Summary
This document proposes the supersedure of an existing airworthiness directive (AD), applicable to McDonnell Douglas Model MD-11 series airplanes equipped with certain Collins LRA-900 radio altimeters. That AD 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 action would require a one-time inspection to determine whether a Collins LRA-900 radio altimeter receiver/transmitter with a certain part number is installed. This action would also require modification of such a radio altimeter. This proposal 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 the proposed 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.