AD 99-10-13

This amendment adopts a new airworthiness directive (AD) that is applicable to certain Boeing Model 737 series airplanes. This action requires repetitive inspections and functional tests of a trailing edge flap limit switch to verify proper operation, and replacement of the existing limit switch with a new limit switch, if necessary. This AD also requires modification of the stabilizer control system, which constitutes terminating action for the repetitive inspections and tests. This amendment is prompted by reports of uncommanded stabilizer trim motion due to failure of the trailing edge flap limit switch. The actions specified in this AD are intended to prevent such failure, which could result in uncommanded (nose down) stabilizer trim motion and consequent reduced controllability of the airplane.

Final rule; request for comments

99-10-13 BOEING: Amendment 39-11166. Docket 99-NM-97-AD. \n\n\tApplicability: Model 737-300, -400, -500, -600, -700, and -800 series airplanes, certificated in any category; equipped with Vickers combined stabilizer trim motors. \n\n\tNOTE 1: This AD applies to each airplane identified in the preceding applicability provision, regardless of whether it has been modified, altered, or repaired in the area subject to the requirements of this AD. For airplanes that have been modified, altered, or repaired so that the performance of the requirements of this AD is affected, the owner/operator must request approval for an alternative method of compliance in accordance with paragraph (e) of this AD. The request should include an assessment of the effect of the modification, alteration, or repair on the unsafe condition addressed by this AD; and, if the unsafe condition has not been eliminated, the request should include specific proposed actions to address it. \n\n\tCompliance: Requiredas indicated, unless accomplished previously. \n\n\tTo prevent failure of the trailing edge flap limit switch, which could result in uncommanded (nose down) stabilizer trim motion and reduced controllability of the airplane, accomplish the following: \n\nInspections and Tests \n\n\t(a)\tPerform a special detailed inspection and functional test to verify proper operation of the S245 trailing edge flap limit switch, in accordance with the applicable Boeing Alert Service Bulletin 737-27A1227 (for Model 737-300, -400, and -500 series airplanes) or 737-27A1228 (for Model 737-600, -700, and -800 series airplanes), both dated April 8, 1999; as applicable; at the time specified in paragraph (a)(1) or (a)(2) of this AD, as applicable. \n\n\t\t(1)\tFor airplanes that have accumulated less than 1,000 total flight hours as of the effective date of this AD: Inspect and test prior to the accumulation of 1,000 total flight hours, or within 10 days after the effective date of this AD, whichever occurs later. Repeat the inspection and test thereafter at intervals not to exceed 300 flight hours, until accomplishment of paragraph (c) of this AD. \n\n\t\t(2)\tFor airplanes that have accumulated 1,000 or more total flight hours as of the effective date of this AD: Inspect and test within 5 days after the effective date of this AD. Repeat the inspection and test thereafter at intervals not to exceed 300 flight hours, until accomplishment of paragraph (c) of this AD. \n\n\tNOTE 2: Any inspection and test of the S245 trailing edge flap limit switch accomplished prior to the effective date of this AD in accordance with the Accomplishment Instructions of either Boeing Alert Service Bulletin 737-27A1227 (for Model 737-300, -400, and -500 series airplanes) or 737-27A1228 (for Model 737-600, -700, and -800 series airplanes), both dated April 8, 1999, as applicable, is considered acceptable for compliance with the initial inspection and test specified in paragraph (a) of this AD. \n\nCorrective Action(b)\tIf any malfunction is detected during any inspection and test required by paragraph (a) of this AD, prior to further flight, replace the existing limit switch with a new limit switch in accordance with the Boeing Alert Service Bulletin 737-27A1227 (for Model 737-300, -400, and -500 series airplanes) or 737-27A1228 (for Model 737-600, -700, and -800 series airplanes), both dated April 8, 1999, as applicable. Repeat the inspection and test thereafter at intervals not to exceed 300 flight hours, until accomplishment of paragraph (c) of this AD. \n\n\t(c)\tWithin 3 months after the effective date of this AD: Incorporate an improved design of the stabilizer control system in accordance with a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA, Transport Airplane Directorate. Incorporation of an improved design, as required by this paragraph, constitutes terminating action for the repetitive inspection and test requirements of this AD. \n\nReporting Requirement \n\n\t(d)\tWithin 10 days after accomplishing the inspection and test required by paragraph (a) of this AD, submit a report of the inspection and test results (positive findings of malfunctioning only) to the Manager, Seattle ACO, FAA, Transport Airplane Directorate, 1601 Lind Avenue SW., Renton, Washington, 98055-4056. The report must include the inspection results, the airplane serial number, and the total number of landings and flight hours on the airplane. Information collection requirements contained in this regulation have been approved by the Office of Management and Budget (OMB) under the provisions of the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et seq.) and have been assigned OMB Control Number 2120-0056. \n\nAlternative Methods of Compliance \n\n\t(e)\tAn alternative method of compliance or adjustment of the compliance time that provides an acceptable level of safety may be used if approved by the Manager, Seattle ACO. Operators shall submit their requests through an appropriate FAA Principal Maintenance Inspector, who may add comments and then send it to the Manager, Seattle ACO. \n\n\tNOTE 3: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Seattle ACO.\n\nSpecial Flight Permits \n\n\t(f)\tSpecial flight permits may be issued in accordance with sections 21.197 and 21.199 of the Federal Aviation Regulations (14 CFR 21.197 and 21.199) to operate the airplane to a location where the requirements of this AD can be accomplished. \n\nIncorporation by Reference \n\n\t(g)\tExcept as provided by paragraph (c) of this AD, the actions shall be done in accordance with Boeing Alert Service Bulletin 737-27A1227, dated April 8, 1999; or Boeing Alert Service Bulletin 737-27A1228, dated April 8, 1999; as applicable. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, Washington 98124-2207. Copies may be inspected at the FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC. \n\n\t(h)\tThis amendment becomes effective on May 27, 1999.

The FAA has received reports of several incidents of uncommanded nose down stabilizer trim motion on certain Boeing Model 737 series airplanes having Vickers type combined manual-autopilot stabilizer trim motors (STM). Investigation revealed the cause as a single point failure in the stabilizer control system, in conjunction with a design deficiency in the STM. Analysis of the S245 trailing edge flap limit switch of the stabilizer control system revealed that the switch had failed due to moisture penetration into the switch contacts, resulting in corrosion and an electrical short circuit. This short circuit caused an erroneously energized STM and subsequent uncommanded stabilizer trim motion in the airplane nose down direction. This condition, if not corrected, could result in reduced controllability of the airplane. \n\nExplanation of Relevant Service Information \n\n\tThe FAA has reviewed and approved Boeing Alert Service Bulletins 737-27A1227 (for Model 737-300, -400, and -500 series airplanes) and 737-27A1228 (for Model 737-600, -700, and -800 series airplanes), both dated April 8, 1999, which describe procedures for repetitive inspections and functional tests of the S245 trailing edge flap limit switch to verify proper operation, and replacement of any malfunctioning limit switch with a new limit switch. \n\nExplanation of the Requirements of the Rule \n\n\tSince an unsafe condition has been identified that is likely to exist or develop on other airplanes of the same type design, this AD is being issued to prevent failure of the S245 trailing edge flap limit switch, and subsequent uncommanded (nose down) stabilizer trim motion, which could result in reduced controllability of the airplane. This AD requires accomplishment of the actions specified in the alert service bulletins described previously. This AD also requires that operators submit a report of findings of malfunctioning to the FAA. \n\n\tAdditionally, this AD requires modification of the stabilizer control system, which constitutes terminating action for the repetitive inspections and tests required by this AD. \n\nInterim Action \n\n\tThis is considered to be interim action until final action is identified, at which time the FAA may consider further rulemaking. \n\nDifferences Between This AD and Alert Service Bulletins \n\n\tOperators should note that, although the alert service bulletins do not specify procedures for terminating action for the repetitive inspections and tests, this AD mandates, within 3 months, incorporation of an improved design of the stabilizer control system as terminating action for the repetitive inspections and tests. \n\n\tThe FAA has determined that long-term continued operational safety will be better assured by design changes to remove the source of the problem, rather than by repetitive inspections and tests. Long-term inspections and tests may not be providing the degree of safety assurance necessary for the transport airplane fleet. This, coupled with a better understanding of the human factors associated with numerous continual inspections, has led the FAA to consider placing less emphasis on inspections and more emphasis on design improvements. Incorporation of an improved design of the stabilizer control system requirement is in consonance with these conditions. \n\nDetermination of Rule's Effective Date \n\n\tSince a situation exists that requires the immediate adoption of this regulation, it is found that notice and opportunity for prior public comment hereon are impracticable, and that good cause exists for making this amendment effective in less than 30 days. \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 99-NM-97-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, Incorporation by reference, 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\n PART 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.§ 39.13 (Amended) \n\t2. Section 39.13 is amended by adding the following new airworthiness directive: