Airworthiness Directives; McDonnell Douglas Model MD-11 Series Airplanes Equipped with Certain Collins LRA-900 Radio Altimeters

This document publishes in the Federal Register an amendment adopting airworthiness directive (AD) T98-24-51 that was sent previously to all known U.S. owners and operators of certain McDonnell Douglas Model MD-11 series airplanes by individual telegrams. This AD requires a revision to the Airplane Flight Manual to prohibit autopilot coupled autoland operations in certain conditions; or, for certain airplanes, replacement of certain Collins LRA-900 radio altimeters with Collins LRA-700 radio altimeters. This action is prompted by a report that a fault in certain Collins LRA-900 radio altimeters could result in an incorrect and unbounded output of radio altitude to other airplane systems. The actions specified by this AD are intended to prevent an undetected anomalous radio altitude signal that is passed along to the flare control law of the flight control computer, which could cause the airplane to flare too high or too low during landing, and consequently result in a hard landing.