.R.L., was conducting a high-altitude performance test flight. The aircraft departed from Pisco airport to evaluate its capabilities in mountainous terrain at altitudes of approximately 4,400 meters.
After 90 minutes of flight, the crew attempted a practice landing on the "Ajopata" Andean plain in the Castrovirreyna province. During the approach, the aircraft descended from 5,034 meters to a hover at approximately 25 meters above the ground. While maintaining a low airspeed of approximately 9 km/h, the aircraft experienced a sudden, uncommanded left turn. Although the pilot attempted to correct the deviation by applying significant right pedal input, the aircraft continued to experience uncommanded turns and eventually entered a right-hand bank.
The helicopter impacted the terrain, resulting in the separation of the main rotor blades and the tail boom from the fuselage. Despite the heavy damage to the airframe and engines, the crew and passengers successfully evacuated through the upper hatch and side sliding door. There were no fatalities.
The investigation
The CIAA investigation examined the flight path via Flight Data Recorder (FDR) analysis, the aircraft'-s flight manuals, and maintenance records. Investigators reviewed the operational limitations for high-altitude flight and the specific aerodynamic characteristics of the Mi-8MSB, noting that the main rotor torque causes the fuselage to tend to rotate left, requiring right pedal input to maintain heading.
Findings
- The primary cause of the accident was Loss of Tail Rotor Effectiveness (LTE) during a low-speed hover.
- The aircraft was operating in a marginal flight condition at high altitude (4,650 meters) during a landing practice.
- The pilot attempted to counteract the uncommanded yaw by applying right pedal up to 97 mm (near the 106 mm maximum), but was unable to maintain directional control.
- The impact caused significant structural failure, including the loss of the tail boom and main rotor blades.