Agusta A109E N109DU incident near Augusta, Georgia

Probable cause

The lack of available engine power and subsequent loss of rotor rpm due to the pilot’s departing with one engine at idle. Contributing to the incident was a corroded engine control wiring harness.

NTSB analysis

The pilot described ascending to a hover over the helipad, conducting a power check, and determining that the helicopter was "ready for flight." The helicopter departed in a northwesterly direction, climbed to an altitude of about 30 feet, where "both engines lost power." The pilot performed a right pedal turn and maneuvered the helicopter back toward the helipad. The helicopter landed short of the helipad, straddling a fence that separated the pad from an adjacent road. The helicopter's main landing gear sustained minor damage, and the helicopter's skin was punctured in a non-structural area. Data downloaded from the digital engine control units (DECU) revealed that a stepper motor fault on the number 2 engine prevented that engine from accelerating above idle speed, and that the engine remained at idle throughout the accident flight. The data further revealed that the number 1 engine was accelerated to flight, but that the “102%NR” switch was not selected prior to takeoff as required by the flight manual. Neither was the number 2 engine trim switch, nor the number 2 engine power unit actuated by the pilot, which was required to accelerate that engine after the stepper motor fault. The number 2 Digital Engine Control Unit (DECU), the number 2 Hydro-Mechanical Unit (HMU), and the wire harnesses (Control and Monitoring – Regulation) that connected them were tested. The DECU and the HMU both functioned as designed with no faults noted. The regulation wire harness was tested and impedance was detected on pin 3 of the stepper motor plug. The jacket of the harness was removed, which revealed "moist deposits" and corrosion on the metallic ground shield. Removal of the shield revealed more deposits and corrosion, as well as cracked and burned insulation on wire 3. The exposed wire on pin 3 allowed a short circuit to ground which resulted in the stepper motor fault. The helicopter was not equipped with an optional "Engine Out" kit that, if installed, provided the pilot with an “ENGINE OUT” audio signal in case, during the takeoff phase, one or both the engines were in IDLE or in OFF.

Text from the NTSB. Open the NTSB docket for NYC08IA244 for the full record.