Airworthiness Directive · Amendment 39-12559 · Sikorsky Aircraft Corporation Model S-76B and S-76C Helicopters
Sikorsky Aircraft Corporation Model S-76B and S-76C Helicopters
Model S-76B helicopters, serial numbers (S/N) 760430, 760441 through 760445, 760448 through 760452, 760454, 760455, 760458, 760462, and 760465, and Model S-76C helicopters, S/N 760420, 760436, 760438, 760440, 760453, 760456, 760457, 760459, 760460, 760461 760463, 760464, 760466 through 760487, certificated in any category.
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.
The aircraft, engines and makers this AD names, by the model designations in its text. A model listed here may still be outside the AD's serial ranges.
