On November 19, 1998, the FAA issued telegraphic AD T98-24-51, which is applicable to certain McDonnell Douglas Model MD-11 series airplanes equipped with certain Collins LRA-900 radio altimeters. That action was prompted by a report from Rockwell Collins that a fault in certain Collins LRA-900 radio altimeters has been identified, which could result in an incorrect and unbounded output of radio altitude to other airplane systems. \n\n\tThe fail-operational autoland installation on McDonnell Douglas Model MD-11 series airplanes utilizes a dual- dual architecture that relies on the self-monitoring capability of the Collins LRA-900 radio altimeters. Any undetected anomalous radio altitude signal that is passed along to the flare control law of the flight control computer (FCC) could cause the initiation of the flare mode at an altitude that is either too high or too low for safe landing during autoland operations. \n\n\tThis fault does not affect airplanes equipped with either an autoland system architecture that utilizes triplex radio altimeter sensors or a dual fail-passive autoland architecture. The triplex radio altimeter sensors are able to "vote out" the undetected radio altimeter anomaly. The dual fail-passive autoland architecture compares both radio altimeters and passively disconnects when the signals do not match (i.e., radio altimeter miscompare). \n\n\tIn light of these findings, the FAA has determined that the reported anomaly is limited to airplanes with fail-operational autoland systems with a dual-dual fail-operational radio altimeter architecture. \n\n\tAn undetected anomalous radio altitude signal that is passed along to the flare control law of the FCC, if not corrected, could cause the airplane to flare too high or too low during landing, and consequently result in a hard landing. \n\nExplanation of Requirements of the Rule \n\n\tSince the unsafe condition described is likely to exist or develop on other airplanes of thesame type design, the FAA issued telegraphic AD T98-24-51 to require a revision to the Limitations Section of the FAA-approved Airplane Flight Manual (AFM) 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. \n\n\tThis is considered to be interim action until final action is identified, at which time the FAA may consider further rulemaking. \n\n\tSince it was found that immediate corrective action was required, notice and opportunity for prior public comment thereon were impracticable and contrary to the public interest, and good cause existed to make the AD effective immediately by individual telegrams issued on November 19, 1998, to all known U.S. owners and operators of certain McDonnell Douglas Model MD-11 series airplanes equipped with certain Collins LRA-900 radio altimeters. These conditions still exist, and the AD is hereby published inthe Federal Register as an amendment to section 39.13 of the Federal Aviation Regulations (14 CFR 39.13) to make it effective to all persons. \n\nComments Invited \n\n\tAlthough this action is in the form of a final rule that involves requirements affecting flight safety and, thus, was not preceded by notice and an opportunity for public comment, comments are invited on this rule. Interested persons are invited to comment on this rule by submitting such written data, views, or arguments as they may desire. Communications shall identify the Rules Docket number and be submitted in triplicate to the address specified under the caption "ADDRESSES." All communications received on or before the closing date for comments will be considered, and this rule may be amended in light of the comments received. Factual information that supports the commenter's ideas and suggestions is extremely helpful in evaluating the effectiveness of the AD action and determining whether additional rulemaking action would be needed. \n\n\tComments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the rule that might suggest a need to modify the rule. All comments submitted will be available, both before and after the closing date for comments, in the Rules Docket for examination by interested persons. A report that summarizes each FAA-public contact concerned with the substance of this AD will be filed in the Rules Docket. \n\n\tCommenters wishing the FAA to acknowledge receipt of their comments submitted in response to this rule must submit a self-addressed, stamped postcard on which the following statement is made: "Comments to Docket Number 98-NM-334-AD." The postcard will be date stamped and returned to the commenter. \n\nRegulatory Impact \n\n\tThe regulations adopted herein will not have substantial direct effects on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government. Therefore, in accordance with Executive Order 12612, it is determined that this final rule does not have sufficient federalism implications to warrant the preparation of a Federalism Assessment. \n\n\tThe FAA has determined that this regulation is an emergency regulation that must be issued immediately to correct an unsafe condition in aircraft, and that it is not a "significant regulatory action" under Executive Order 12866. It has been determined further that this action involves an emergency regulation under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979). If it is determined that this emergency regulation otherwise would be significant under DOT Regulatory Policies and Procedures, a final regulatory evaluation will be prepared and placed in the Rules Docket. A copy of it, if filed, may be obtained from the Rules Docket at the location provided under the caption "ADDRESSES." \n\nList of Subjects in 14 CFR Part 39 \n\n\tAir transportation, Aircraft, Aviation safety, Safety. \n\nAdoption of the Amendment \n\n\tAccordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration amends part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows: \n\nPART 39 - AIRWORTHINESS DIRECTIVES \n\n\t1. The authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. 106(g), 40113, 44701. \n\n§ 39.13 (Amended) \n\n\t2. Section 39.13 is amended by adding the following new airworthiness directive: