On November 12, 2019, about 2008 mountain standard time, an AgustaWestland AW109SP helicopter, N271HC, was substantially damaged during cruise flight near Spanish Fork, Utah. The pilot and two flight nurses were not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 135 air ambulance flight. According to the pilot, the crew departed their base about 1952 and was bound for a motor vehicle accident about 22 minutes away. About 5 minutes from the destination, the flight nurse, seated in the left front seat, attempted to contact personnel on the ground to coordinate their landing. Rather than step on the push-to-talk (PTT) button located on the cabin floor to activate the radio microphone, he inadvertently stepped on the left anti-torque pedal, which resulted in a rapid yaw to the left. The pilot re-established straight and level flight and landed the helicopter uneventfully. The pilot did not observe any unusual signatures on the exterior of the helicopter during a walkaround inspection after they landed at their destination following the accident; however, he did not inspect the tail rotor blades or tailboom. The rest of the flight was uneventful, and the pilot did not report any anomalies with the tail rotor blades or tailboom during the subsequent flights that took place during his shift. After the accident crew arrived at the company’s home base and went off-duty, the oncoming pilot conducted a walkaround inspection in daylight conditions and discovered damage to the tailboom as he approached the helicopter. The helicopter was immediately taken out of service and ferried to the company’s maintenance facility for repair. The nurse who was seated in the left front seat during the accident flight normally rode in the rear cabin of the helicopter, but the crew decided that he would sit in the cockpit during the accident flight to make room for the patient they intended to pick up. The tail rotor blades, tail rotor gearbox, pitch change slider assembly, and the tail rotor hub assembly were examined by the NTSB materials laboratory and the components exhibited no anomalies. The helicopter was equipped with an Appareo flight data monitoring (FDM) system, which indicated that the yaw was about 11°. An NTSB performance study using data retrieved from the FDM corroborated the pilot’s statement and showed that the helicopter rapidly turned left from a heading of 153° to 142° in 1.25 seconds. When the pilot countered the movement with right tail rotor pedal, the helicopter returned to 151° in 2 seconds. During this time, the helicopter rapidly rolled from level flight to -15° to 15° and pitched up 2° before returning to its original altitude. The helicopter manufacturer provided flight handling qualities data obtained during the certification of the accident helicopter. A dynamic stability test point was performed in level flight at 0.9Vh ± 10° yaw where Vh is the calculated maximum horizontal airspeed based on atmospheric conditions. This test involved a 15% pedal input over 0.5 second, which resulted in a 6º-to-15º per second sideslip deviation. A simulation was performed using the accident helicopter’s parametric data and ambient conditions showed that to match the yaw rate and magnitude, a 40% left pedal input (90% pedal position) was required. This resulted in a 20° per second deviation, which exceeded the parameters tested for during certification.