Airworthiness Directives; Boeing Model 737-100, -200, -300, -400, and -500 Series Airplanes

Comments Invited

Interested persons are invited to participate in the making of the proposed 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 above. All communications received on or before the closing date for comments, specified above, will be considered before taking action on the proposed rule. The proposals contained in this notice may be changed in light of the comments received. Comments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the proposed 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 summarizing each FAA-public contact concerned with the substance of this proposal will be filed in the Rules Docket. Commenters wishing the FAA to acknowledge receipt of their comments submitted in response to this notice must submit a self-addressed, stamped postcard on which the following statement is made: ``Comments to Docket Number 98-NM-251-AD.'' The postcard will be date stamped and returned to the commenter.

Availability of NPRMs

Any person may obtain a copy of this NPRM by submitting a request to the FAA, Transport Airplane Directorate, ANM-114, Attention: Rules Docket No. 98-NM-251-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056.

Discussion

The FAA has received reports indicating that tie bolts have failed on certain BFGoodrich wheel assemblies that are installed on the main landing gear (MLG) of Boeing Model 737-100, -200, -300, -400, and -500 series airplanes. Most of the incidents of

failure of multiple tie bolts (that is, failure of more than one bolt on a single wheel) resulted only in flat tires. However, since 1989, there have been several incidents that resulted in high-energy release of the wheel rim and consequent damage to the airplane. In one incident, failure of both tires on the left MLG resulted in a rejected takeoff (RTO). Failure of the wheel tie bolts has been attributed to fatigue, which may be caused by improper torquing of the tie bolts. Although the specific cause of failure has not been identified on a few wheel tie bolts, other causes of failure have been identified as improper maintenance, wear, corrosion, or a combination of several factors. Failure of multiple wheel tie bolts, if not corrected, could result in failure of the wheel rim, rapid release of tire pressure, and possible consequent damage to the airplane and injury to passengers and flightcrew.

Explanation of Relevant Service Information

The FAA has reviewed and approved BFGoodrich Aerospace Service Bulletins 3-1439-32-13 and 3-1398-32-16, both dated August 20, 1993. The service bulletins describe procedures for corrective actions if certain wheel assemblies are installed on the MLG. The corrective actions include modification of the wheel assembly by replacement of existing tie bolts, nuts, and washers with new, improved Inconel tie bolts and nuts, and new, thicker washers. The service bulletins also describe new torque values, procedures for inspecting the new tie bolts to ensure that they did not crack during torquing, and a procedure for stamping a new part number on the wheel assembly once it has been modified. Accomplishment of the actions specified in the service bulletins is intended to adequately address the identified unsafe condition.

Explanation of Requirements of Proposed Rule

Since an unsafe condition has been identified that is likely to exist or develop on other products of this same type design, the proposed AD would require a one-time visual inspection of the MLG wheel assemblies to determine whether certain part numbers are installed on the airplane, and follow-on corrective actions, if necessary. If certain part numbers are installed, the proposed AD would require repetitive replacement of all tie bolts, nuts, and washers of the MLG wheel assembly with new parts; or repetitive visual inspections to detect fractures of the tie bolts of the MLG wheel assembly, and replacement of discrepant parts with new parts. Alternatively, the proposed AD would require revisions to the FAA-approved maintenance program to require one of those actions. If those certain part numbers are installed, the proposed AD also would require eventual accomplishment of corrective actions specified in the service bulletins described previously, except as discussed below. Accomplishment of these corrective actions would terminate the requirements of this AD.

Differences Between Proposed Rule and Service Bulletins

Operators should note that the service bulletins specify replacement of wheel tie bolts with new parts only when broken wheel tie bolts have been found. This proposed AD would require, at every fifth tire change, replacement of the existing wheel tie bolts, washers, and nuts with parts having the same part number, or repetitive visual inspections at intervals not to exceed 100 flight cycles; or alternatively, revisions to the FAA-approved maintenance program to require one of these actions. The service bulletins also specify that existing parts can continue to be used if there are no discrepancies in those parts. This proposed AD would require eventual modification of affected wheel assemblies by replacement of existing wheel tie bolts, washers, and nuts with new, improved parts, which would constitute terminating action for the requirements of this AD. The FAA has determined that long-term continued operational safety would be better assured by design changes to remove the source of the problem, rather than by repetitive inspections. Long-term inspections may not provide 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. The proposed modification is in consonance with these conditions.

Explanation of the Applicability of the Rule

The FAA notes that its general policy is that, when an unsafe condition results from the installation of an appliance or other item that is installed in only one particular make and model of airplane, an AD is issued so that it is applicable to the airplane, rather than the item. The reason for this is simple: making the AD applicable to the airplane model on which the item is installed ensures that operators of those airplanes will be notified directly of the unsafe condition and the action required to correct it. While it is assumed that an operator will know the models of airplanes that it operates, there is a potential that the operator will not know or be aware of specific items that are installed on its airplanes. Therefore, calling out the airplane model as the subject of the AD prevents ``unknowing non- compliance'' on the part of the operator.

Cost Impact

There are approximately 460 airplanes of the affected design in the worldwide fleet. The FAA estimates that 118 airplanes of U.S. registry would be affected by this proposed AD. It would take approximately 1 work hour per airplane to accomplish the proposed one-time inspection, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the one-time inspection proposed by this AD on U.S. operators is estimated to be $7,080, or $60 per airplane. The cost impact figure discussed above is based on assumptions that no operator has yet accomplished any of the proposed requirements of this AD action, and that no operator would accomplish those actions in the future if this AD were not adopted. Should an operator elect to accomplish the repetitive visual inspection, it would take approximately 1 work hour per airplane to accomplish, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the repetitive inspections, if accomplished, is estimated to be $60 per airplane, per inspection cycle. Should an operator elect to accomplish the replacement, such replacement would require no additional work hours if accomplished during a regularly scheduled tire change. Required parts would cost $2,840 per airplane ($710 per wheel). Based on these figures, the cost impact of the replacement, if accomplished, is estimated to be $2,840 per airplane, per replacement cycle. Should an operator elect to accomplish the revisions to the FAA- approved maintenance program, it would take approximately 1 work hour per airplane to accomplish, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the maintenance program revision, if accomplished, is estimated to be $60 per airplane. Should an operator be required to accomplish the necessary modification of the wheel assembly, it would require no additional work hours per airplane,

if the modification is accomplished during a regularly scheduled tire change. Required parts would cost $4,848 per airplane ($1,212 per wheel). Based on these figures, the cost impact of any necessary modification is estimated to be $4,848 per airplane.

Regulatory Impact

The regulations proposed herein would 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 proposal would not have sufficient federalism implications to warrant the preparation of a Federalism Assessment. For the reasons discussed above, I certify that this proposed regulation (1) is not a ``significant regulatory action'' under Executive Order 12866; (2) is not a ``significant rule'' under the DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and (3) if promulgated, will not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act. A copy of the draft regulatory evaluation prepared for this action is contained in the Rules Docket. A copy of it may be obtained by contacting the Rules Docket at the location provided under the caption ADDRESSES.

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Safety.

The Proposed Amendment

Accordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration proposes to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows:

PART 39--AIRWORTHINESS DIRECTIVES

1. The authority citation for part 39 continues to read as follows:

Authority: 49 U.S.C. 106(g), 40113, 44701.

Sec. 39.13 [Amended]

2. Section 39.13 is amended by adding the following new airworthiness directive:

Boeing: Docket 98-NM-251-AD. Applicability: All Model 737-100, -200, -300, -400, and -500 series airplanes; certificated in any category.

Note 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.

Compliance: Required as indicated, unless accomplished previously. To prevent failure of multiple tie bolts of main landing gear (MLG) wheel assemblies, which could result in failure of the wheel rim, rapid release of tire pressure, and possible consequent damage to the airplane and injury to passengers and flightcrew, accomplish the following: (a) Within 60 days after the effective date of this AD, perform a one-time visual inspection of the MLG wheel assemblies to determine the part number (P/N) of each assembly. If no wheel assembly manufactured by BFGoodrich Aerospace and having P/N 3-1398- 1, 3-1439-2, or 3-1439-3 is installed on the airplane, no further action is required by this AD. (b) Except as provided by paragraph (d) of this AD, if any MLG wheel assembly manufactured by BFGoodrich Aerospace and having P/N 3-1398-1, 3-1439-2, or 3-1439-3 is installed on the airplane, within 60 days after the effective date of this AD, accomplish the actions specified by paragraph (b)(1) or (b)(2) of this AD. (1) Accomplish the actions specified by (b)(1)(i) or (b)(1)(ii) of this AD. (i) Replace all tie bolts, nuts, and washers of the MLG wheel assembly with parts having the same P/N's, in accordance with the BFGoodrich component maintenance manual. Thereafter, repeat the replacement of tie bolts, nuts, and washers, at intervals not to exceed 5 tire changes, until the actions specified by paragraph (b)(2) or paragraph (c) of this AD have been accomplished. Or (ii) Perform a visual inspection to detect fractures of any of the 16 tie bolts on each MLG wheel assembly, in accordance with the Boeing 737 airplane maintenance manual. Thereafter, repeat the inspection at intervals not to exceed 100 flight cycles until the actions specified by paragraph (b)(2) or paragraph (c) of this AD have been accomplished. If any fracture of any tie bolt is found during any inspection performed in accordance with this requirement, prior to further flight, replace the tie bolt, nut, and washer, in accordance with the BFGoodrich component maintenance manual, with new parts having the same P/N's. (2) Revise the FAA-approved maintenance program as specified in paragraph (b)(2)(i) or (b)(2)(ii) of this AD. (i) Revise the FAA-approved maintenance program to require replacement of all tie bolts, nuts, and washers of the MLG wheel assembly with parts having the same P/N's, in accordance with the BFGoodrich component maintenance manual, at intervals not to exceed 5 tire changes. Or (ii) Revise the FAA-approved maintenance program to require a visual inspection to detect fractures of any of the 16 tie bolts on each MLG wheel assembly, in accordance with the Boeing 737 airplane maintenance manual, at intervals not to exceed 100 flight cycles. If any fracture of any tie bolt is found during any inspection performed in accordance with this requirement, prior to further flight, replace the tie bolt, nut, and washer, in accordance with the BFGoodrich component maintenance manual, with new parts having the same P/N's.

Note 2: After the maintenance program has been revised to include the procedures specified in paragraph (b)(2)(i) or (b)(2)(ii) of this AD, operators are not required to subsequently record AD compliance each time the replacement or inspection is performed.

(c) If any MLG wheel assembly manufactured by BFGoodrich Aerospace and having P/N 3-1398-1, 3-1439-2, or 3-1439-3 is installed on the airplane: Except as provided by paragraph (d) of this AD, within 2 years after the effective date of this AD, modify any BFGoodrich Aerospace wheel assembly, having P/N 3-1398-1, 3- 1439-2, or 3-1439-3; by replacing all existing tie bolts, nuts, and washers, with new, improved parts; and by converting the P/N of the MLG wheel assembly to 3-1398-2 (for BFGoodrich wheel assemblies having the old P/N 3-1398-1), 3-1439-5 (for BFGoodrich wheel assemblies having the old P/N 3-1439-2), or 3-1439-6 (for BFGoodrich wheel assemblies having the old P/N 3-1439-3), as applicable; in accordance with BFGoodrich Aerospace Service Bulletin 3-1439-32-13, or BFGoodrich Aerospace Service Bulletin 3-1398-32-16, both dated August 20, 1993, as applicable. Such modification constitutes terminating action for the requirements of this AD, and the FAA- approved maintenance program procedures specified by paragraph (b)(2) of this AD may be removed following accomplishment of the requirements of this paragraph. (d) Airplanes on which the modification required by paragraph (c) of this AD is accomplished within the compliance time specified in paragraph (b) of this AD are not required to accomplish the actions required by paragraph (b). (e) An 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 Aircraft Certification Office (ACO), FAA, Transport Airplane Directorate. 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.

Note 3: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Seattle ACO.

(f) Special 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.

Issued in Renton, Washington, on November 10, 1998. Darrell M. Pederson, Acting Manager, Transport Airplane Directorate, Aircraft Certification Service. [FR Doc. 98-30767 Filed 11-17-98; 8:45 am] BILLING CODE 4910-13-P