01-10729. Airworthiness Directives; Boeing Model 737 Series Airplanes

AGENCY:

Federal Aviation Administration, DOT.

ACTION:

Notice of proposed rulemaking (NPRM).

SUMMARY:

This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Boeing Model 737 series airplanes. This proposal would require initial and repetitive inspections of certain areas of the wing spars to detect cracking or corrosion; and follow-on corrective actions and repair, if necessary. This proposal is prompted by reports of cracks and corrosion in the upper chord of the front and rear spars of the wing and reports of cracks propagating from previously repaired areas. The actions specified in the proposed AD are intended to detect and correct such cracking or corrosion of the upper and lower chords of the wing spars, which could result in reduced structural integrity of the wing.

DATES:

Comments must be received by June 15, 2001.

ADDRESSES:

Submit comments in triplicate to the Federal Aviation Administration (FAA), ANM-114, Attention: Rules Docket No. 99-NM-367-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056. Comments may be inspected at this location between 9:00 a.m. and 3:00 p.m., Monday through Friday, except Federal holidays. Comments may be submitted via fax to [phone]. Comments may also be sent via the Internet using the following address: [e-mail]. Comments sent via fax or the Internet must contain “Docket No. 99-NM-367-AD” in the subject line and need not be submitted in triplicate. Comments sent via the Internet as attached electronic files must be formatted in Microsoft Word 97 for Windows or ASCII text.

The service information referenced in the proposed rule may be obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, Washington 98124-2207. This information may be examined at the FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington.

FOR FURTHER INFORMATION CONTACT:

Nenita Odesa, Aerospace Engineer, Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone [phone]; fax [phone].

SUPPLEMENTARY INFORMATION:

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 action 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 action must submit a self-addressed, stamped postcard on which the following statement is made: “Comments to Docket Number 99-NM-367-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. 99-NM-367-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056.

Discussion

The FAA has received numerous reports of cracking and corrosion in the upper chord of both the front and rear spars of the wing. Cracks were reported by 18 operators in the upper and rear spar chord on 38 airplanes. In addition, 5 operators reported cracks propagating from repairs that were accomplished previously on 6 airplanes per Boeing Service Bulletin 737-57-1067, Revision 3, dated May 24, 1990, or earlier revisions. Findings indicate that 2 of those cracks were found in the front spar upper chord, and 4 cracks in the rear spar upper chord. Corrosion also was found in the external surfaces of the upper and lower chords of the front and rear spars on 64 airplanes. Such cracking and corrosion, if not corrected, could result in reduced structural integrity of the wing.

Explanation of Relevant Service Information

The FAA has reviewed and approved Boeing Service Bulletin 737-57-1067, Revision 4, dated November 7, 1991. Revision 4 adds initial and repetitive inspections for airplanes on which a previous repair to the upper chord of the front or rear spar was made per Revision 3, or earlier revisions, of the service bulletin. Revision 4 includes the following procedures:

• An initial (detailed) visual inspection for cracks of the external surfaces of the upper and lower chords of the front spar outboard of the engine nacelle, upper and lower chords of the rear spar, and the lower chord of the front spar inboard of the engine nacelle.
• An initial eddy current inspection for cracks of the external surfaces of the vertical legs of the upper chords of the front and rear spars.
• Repetitive close (detailed) visual inspections for cracks and corrosion of the external surfaces of the upper and lower chords of the front and rear spars.
• Repetitive eddy current inspections for cracks of the external surfaces of the vertical legs of the upper chords of the front and rear spars.
• Follow-on corrective actions that include cleaning the exposed front and rear spar cavities, removing corrosion by blending out the damaged surface areas, and applying a corrosion-inhibiting compound to the accessible areas of the upper and lower chords of the front and rear spars.
• Crack and corrosion repair that includes removing corrosion; inspecting the rework area for cracks, and repairing any cracks found in the specific chord surfaces of the front and rear spars, which includes installing a nesting angle if necessary and refinishing the blend out area; and applying a corrosion-inhibiting compound.

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 accomplishment of the actions specified in the service described previously, except as discussed below.

Differences Between Proposed Rule and Service Bulletin

Operators should note that this proposed AD differs from the service bulletin in that it would NOT allow the following actions:

1. Repair of certain damage (that exceeds the limits specified in the service bulletin) to be accomplished by contacting Boeing for repair instructions; rather, it would require that such repairs be accomplished per a method approved by the FAA.

2. Flight with a horizontal crack in the upper chords of the front or rear spars; rather, it would require repair per a method approved by the FAA.

3. “Close visual inspections”; rather, it would require “detailed visual inspections.” A note has been added following paragraph (a) of this proposed AD to clarify this.

Operators also should note that Part II of the service bulletin does not include repair procedures if a horizontal crack is found in the upper chords of the front or rear spars. However, paragraph (d)(3) of this proposed AD requires repair of such cracking per a method approved by the FAA, as cited in paragraph (f) of this AD.

Operators also should note that the service bulletin specifies initial detailed visual and eddy current inspections “upon the accumulation of 20 years airframe age or within 1 year after receipt of Revision 3 of the service bulletin.” However, since all airplanes in the applicability of this AD are over 20 years old, the FAA has determined that it is unnecessary to include the specification for the airframe age in this proposed AD.

Clarification of Action Required for Previous Repair

The Accomplishment Instructions of the previously referenced service bulletin do not specifically include procedures for repetitive eddy current inspections for airplanes on which a previous repair to the upper chord of the front or rear spar was made per Boeing Service Bulletin 737-57-1067, Revision 3, or earlier revisions. However, such action is specified in the service bulletin in an “Action” paragraph of the Summary, and in paragraph C., “Description.” Paragraphs (d)(3), (d)(4)(ii), and (e) of this AD specify repair per a method approved by the FAA, which is cited in paragraph (f) of this AD.

Cost Impact

There are approximately 168 Boeing Model 737 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 45 airplanes of U.S. registry would be affected by this proposed AD.

The FAA estimates that it would take approximately 30 work hours per airplane to do the initial detailed visual and eddy current inspections, and that the average labor rate is $60 per work hour. Based on these figures, the cost impact of the initial inspections on U.S. operators is estimated to be $81,000, or $1,800 per airplane.

The FAA estimates that it would take approximately 30 work hours per airplane to do the repetitive inspections, and that the average labor rate is $60 per work hour. Based on these figures, the cost impact of the proposed AD on U.S. operators is estimated to be $81,000, or $1,800 per airplane, per inspection cycle.

The cost impact figures discussed above are based on assumptions that no operator has yet accomplished any of the proposed requirements of this AD action, and that no operator would do those actions in the future if this AD were not adopted.

Regulatory Impact

The regulations proposed herein would not have a substantial direct effect 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, it is determined that this proposal would not have federalism implications under Executive Order 13132.

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.

§ 39.13

[Amended]

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

Boeing: Docket 99-NM-367-AD.

Applicability: Model 737 series airplanes, line numbers 1 through 310 inclusive, and 323; 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 per paragraph (g) 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 detect and correct cracking or corrosion of the upper and lower chords of the front and rear spars of the wing, which could result in reduced structural integrity of the wing, do the following:

Initial Detailed Visual and Eddy Current Inspections (Part I)

(a) Within 12 months after the effective date of this AD: Do an initial detailed visual inspection to detect cracking or corrosion of the upper and lower chords of the front and rear spars, and an eddy current inspection to detect cracking of the vertical legs of the upper chords of the front and rear spars, per Part I of the Accomplishment Instructions of Boeing Service Bulletin 737-57-1067, Revision 4, dated November 7, 1991. Before further flight following the inspections, do the follow-on corrective actions required by paragraph (d) of this AD.

Note 2:

For the purposes of this AD, a detailed visual inspection is defined as: “An intensive examination of a specific structural area, system, installation, or assembly to detect damage, failure, or irregularity. Available lighting is normally supplemented with a direct source of good lighting at an intensity deemed appropriate by the inspector. Inspection aids such as mirrors, magnifying lenses, etc. may be used. Surface cleaning and elaborate access procedures may be required.”

Repetitive Detailed Visual and Eddy Current Inspections (Part II)

(b) Repeat the initial detailed visual inspection required by paragraph (a) of this AD at intervals not to exceed 12 months per Part II of the Accomplishment Instructions of Boeing Service Bulletin 737-57-1067, Revision 4, dated November 7, 1991. Before further flight following the inspection, do the follow-on corrective actions required by paragraph (d) of this AD.

(c) Repeat the initial eddy current inspection required by paragraph (a) of this AD at intervals not to exceed 48 months per Part II of the Accomplishment Instructions of Boeing Service Bulletin 737-57-1067, Revision 4, dated November 7, 1991. Before further flight following the inspection, do the follow-on corrective actions required by paragraph (d) of this AD.

Follow-on Corrective Actions (Parts I, II, and III)

(d) Do the follow-on corrective actions (including cleaning spar cavities, removing corrosion, and applying corrosion-inhibiting compound) required by paragraphs (d)(1), (d)(2), (d)(3), and (d)(4) of this AD, as applicable.

(1) If no cracking or corrosion is found, apply a corrosion-inhibiting compound to the accessible areas of the upper and lower chords of both the front and rear spars per Part I or Part II of the Accomplishment Instructions of Boeing Service Bulletin 737-57-1067, Revision 4, dated November 7, 1991, as applicable.

(2) If any corrosion is found, repair per Part III of the Accomplishment Instructions of Boeing Service Bulletin 737-57-1067, Revision 4, dated November 7, 1991.

(3) If a horizontal crack is found in the upper chords of the front or rear spars, repair per paragraph (f) of this AD.

(4) If any cracking is found other than that identified in paragraph (d)(3) of this AD, repair per paragraph (d)(4)(i) or (d)(4)(ii) of this AD, as applicable.

(i) If damage of the chords of the front or rear spar is within the limits specified in the service bulletin, before further flight, repair per Part III of the Accomplishment Instructions of Boeing Service Bulletin 737-57-1067, Revision 4, dated November 7, 1991.

(ii) If damage of the chords of the front or rear spar exceeds the limits specified in the service bulletin, before further flight, repair per paragraph (f) of this AD.

Initial and Repetitive Eddy Current Inspections of Previous Repairs

(e) For airplanes on which a previous repair to the upper chord of the front or rear spar was made per Boeing Service Bulletin 737-57-1067, Revision 3, dated May 24, 1990, or earlier revisions: Within 12 months after the effective date of this AD, do an eddy current inspection of the repair area to detect cracking per a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA. Repeat this inspection thereafter at intervals not to exceed 12 months. If any discrepancy is found, before further flight, repair per paragraph (f) of this AD. For a repair method to be approved by the Manager, SACO, as required by this paragraph, the approval letter must specifically reference this AD.

Repair

(f) Repair (including removing corrosion; inspecting the rework area for cracks; refinishing the blend-out area; installing a nesting angle repair; and applying chemical film treatment, primer, sealant, and corrosion-inhibiting compound) any discrepancy specified in paragraphs (d)(3), (d)(4)(ii), and (e) of this AD, per a method approved by the Manager, Seattle ACO; or per data meeting the type certification basis of the airplane approved by a Boeing Designated Engineering Representative who has been authorized by the Manager, Seattle ACO, to make such findings. For a repair method to be approved by the Manager, Seattle ACO, as required by this paragraph, the approval letter must specifically reference this AD.

Alternative Methods of Compliance

(g) 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 ACO, FAA. 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.

Special Flight Permit

(h) Special flight permits may be issued per §§ 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 April 24, 2001.

Donald L. Riggin,

Acting Manager, Transport Airplane Directorate, Aircraft Certification Service.