Fort Worth, TX, USA
N6040Y
BELL 407
The flight instructor was demonstrating the emergency procedure for a full-authority digital engine control (FADEC) failure that would cause it to switch from auto to manual mode as part of the helicopter manufacturer's training program. The instructor stated that the helicopter was in a stable 5-ft hover when he was demonstrating the maneuver. To begin the demonstration, the flight instructor intentionally switched the FADEC to the manual mode. After about 10 seconds, the engine speed began to increase, which resulted in a corresponding increase in the main rotor speed. The instructor increased the collective control input in an attempt to control the engine and rotor speeds. The helicopter subsequently climbed and began to shake violently. The instructor initiated a descent for landing with an excessive descent rate, which resulted in damage to the landing skids and tailboom upon touchdown. The helicopter flight manual emergency procedure for a FADEC failure and the training program procedure for the FADEC manual mode demonstration both noted that the engine and rotor speeds must be maintained between 95% and 100% with the collective and throttle controls. The demonstration procedure also noted that the throttle control should be in the fly position when the FADEC is placed into manual mode. A review of engine data recovered from the FADEC indicated that the throttle was not in the fly position before beginning the demonstration, which likely resulted in a greater increase in engine and rotor speeds than would have been expected. The flight instructor did not promptly manage the increase in the engine and rotor speeds, which resulted in an inadvertent overspeed condition and subsequent airframe vibrations that prompted the need for an emergency landing.
On January 24, 2017, about 0845 central standard time, a Bell 407 helicopter, N6040Y, was substantially damaged during a hard landing at the Bell Training Facility Heliport (3XS7), Fort Worth, Texas. The flight instructor and pilot receiving instruction were not injured. The helicopter was registered to and operated by Bell Helicopter Textron as a 14 Code of Federal Regulations Part 91 instructional flight. Visual meteorological conditions prevailed at the time of the accident. The flight was not operated on a flight plan. The flight originated from the Bell Helicopter Hurst Heliport about 0820.The flight instructor stated that he was demonstrating the emergency procedure for a failure of the full authority digital engine control (FADEC) unit from auto to manual mode. The helicopter was in a stable 5-foot hover at the time. To begin the demonstration, the FADEC was intentionally switched into manual mode. After approximately 10 seconds, the engine speed began to increase, which resulted in a corresponding increase in the main rotor speed. The instructor increased the collective control input in an attempt to control the engine and rotor speeds. The helicopter subsequently climbed to about 25 feet above ground level (agl) and began to shake violently. The instructor initiated a descent for landing; however, the helicopter contacted the ground with a sufficient descent rate to "spread the skids." The helicopter was shut down and the pilots exited without injuries. Maintenance personnel noticed a crease in the tailboom after it was repainted as part of the repair process. The helicopter maintenance manual stated that creases in tailboom skins are not permitted under any circumstances. In such instances, the tailboom must be repaired or replaced. Data recovered from the FADEC unit revealed that, at the time the FADEC was switched from auto to manual mode, the throttle position (PLA) was 98%, the rotor speed was 100%, and the fuel flow 264 pounds per hour. About 3 seconds later, the rotor speed, engine speed, and fuel flow began to increase. The rotor speed reached a maximum of 118%. The throttle position was subsequently reduced with a corresponding decrease in engine and rotor speeds. The helicopter flight manual stated that in the case of a FADEC failure, the pilot must maintain the engine and rotor speeds between 95% and 100% with the collective and throttle controls, and land as soon as practical. The procedure included a warning that depending on the flight profile and power setting at the time of the failure, the transition to manual mode may result in an increase (overspeed) or a decrease (underspeed) of the engine/rotor speed within 2 to 7 seconds after the failure warning, requiring positive movements of the collective and throttle. The procedure for the FADEC manual mode demonstration noted that the throttle control should be in the fly position when the FADEC is placed into manual mode. The engine and rotor speeds should be maintained between 95% and 100% during the maneuver. According to the helicopter manufacturer, the throttle fly position corresponded to a PLA of 90% in the FADEC data. As a result, a PLA of 98% was consistent with the throttle not being in the fly position at the time of the FADEC demonstration. Without the throttle in the fly position, when the FADEC is switched into manual mode, the unit will adjust fuel flow to that commanded by the throttle setting. In the accident scenario, this was an increase in fuel flow from that being provided by the FADEC in auto mode to maintain the hover, which caused the engine and rotor speeds to increase. As a result of the accident, the helicopter manufacturer revised the training program to perform the FADEC manual mode training in a flight simulator/training device rather than in the helicopter.
The flight instructor's failure to prevent the engine/rotor overspeed condition during the demonstration maneuver and the excessive descent rate during the emergency landing, which resulted in a hard landing. Contributing to the engine/rotor overspeed condition was the flight instructor's failure to comply with the demonstration procedure.
Source: NTSB Aviation Accident Database
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