Airworthiness Directives; Turbomeca Makila 1A and 1A1 Turboshaft Engines

The FAA is superseding an existing airworthiness directive (AD) for Turbomeca Makila 1A, 1A1, and 1A2 turboshaft engines. That AD currently requires replacing certain digital electronic control units (DECUs) and electronic control units (ECUs) with modified DECUs and ECUs. This AD applies only to Makila 1A and 1A1 turboshaft engines, and requires replacing the selector-comparator board in the ECU with a board incorporating Turbomeca modification TU 250. This AD results from recent unexplained reversions of the ECU to the 65% N1 back-up mode. We are issuing this AD to prevent dual-engine continued operation at 65% N1 after reversion of the ECU to the 65% N1 back-up mode due to temporary loss of N2 speed signal, which could lead to inability to continue safe flight, emergency autorotation landing, or an accident.