Airworthiness Directives; Turbomeca Turmo IV A and IV C Series Turboshaft Engines

The FAA is superseding an existing airworthiness directive (AD) for Turbomeca Turmo IV A and IV C series turboshaft engines. That AD currently requires borescope and eddy current inspections or ultrasonic inspections of centrifugal compressor intake wheel blades for cracks and evidence of corrosion pitting, and replacement with serviceable parts. This AD requires the same actions, but would require borescope inspections at more frequent intervals for certain engines. This AD results from Turbomeca's review of the engines' service experience that determined more frequent borescope inspections are required on engines not modified to the TU 191, TU 197, or TU 224 standard. We are issuing this AD to prevent centrifugal compressor intake wheel blade cracks, which can result in engine in-flight power loss, engine shutdown, or forced landing.