What happened
On January 13, 2013, an Mi-171 helicopter, registration HK-4575, was performing a maintenance test flight at Base Costayaco in the Putumayo department of Colombia. The flight was intended to verify flight controls following the replacement of the tail rotor's 90° gearbox.
During the initial hover checks, the pilot noted unusual pedal conditions. As the aircraft began its takeoff, a loud, continuous noise and high-frequency vibration emerged. The pilot attempted an immediate emergency landing, but encountered obstacles that forced the aircraft to continue the climb. During the subsequent flight, the vibration became so intense that the tail rotor blades struck the tail boom, fracturing the structure near the 45° gearbox. The helicopter lost control, yawing sharply and descending from approximately 3/0 meters, ultimately impacting the terrain on its right side. All 8 occupants, including 3 crew members and 5 passengers, were able to evacuate the aircraft without fire, though the crew sustained 2 serious injuries.
The investigation
The GRIAA investigation established that the aircraft had undergone significant maintenance in the days prior to the accident. A technician had replaced the 90° gearbox, a task performed outdoors in high temperatures. The investigation found that the technician relied on a CD-ROM for maintenance manual references because no laptop was available at the worksite, and the work was completed without a final inspection by the Authorized Technical Inspector (ALT).
Investigators examined the maintenance procedures and found that the technician had performed the assembly of the tail rotor blades to their respective hubs using pins and safety nuts. The investigation also reviewed the pilot's training, medical certifications, and the organizational culture of the operator, Helistar.
Findings
- The primary cause of the accident was the incorrect installation of the three tail rotor blades, which generated high-frequency vibrations leading to the structural failure of the tail boom.
- Maintenance tasks were performed using a repetitive, memorized approach rather than strictly following the maintenance manual steps.
- The inspection of the completed work by the ALT was inadequate.
- The pre-flight inspection failed to identify the incorrect blade installation.
- The maintenance environment was suboptimal, involving outdoor work in high temperatures and a lack of proper digital access to manuals.
- A design deficiency was noted: despite manufacturer markings, the tail rotor blade assembly allows for installation in both directions, creating a risk of human error.
- There were self-imposed pressures on the maintenance staff to complete the repairs quickly to meet the contractor's schedule.