Summary
This document revises an earlier proposed airworthiness directive (AD), applicable to Boeing Model 747-400, 777-200, and 777- 300 series airplanes, that would have required an inspection of the flight deck humidifier to determine certain part numbers. That proposed AD also would have required, 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, if necessary. For other airplanes, that proposed AD would have required replacing the cell stack with a supplier-tested cell stack, or replacing the cell stack with a blanking plate and subsequently deactivating the humidifier system, if necessary. The proposed AD also would have allowed blanking plates to be replaced with cell stacks. This new action revises the proposed rule by adding airplanes to the applicability, requiring an inspection of the flight deck humidifierto determine certain part numbers on certain airplanes, and requiring replacement of the cell stack on certain other airplanes. The actions specified by this new proposed 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.