Airworthiness Directives; Boeing Model 747-400 Series Airplanes

This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 747-400 series airplanes, that requires removal of existing inertial reference units (IRU) and installation of modified IRU's. This amendment is prompted by a report of the failure of the left and center IRU's on a single flight. The actions specified by this AD are intended to prevent loss of multiple IRU's in flight, which could result in the loss of navigation data during flight. This could compromise the ability of the flight crew to maintain the safe flight and landing of the airplane.