00-20248. Airworthiness Directives; Boeing Model 737 Series Airplanes

AGENCY:

Federal Aviation Administration, DOT.

ACTION:

Supplemental notice of proposed rulemaking; reopening of comment period.

SUMMARY:

This document revises an earlier proposed airworthiness directive (AD), applicable to certain Boeing Model 737 series airplanes, that would have required inspections of certain bonded skin panels to detect delamination of the skin doublers (tear straps) from the skin panels; and follow-on corrective actions, if necessary. That proposal was prompted by reports that certain skin doublers were delaminated from their skin panels due to improper processing of certain skin panels. This new action would revise the proposed rule by referencing new service information that incorporates new inspection procedures and corrective actions, and recommends new compliance times. This new action also would remove airplanes from the applicability. The actions specified by this new proposed AD are intended to prevent skin doublers from delaminating from their skin panels, which could result in fatigue cracks in the skin doublers and skin panels, and consequent rapid decompression of the airplane.

DATES:

Comments must be received by September 5, 2000.

ADDRESSES:

Submit comments in triplicate to the Federal Aviation Administration (FAA), Transport Airplane Directorate, ANM-114, Attention: Rules Docket No. 98-NM-11-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. 98-NM-11-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, Transport Airplane Directorate, 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 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-11-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-11-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056.

Discussion

A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to add an airworthiness directive (AD), applicable to certain Boeing Model 737 series airplanes, was published as a notice of proposed rulemaking (NPRM) in the Federal Register on January 11, 1999 (64 FR 1549). That NPRM would have required inspections of certain bonded skin panel assemblies to detect delamination of the skin doublers (tear straps) from the skin panels; and follow-on corrective actions, if necessary. That NPRM was prompted by reports indicating that certain skin doublers were delaminated from their skin panels due to improper processing of certain skin panels. That condition, if not corrected, could result in fatigue cracks in the skin doublers and skin panels, and consequent rapid decompression of the airplane.

Explanation of New Relevant Service Information

Since the issuance of the NPRM, the FAA has reviewed and approved Boeing Service Bulletin 737-53-1179, Revision 1, dated September 30, 1999. (Boeing Service Bulletin 737-53-1179, dated June 22, 1995, was referenced as the appropriate source of service information in the NPRM.) Revision 1 of the service bulletin adds airplanes having line numbers (L/N) 2726 through 2869 inclusive to the effectivity listing. Among other things, Revision 1 of the service bulletin also introduces new inspection procedures and corrective actions, and recommends extension of certain compliance times and repetitive inspection intervals.

The service bulletin describes the area to be inspected as skin panel assemblies from 259 to 1016, on both sides of the airplane, except between stringers 10 and 14 between body stations 540 and 727. The procedures described in Revision 1 include repetitive external visual inspections to detect cracks or corrosion. Also, Revision 1 describes procedures for repetitive detailed visual inspections to detect cracks or corrosion at stringer 17, the window belts, and the lap and butt joints.

Revision 1 of the service bulletin classifies affected skin panels as either “Zone A” or “Zone B,” and describes procedures for a one-time internal inspection of “Zone A” skin panels, and a one-time internal or external ultrasonic inspection of “Zone B” skin panels, to detect cracks, corrosion, and delamination. The internal inspection includes a close visual inspection to detect delamination or corrosion and a visual inspection using a separation tool to detect weak or delaminated bonds. Accomplishment of the internal inspection eliminates the need for the repetitive external inspections.

The service bulletin describes procedures for corrective actions if cracking, corrosion, or delamination is found. Corrective actions include replacement of the affected skin panel with a new or serviceable skin panel manufactured by Boeing after a certain date. In lieu of replacement of the skin panel, the service bulletin also describes procedures for various follow-on inspections of the affected skin panel (including internal visual, low and high frequency eddy current, and ultrasonic inspections), and repair of cracks, corrosion, or delamination, as applicable.

Accomplishment of the actions specified in Revision 1 of the service bulletin is intended to adequately address the unsafe condition.

Comments

Due consideration has been given to the comments received in response to the NPRM. Certain comments have resulted in changes to the proposal that are reflected in this supplemental NPRM. The FAA finds that certain other comments received in response to the NPRM are no longer relevant to this proposal because of the introduction of Revision 1 of the service bulletin. Certain other comments that are still relevant but have not resulted in any change to the proposal will be addressed in the final rule, along with any additional comments received in response to this supplemental NPRM.

Request To Revise Compliance Time and Repetitive Interval for External Inspections

One commenter, the manufacturer, requests that the FAA revise paragraph (a)(2) of the NPRM to eliminate the statement of the grace period in calendar time (18 months after the effective date of this AD). The commenter states that disbonding is a function of flight cycles rather than calendar time, and therefore, a compliance time based on calendar time is not appropriate.

The FAA concurs with the commenter's request that the compliance threshold based on calendar time should be eliminated, though not for the reason stated by the commenter. The external visual inspections to which the commenter refers are intended to detect fatigue cracks, not disbonding, and growth of fatigue cracks is a function of flight cycles, not calendar time. Therefore, grace periods in calendar time have not been included for the actions specified in this supplemental NPRM.

Request To Clarify Affected Replacement Skin Panels

One commenter requests that the FAA revise the proposal to make the actions specified in the proposed AD applicable only to replacement skin panels fabricated by Boeing. The commenter states that many operators have fabricated their own replacement skin panels that are not affected by the proposed AD.

The FAA concurs with the commenter's request. Skin panels fabricated by operators are not affected by the proposed AD. Therefore, paragraph (b) of this supplemental NPRM specifies that the actions required by that paragraph apply only to affected airplanes “on which any bonded skin panel was replaced with a new or serviceable, Boeing-built, bonded skin panel prior to October 1, 1997.” In addition, the applicability statement of this supplemental NPRM has been revised accordingly.

Request To Correct Typographical Error

Two commenters request that the FAA revise paragraph (e) of the NPRM to correct a typographical error. Paragraph (e) of the NPRM states that corrective action for any detected crack was specified in paragraphs (b)(1) and (b)(2) of the proposed AD. The commenters state that the reference for corrective actions should have been to paragraphs (e)(1) and (e)(2) of the proposed AD. The FAA concurs with the intent of the commenter's request. Paragraphs (e), (e)(1), and (e)(2) of the NPRM are restated as paragraphs (c), (c)(1), and (c)(2) of the supplemental NPRM, and these paragraphs have been revised with correct references.

Explanation of Applicability

The applicability statement of the original NPRM includes Model 737-100, -200, -200C, -300, -400, and -500 series airplanes; having line numbers 1 through 3072 inclusive. The FAA set this applicability in consideration of the possibility that any airplane delivered prior to October 1, 1997, might have an improperly processed skin panel installed. The FAA considered line number 3072 to correspond to a delivery date of October 1, 1997. Since the issuance of the NPRM, the FAA has determined that line number 2947 corresponds to a delivery date of October 1, 1997. Therefore, the applicability statement of this supplemental NPRM includes certain Model 737 series airplanes having line numbers 1 through 2947 inclusive.

Explanation of New Requirements of Supplemental NPRM

Since an unsafe condition has been identified that is likely to exist or develop on other products of this same type design, this supplemental NPRM would require accomplishment of the actions specified in Boeing Service Bulletin 737-53-1179, Revision 1, described previously, except as discussed below.

Differences Between Supplemental NPRM and Service Bulletin

Operators should note that Table 3 of Figure 2 of the service bulletin specifies actions for “airplanes up to [line number] 2869 with skins replaced with Boeing panels fabricated prior to April 1, 1997.” However, paragraph (b) of this supplemental NPRM applies to “airplanes having [line numbers] 1 through 2947 inclusive, on which any bonded skin panel was replaced with a new or serviceable, Boeing-built, bonded skin panel prior to October 1, 1997.” An operator may not be able to accurately determine the date of manufacture of a replacement skin panel, but the operator would be able to determine the date of installation of a replacement panel. The FAA finds that making this proposed rule applicable to replacement panels installed prior to October 1, 1997, would meet the intent of the service bulletin by ensuring that this supplemental NPRM would apply to replacement skin panels manufactured by Boeing prior to April 1, 1997.

Operators also should note that, although the service bulletin specifies that the manufacturer may be contacted for disposition of certain repair conditions, this supplemental NPRM would require the repair of those conditions to be accomplished in accordance with a method approved by the FAA; or in accordance with data meeting the type certification basis of the airplane approved by a Boeing Company Designated Engineering Representative who has been authorized by the Manager, Seattle Aircraft Certification Office, to make such findings.

Additionally, the service bulletin specifies that certain actions may be accomplished in accordance with “an equivalent” procedure. However, this supplemental NPRM would require that those actions be accomplished in accordance with the procedures specified in Part 6, Subject 51-00-00, Figure 4, of the 737 Nondestructive Test Manual. An “equivalent” procedure may be used only if approved as an alternative method of compliance in accordance with the provisions of paragraph (f) of the proposed AD.

Conclusion

Since these changes expand the scope of the originally proposed rule, it is necessary to reopen the comment period to provide additional opportunity for public comment.

Cost Impact

There are approximately 2,083 airplanes of the affected design in the worldwide fleet. The FAA estimates that 863 airplanes of U.S. registry would be affected by this proposed AD.

It would take approximately 10 work hours per airplane to accomplish the proposed external general visual and detailed visual inspection, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of this inspection proposed by this AD on U.S. operators is estimated to be $517,800, or $600 per airplane, per inspection cycle.

It would take approximately 360 work hours per airplane to accomplish the proposed internal visual and ultrasonic inspection, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of this inspection proposed by this AD on U.S. operators is estimated to be $18,640,800, or $21,600 per airplane.

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 accomplish those actions in the future if this AD were not adopted. The cost impact figures discussed in AD rulemaking actions represent only the time necessary to perform the specific actions actually required by the AD. These figures typically do not include incidental costs, such as the time required to gain access and close up, planning time, or time necessitated by other administrative actions.

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 98-NM-11-AD.

Applicability: Model 737-100, -200, -200C, -300, -400, and -500 series airplanes; line numbers (L/N) 1 through 2947 inclusive; certified 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 (f) 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 delamination of the skin doublers (tear straps) from the skin panels, which could result in fatigue cracks in the skin doublers and the skin panels, and consequent rapid decompression of the airplane, accomplish the following:

Initial and Repetitive Inspections (L/N 611 through 2725 inclusive)

(a) For airplanes having L/N 611 through 2725 inclusive: Accomplish the actions required by paragraphs (a)(1) and (a)(2) of this AD on any bonded skin panel assembly that has NOT been replaced with any new or serviceable bonded skin panel assembly, in accordance with Boeing Service Bulletin 737-53-1179, Revision 1, dated September 30, 1999.

Note 2:

For the purposes of this AD, bonded skin panels consist of skin doublers (tear straps) that are bonded to skin panels located above stringer S-26 from body station (BS) 259 to BS 1016 on both sides of the airplane.

Note 3:

If the skin panel is solid with no doublers (tear straps) bonded to it, the inspections required by this AD are not necessary for that skin panel.

(1) Prior to the accumulation of 20,000 total flight cycles, or within 5,000 flight cycles after the effective date of this AD, whichever occurs later, accomplish paragraphs (a)(1)(i) and (a)(1)(ii) of this AD, in accordance with the Accomplishment Instructions of the service bulletin. Repeat the inspections thereafter at intervals not to exceed 5,000 flight cycles, until accomplishment of paragraph (a)(2) of this AD.

(i) Perform an external general visual inspection of all affected areas NOT specified in paragraph (a)(1)(ii) to detect cracks or corrosion of bonded skin panels.

(ii) Perform a detailed visual inspection to detect cracks or corrosion of bonded skin panels at stringer 17, window belts, lap joints, and butt splice joints.

Note 4:

For the purposes of this AD, a general visual inspection is defined as: “A visual examination of an interior or exterior area, installation, or assembly to detect obvious damage, failure, or irregularity. This level of inspection is made under normally available lighting conditions such as daylight, hangar lighting, flashlight, or drop-light, and may require removal or opening of access panels or doors. Stands, ladders, or platforms may be required to gain proximity to the area being checked.”

Note 5:

For the purposes of this AD, a detailed visual inspection is defined as: “An intensive visual 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 intensity deemed appropriate by the inspector. Inspection aids such as mirror, magnifying lenses, etc., may be used. Surface cleaning and elaborate access procedures may be required.”

(2) Prior to the accumulation of 40,000 total flight cycles, but after the accumulation of 4,500 total flight cycles; or within 10,000 flight cycles after the effective date of this AD; whichever occurs later; accomplish paragraphs (a)(2)(i) and (a)(2)(ii) of this AD. Accomplishment of the requirements of paragraphs (a)(2)(i) and (a)(2)(ii) of this AD constitutes terminating action for the repetitive inspection requirement of paragraph (a)(1) of this AD.

(i) For “Zone A” areas (as defined in the “Inspection” section and Figure 2 of the service bulletin): Perform a one-time internal general visual inspection to detect cracks or corrosion of bonded skin panels, or delamination of the skin doublers from the bonded skin panels.

(ii) For “Zone B” areas (as defined in the “Inspection” section and Figure 2 of the service bulletin): Perform an internal or external ultrasonic inspection to detect cracks or corrosion of bonded skin panels, or delamination of the skin doublers from the bonded skin panels.

Note 6:

Internal inspections accomplished prior to the effective date of this AD in accordance with the original issue of Boeing Service Bulletin 737-53-1179, dated June 22, 1995, are acceptable for compliance with paragraph (a)(2) of this AD, provided that they were accomplished after the accumulation of 4,500 total flight cycles.

Note 7:

For the purposes of this AD, the one-time internal inspection includes an internal detailed visual inspection, a second internal detailed visual inspection while trying to separate the skin doublers from the skin panels, and an ultrasonic inspection.

Initial and Repetitive Inspections (L/N 1 through 2947 inclusive)

(b) For airplanes having L/N 1 through 2947 inclusive, on which any bonded skin panel was replaced with a new or serviceable, Boeing-built, bonded skin panel prior to October 1, 1997: Accomplish the actions required by paragraphs (b)(1) and (b)(2) of this AD, in accordance with Boeing Service Bulletin 737-53-1179, Revision 1, dated September 30, 1999.

(1) Within 20,000 flight cycles after replacement of the bonded skin panel, or within 5,000 flight cycles after the effective date of this AD, whichever occurs later, accomplish paragraphs (b)(1)(i) and (b)(1)(ii) of this AD, in accordance with the Accomplishment Instructions of the service bulletin. Repeat the inspections thereafter at intervals not to exceed 5,000 flight cycles, until accomplishment of paragraph (b)(2) of this AD.

(i) Perform an external general visual inspection of all affected areas NOT specified in paragraph (b)(1)(ii) to detect cracks or corrosion of bonded skin panels.

(ii) Perform a detailed visual inspection to detect cracks or corrosion of bonded skin panels at stringer 17, window belts, lap joints, and butt splice joints.

(2) Within 40,000 total flight cycles after skin panel replacement, but after the accumulation of 4,500 flight cycles after such replacement; or within 10,000 flight cycles after the effective date of this AD; whichever occurs later; accomplish paragraphs (b)(2)(i) and (b)(2)(ii) of this AD. Accomplishment of the requirements of paragraphs (b)(2)(i) and (b)(2)(ii) of this AD constitutes terminating action for the repetitive inspection requirement of paragraph (b)(1) of this AD.

(i) For “Zone A” areas (as defined in the “Inspection” section and Figure 2 of the service bulletin): Perform a one-time internal general visual inspection to detect cracks or corrosion of bonded skin panels or delamination of the skin doublers from the bonded skin panels.

(ii) For “Zone B” areas (as defined in the “Inspection” section and Figure 2 of the service bulletin): Perform an internal or external ultrasonic inspection to detect cracks or corrosion of bonded skin panels, or delamination of the skin doublers from the bonded skin panels.

Corrective Actions

(c) If any crack, corrosion, or delamination is detected during any inspection required by paragraph (a) or (b) of this AD, prior to further flight, accomplish the actions required by either paragraph (c)(1) or (c)(2) of this AD.

(1) Replace the cracked, corroded, or delaminated skin panel with a new or serviceable skin panel manufactured by Boeing on or after April 1, 1997, in accordance with Figure 2 of Boeing Service Bulletin 737-53-1179, Revision 1, dated September 30, 1999.

(2) Accomplish corrective actions (including additional inspections and repairs) in accordance with Figure 2 and the “Delamination and Crack Repair” section of Boeing Service Bulletin 737-53-1179, Revision 1, dated September 30, 1999, except as provided by paragraph (d) of this AD.

(d) Where Boeing Service Bulletin 737-53-1179, Revision 1, dated September 30, 1999, specifies that repair of a cracked or delaminated skin panel is to be accomplished in accordance with instructions received from Boeing, this AD requires that the repair be accomplished in accordance with a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA; or in accordance with data meeting the type certification basis of the airplane approved by a Boeing Company 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.

Operator's Equivalent Procedures

(e) Where Boeing Service Bulletin 737-53-1179, Revision 1, dated September 30, 1999, specifies that the actions required by this AD may be accomplished in accordance with an “equivalent” procedure, the actions must be accomplished in accordance with the chapter of the Boeing 737 Nondestructive Test Manual specified in the service bulletin.

Alternative Methods of Compliance

(f) 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. 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 8:

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

Special Flight Permits

(g) 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 August 3, 2000.

Donald L. Riggin,

Acting Manager, Transport Airplane Directorate, Aircraft Certification Service.