Summary
We propose to adopt a new airworthiness directive (AD) for all Turbomeca S.A. Arriel 2B turboshaft engines. This proposed AD was prompted by a report of an uncommanded in-flight shutdown (IFSD) on a single-engine helicopter, caused by a low returning spring rate of the needle of the hydro-mechanical metering unit (HMU). This proposed AD would require removing any pre-modification (mod) TU 158 HMU and replacing with a part eligible for installation. We are proposing this AD to prevent failure of the HMU, failure of the engine, IFSD, and loss of the helicopter.