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

This amendment supersedes an existing airworthiness directive (AD), applicable to certain McDonnell Douglas Model MD-11 series airplanes equipped with certain Collins LRA-900 radio altimeters, that currently 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 amendment also requires a one-time inspection to determine whether a Collins LRA-900 radio altimeter receiver/transmitter with a certain part number is installed, and modification of such a radio altimeter. This amendment is prompted by reports indicating that a fault in Collins LRA-900 radio altimeters having a certain part number could result in an incorrect and unbounded output of radio altitude to other airplanes. 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. This action is intended to address the identified unsafe condition.