Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Boeing Model 747-400 series airplanes. This proposal would require removal of existing inertial reference units (IRU) and installation of modified IRU's. This proposal is prompted by a report of the failure of the left and center IRU's on a single flight. The actions specified by the proposed 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.