What happened
On June 22, 2014, an AgustaWestland AW109SP, registration HB-ZRU, was conducting a search mission for a missing person near Cortaccio, in the municipality of Brissago. After completing an initial flight to transport personnel to a landing site, the pilot began a solo return flight to Brissago to pick up additional passengers.
During this flight, the pilot executed a significant nose-down maneuver, initiating a steep descent with rapidly increasing airspeed while turning to the right. As the aircraft accelerated, the pilot noted a high rate of descent and found the right rudder pedal at its full limit. The aircraft's nose began to yaw toward the left. In an attempt to correct the rotation, the pilot applied further right pedal, but the control authority was exhausted.
Following an improper control correction, the aircraft entered a sharp right bank of up to 78 degrees, reaching a load factor of 2.2 g. During this sequence, the rotor speed exceeded its maximum allowable limit, reaching 120% of the rated speed. The pilot eventually regained control of the aircraft and elected to continue the mission.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight data from the Electronic Flight Instrument System (EFIS) and the Data Acquisition Unit (DAU). The investigation confirmed that the aircraft's weight and center of gravity were within the limits specified by the Rotorcraft Flight Manual (RFM). There were no pre-existing technical defects found in the aircraft. The investigation also reviewed the pilot's experience and the operational procedures in place at the time of the event.
Findings
- The primary cause of the serious incident was that during a steep descent with rapidly increasing speed in a right turn, the available right pedal travel was insufficient to maintain directional control around the yaw axis.
- This loss of directional control, combined with an inappropriate control correction, led to a temporary loss of aircraft control and a subsequent rotor overspeed.
- The incident was further contributed to by the fact that the helicopter was flying near, or potentially outside, its certified flight envelope limits.