Airworthiness Directives; Boeing Model 747-400, 777-200, and 777-300 Series Airplanes

This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 747-400, 777-200, and 777-300 series airplanes. This AD requires, for certain airplanes, replacing the cell stack of the flight deck humidifier with a supplier-tested cell stack, or replacing the cell stack with a blanking plate and subsequently deactivating the flight deck humidifier. For certain other airplanes, this AD requires an inspection of the flight deck humidifier to determine certain part numbers and replacing the cell stack if necessary. This AD also allows blanking plates to be replaced with cell stacks. The actions specified by this AD are intended to prevent an increased pressure drop across the humidifier and consequent reduced airflow to the flight deck, which could result in the inability to clear any smoke that might appear in the flight deck. This action is intended to address the identified unsafe condition.