Introduction
On November 12, 2019, at about 2008 mountain standard time, an AgustaWestland AW109SP helicopter, registration N271HC, was substantially damaged during cruise flight near Spanish Fork, Utah. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 air ambulance flight. The pilot and two flight nurses on board were not injured.
Flight Details
The crew departed their base at about 1952, responding to a motor vehicle accident approximately 22 minutes away. About five minutes from the destination, the flight nurse seated in the left front seat attempted to contact ground personnel to coordinate the landing. Instead of using 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. This action resulted in a rapid yaw to the left.
Incident Sequence
The pilot re-established straight and level flight and landed the helicopter without further incident. After landing at the destination, the pilot conducted a walkaround inspection but did not examine the tail rotor blades or tailboom. No unusual external signatures were observed. The remaining portion of the flight was uneventful, and the pilot reported no anomalies during subsequent flights that shift. After the accident crew returned to home base and went off-duty, the oncoming pilot discovered damage to the tailboom during a daylight walkaround. The helicopter was immediately taken out of service and ferried to the company’s maintenance facility for repair.
Investigation Findings
The nurse involved typically rode in the rear cabin but was seated in the cockpit on the accident flight to accommodate the intended patient. The National Transportation Safety Board (NTSB) materials laboratory examined the tail rotor blades, tail rotor gearbox, pitch change slider assembly, and tail rotor hub assembly, finding no anomalies. The helicopter was equipped with an Appareo flight data monitoring (FDM) system. The FDM data indicated a yaw of about 11°. An NTSB performance study using that data corroborated the pilot’s account, showing the helicopter turned left from a heading of 153° to 142° in 1.25 seconds. When the pilot applied right tail rotor pedal, the helicopter returned to 151° in 2 seconds. During this maneuver, the helicopter 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 from certification testing for the accident model. A dynamic stability test point was conducted at 0.9Vh (calculated maximum horizontal airspeed based on atmospheric conditions) with a ±10° yaw excursion. A 15% pedal input over 0.5 seconds resulted in a sideslip deviation of 6 to 15 degrees per second. A simulation using the accident helicopter’s parametric data and ambient conditions showed that a 40% left pedal input (90% pedal position) was required to match the observed yaw rate and magnitude. This resulted in a 20° per second yaw deviation, exceeding the parameters tested during certification.