Airworthiness Directives; Sikorsky Aircraft Corporation (Sikorsky) Model S76A, B, and C Helicopters

This amendment supersedes an existing emergency airworthiness directive (EAD) for the specified Sikorsky model helicopters. The EAD requires inspecting the LITEF Attitude Heading and Reference System (AHRS) unit of the navigation system to determine if it is at a Mod Status "18." If either AHRS unit is at Mod Status "18," the EAD requires installing placards on the instrument panel to prohibit single pilot instrument flight rule (IFR) and single pilot night flight and reducing airspeeds to 120 knots indicated airspeed (KIAS) if both autopilots uncouple during instrument meteorological conditions (IMC) or night flight. The EAD also requires inserting minimum crew and airspeed limitations into the Limitations section of the applicable Rotorcraft Flight Manual (RFM) to limit the minimum flight crew to 2 pilots for night flight and IFR flight and to reduce airspeed to 120 KIAS if both autopilots uncouple during IMC or night flight. This amendment contains the same requirements but draws the appropriate distinctions between IFR and IMC as used in the intended operating limitations. Also, unlike the EAD, this AD states the airspeed must be reduced to 120 KIAS if both autopilots uncouple during IMC or night flight. Further, we are removing the limitation contained in the Active Temporary Revisions relating to pilots keeping their hands and feet near the flight controls. This AD was prompted by the need to supersede the EAD to state the distinction between IFR and IMC as used in the operating limitations and to reduce the airspeed to 120 KIAS if both autopilots uncouple during IMC or night flight. The actions specified by this AD are intended to implement operating limitations based on an anomaly in the AHRS related to the 26 volt AC inverter that could result in a decoupling of both autopilots and to prevent loss of control of the helicopter during IMC and during night flight.