Summary
We propose to supersede an existing airworthiness directive (AD) that applies to certain Turbomeca S.A. Arriel 2B and 2B1 turboshaft engines. The existing AD currently requires accomplishment of the TU166 modification. Since we issued that AD, we became aware of an accident involving an engine in-flight shutdown on a twin-engine helicopter powered by two Arriel 2S2 engines. This proposed AD would add the Arriel 2S2 engine to the applicability of engines requiring the TU166 modification with a different compliance time. We are proposing this AD to prevent rupture of a gas generator (GG) turbine blade, which could result in an uncommanded in-flight shutdown and a forced landing or accident.