Airworthiness Directives; Sikorsky Aircraft Corporation Model S-76B and S-76C Helicopters

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This amendment adopts a new airworthiness directive (AD) for Sikorsky Aircraft Corporation (Sikorsky) Model S-76B and S-76C helicopters. This action requires modifying the electrical power distribution system to change the source of the 28 volts supplied to Nos. 1 and 2 attitude and heading reference (AHRS) WARN circuits. This amendment is prompted by a ground test simulating loss of the essential bus by pulling both bus tie circuit breakers and switching off both batteries. As a result of this action, both autopilots went off-line. The actions specified in this AD are intended to prevent an AHRS fail signal to both autopilots due to a failure of the essential bus, loss of autopilot functions, and subsequent loss of control of the helicopter.