Airworthiness Directives; The Boeing Company Airplanes

We are adopting a new airworthiness directive (AD) for certain The Boeing Company Model 787-8 airplanes. This AD was prompted by reports of the accumulation of very fine particle deposits in the power control unit (PCU) electro-hydraulic servo valves (EHSVs) used in the flight control system; this accumulation caused degraded performance due to reduced EHSV internal hydraulic supply pressures, resulting in the display of PCU fault status messages from the engine indication and crew alerting system (EICAS). This AD requires installing markers to limit the hydraulic system fluid used to a specific brand, doing hydraulic fluid tests of the hydraulic systems, replacing hydraulic system fluid if necessary, and doing all applicable related investigative and corrective actions. We are issuing this AD to address the unsafe condition on these products.