Summary
The FAA proposes to supersede an existing airworthiness directive (AD) for Turbomeca Makila 1 A, 1 A1, and 1 A2 turboshaft engines. That AD currently requires replacing certain digital electronic control units (DECUs) and electronic control units (ECUs) with modified DECUs and ECUs. This proposed AD would apply only to Makila 1 A and 1 A1 turboshaft engines, and would require replacing the selector-comparator board in the ECU with a board incorporating Turbomeca modification TU 250. This proposed AD results from recent unexplained reversions of the ECU to the 65% N1 back-up mode. We are proposing this AD to prevent dual-engine reversion of the ECU to the 65% N1 back-up mode, which could lead to inability to continue safe flight, emergency autorotation landing, or an accident.