RULES and REGULATIONS

DEPARTMENT OF TRANSPORTATION

14 CFR Part 39

(Docket No. 92-NM-230-AD; Amdt. 39-8594; AD 93-11-02)

Airworthiness Directives; Boeing Model 707 and 720 Series Airplanes

Friday, June 4, 1993

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final rule.

SUMMARY: This amendment supersedes an existing airworthiness directive (AD), applicable to certain Boeing 

Model 707 and 720 series airplanes, that currently requires repetitive inspections to detect cracks in the inboard nacelle strut midspar fittings, and replacement, if necessary. This amendment adds a requirement to replace the fittings 

with improved fittings; when accomplished, such replacement constitutes terminating action for the repetitive inspections. This amendment is prompted by the development of improved fittings that will correct the fatigue cracking 

problem addressed in the existing AD. The actions specified by this AD are intended to prevent separation of an 

inboard engine from an airplane.

DATES: Effective July 6, 1993.

The incorporation by reference of certain publications listed in the regulations was approved previously by the Director of the Federal Register as of October 8, 1992 (57 FR 43890, September 23, 1992).

ADDRESSES: The service information referenced in this AD may be obtained from Boeing Commercial Airplane 

Group, P.O. Box 3707, Seattle, Washington 98124-2207. This information may be examined at the Federal Aviation 

Administration (FAA), Transport Airplane Directorate, Rules Docket, 1601 Lind Avenue, SW., Renton, Washington; 

or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.

FOR FURTHER INFORMATION CONTACT: Stanton R. Wood, Aerospace Engineer, Airframe Branch, 

ANM-120S, FAA, Transport Airplane Directorate, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., 

Renton, Washington 98055-4056; telephone (206) 227-2772; fax (206) 227-1181.

SUPPLEMENTARY INFORMATION: A proposal to amend part 39 of the Federal Aviation Regulations by superseding AD 92-19-15, Amendment 39-8373 (57 FR 43890, September 23, 1992), which is applicable to certain Boeing 

Model 707 and 720 series airplanes, was published in the Federal Register on January 27, 1993 (58 FR 6198). The 
action proposed to require repetitive inspections to detect cracks in the inboard nacelle strut midspar fittings, and 

replacement, if necessary; and replacement of the fittings with improved fittings, which, when installed, would constitute terminating action for the repetitive inspections.

Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received.

One commenter supports the proposed rule.

One commenter, Boeing Commercial Airplane Group, requests that the FAA revise the applicability of the proposed 

rule to exclude airplanes that are not modified with hush kits. Boeing states that accomplishment of the structural 

modifications required by AD 91-07-19 (56 FR 13073, March 29, 1991) and the repetitive inspections required by AD

92-19-15 will maintain the structural integrity of the subject parts on airplanes that are not modified with hush kits; 

therefore, those airplanes should not be included in the applicability of the proposed rule.

The FAA does not concur with the commenter's request to exclude airplanes from the applicability of the rule that are 

not modified with hush kits. While AD 91-07-19 requires certain structural modifications of the affected airplanes, 

those modifications are not required until after an airplane reaches its design life goal. The FAA finds that the affected 

airplanes must be modified prior to reaching the design life goal.

In addition, the FAA has determined that long term continued operational safety will be better assured by actual 

replacement of the midspar fittings on these airplanes with new, improved fittings to remove the source of the problem, rather than by repetitive inspections, as required by AD 92-19-15. These long term inspections 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 repetitive inspections, has led the FAA to consider placing 

less emphasis on special procedures and more emphasis on design improvements. The replacement requirement of this 

AD is in consonance with these considerations.

Boeing also requests that the FAA revise the proposed rule to require repetitive inspections of airplanes modified with 

hush kits following accomplishment of the proposed terminating action. Boeing states that a fatigue analysis of the 

failed parts was conducted to establish the integrity of the replacement part. Boeing adds that the analysis was based 

on the history of part failures only on Boeing Model 707 and 720 series airplanes designed and built to the original 

certification basis. Results of that fatigue analysis revealed that a significant portion (over 33 percent) of the fatigue 

damage occurred due to flight conditions involving lateral loading. Boeing indicates that any nacelle configuration 

that increases inlet length and area produces higher loading, particularly lateral loading, which is the case with the 

airplanes modified with hush kits.

Boeing emphasizes that the FAA recognized the differences in fatigue loading and crack growth rates on the nacelle 

fittings between baseline airplanes and airplanes modified with hush kits by requiring shorter inspection intervals in 

AD 92-19-15 for the latter group. Consequently, Boeing questions the FAA's reasoning in not recognizing the differences in the fatigue life of the replacement fittings between these two nacelle configurations in regard to the proposed terminating action. Boeing concludes that the FAA is taking an unnecessary risk in allowing replacement of the 
midspar fittings to constitute terminating action for airplanes modified with hush kits.

The FAA does not concur with the commenter's request to revise the proposed rule to require post-modification inspections of airplanes modified with hush kits. The FAA finds that accomplishment of the actions required by this 

final rule addresses adequately the identified fatigue cracking problem in the area of the midspar fittings. Currently, 

the FAA is unaware of any service difficulties associated with the installation of the new improved fittings on these 

airplanes. In addition, AD 85-12-01 (50 FR 26690, June 28, 1985) requires periodic inspections of this area on airplanes that have been modified with hush kits, as well as those that have not been modified with hush kits, which 

would detect any cracking that may develop subsequently in the midspar fittings. If the FAA receives reports indicating that cracks have been detected in the new improved fittings, further rulemaking may be considered to address 

that service difficulty.

After careful review of the available data, including the comments noted above, the FAA has determined that air safety 

and the public interest require the adoption of the rule as proposed.

There are approximately 187 Model 707 and 720 series airplanes of the affected design in the worldwide fleet. The 

FAA estimates that 53 airplanes of U.S. registry will be affected by this AD, that it will take approximately 260 work 

hours per airplane to accomplish the required actions, and that the average labor rate is $55 per work hour. Required 

parts will cost approximately $7,800 per airplane. Based on these figures, the total cost impact of the AD on U.S. 

operators is estimated to be $1,171,300, or $22,100 per airplane. This total cost figure assumes that no operator has yet 

accomplished the requirements of this AD.

The 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.

For the reasons discussed above, I certify that this action (1) is not a “major rule” under Executive Order 12291; (2) is 

not a “significant rule” under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and (3) 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 final evaluation has been prepared for this action and it is contained in the 

Rules Docket. A copy of it may be obtained from the Rules Docket at the location provided under the caption 

“ADDRESSES.”

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.

Adoption of the Amendment

Accordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration 

amends 14 CFR part 39 of the Federal Aviation Regulations as follows:
PART 39—AIRWORTHINESS DIRECTIVES1. The authority citation for part 39 continues to read as follows:

Authority: 49 U.S.C. App. 1354(a), 1421 and 1423; 49 U.S.C. 106(g); and 14 CFR 11.89.

14 CFR § 39.13

§39.13 (Amended)

14 CFR § 39.13

2. Section 39.13 is amended by removing amendment 39-8373 (57 FR 43890, September 23, 1992), and by adding a 

new airworthiness directive (AD), amendment 39-8594, to read as follows:

93-11-02 Boeing: Amendment 39-8594.Docket 92-NM-230-AD.Supersedes AD 92-19-15, Amendment 39-8373.

Applicability: Model 707 and 720 series airplanes; line numbers 1 through 906, inclusive; including Model 707 and 

720 series airplanes that have been modified in accordance with Supplemental Type Certificates (STC's) SA2699NM, 

SA3595NM, or SA4015NM; certificated in any category.

Compliance: Required as indicated, unless accomplished previously.

To prevent separation of an inboard engine from an airplane, accomplish the following:

(a) Perform a close visual inspection of the midspar fittings to detect cracks in accordance with Boeing Service Bulletin 3183, Revision 4, dated July 8, 1992, unless accomplished previously in accordance with Boeing Service Bulletin 

3183, Revision 2, dated January 28, 1988, at the time specified in paragraph (a)(1), (a)(2), (a)(3), or (a)(4), of this AD, 

as applicable.

(1) For Model 707-300B/C series airplanes modified by installation of a “hush kit” in accordance with Supplemental 

Type Certificate (STC) SA2699NM, at the later of the times specified in paragraph (a)(1)(i) or (a)(1)(ii) of this AD:

(i) Prior to the accumulation of 12,000 flight hours; or

(ii) Within 650 flight hours or 250 flight cycles after October 8, 1992 (the effective date of AD 92-19-15, Amendment 

39-8373), whichever occurs first.

(2) For Model 707-100B and 720B series airplanes modified by installation of a “hush kit” in accordance with STC 

SA3595NM or SA4015NM, at the later of the times specified in paragraph (a)(2)(i) or (a)(2)(ii) of this AD:

(i) Prior to the accumulation of 12,000 flight hours; or
(ii) Within 650 flight hours or 325 flight cycles after October 8, 1992 (the effective date of AD 92-19-15, Amendment 

39-8373), whichever occurs first.

(3) For Model 707-300, 707-400, and 707-300B/C series airplanes, at the later of the times specified in paragraph 

(a)(3)(i) or (a)(3)(ii) of this AD:

(i) Prior to the accumulation of 12,000 flight hours; or

(ii) Within 1,100 flight hours or 400 flight cycles after October 8, 1992 (the effective date of AD 92-19-15, 

Amendment 39-8373), whichever occurs first.

(4) For Model 707-100, 707-100B, 707-200, 720 and 720B series airplanes, at the later of the times specified in 

paragraph (a)(4)(i) or (a)(4)(ii) of this AD:

(i) Prior to the accumulation of 12,000 flight hours; or

(ii) Within 1,100 flight hours or 550 flight cycles after October 8, 1992 (the effective date of AD 92-19-15, 

Amendment 39-8373), whichever occurs first.

(b) Repeat the inspection required by paragraph (a) of this AD thereafter at the intervals specified in paragraph (b)(1), 

(b)(2), (b)(3), or (b)(4) of this AD, as applicable, in accordance with Boeing Service Bulletin 3183, Revision 4, dated 

July 8, 1992.

(1) For Model 707-300B/C series airplanes modified by installation of a “hush kit” in accordance with STC 

SA2699NM: At intervals not to exceed 750 flight hours or 300 flight cycles, whichever occurs first.

(2) For Model 707-100B and 720B series airplanes modified by installation of a “hush kit” in accordance with STC 

SA3595NM or SA4015NM: At intervals not to exceed 750 flight hours or 375 flight cycles, whichever occurs first.

(3) For Model 707-300, -400, and -300B/C series airplanes: At intervals not to exceed 1,200 flight hours or 450 flight 

cycles, whichever occurs first.

(4) For Model 707-100, 707-100B, 707-200, 720, and 720B series airplanes: At intervals not to exceed 1,200 flight 

hours or 600 flight cycles, whichever occurs first.

(c) If any cracks are found in the midspar support fittings as a result of the inspections required by paragraph (a) or (b) 

of this AD, prior to further flight: Conduct an eddy current inspection, replace both midspar support fittings, and 

modify or repair all other structure specified in the service bulletin, in accordance with Boeing Service Bulletin 3183, 

Revision 1, dated May 13, 1977, or Revision 2, dated January 28, 1988. If a currently-installed fitting is replaced by a 

new fitting of the same part number, that new fitting must be inspected in accordance with the compliance times and 
procedures required by paragraphs (a) and (b) of this AD.

(d) Replace the midspar fittings with new, improved fittings, in accordance with Boeing Service Bulletin 3183, Revision 2, dated January 28, 1988, at the later of the times specified in paragraph (d)(1) or (d)(2) of this AD.

(1) Prior to the accumulation of 12,000 flight hours; or

(2) Within 4 years or 4,800 flight cycles after the effective date of this AD, whichever occurs first.

(e) Replacement of the midspar fittings with new, improved fittings, in accordance with Boeing Service Bulletin 3183, 

Revision 2, dated January 28, 1988, constitutes terminating action for the inspections required by this AD.

(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 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: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may 

be obtained from the Seattle ACO.

(g) Special flight permits may be issued in accordance with FAR 21.197 and 21.199 to operate the airplane to a location where the requirements of this AD can be accomplished.

(h) The inspection, replacement, modification, and repair shall be done in accordance with Boeing Service Bulletin 

3183, Revision 1, dated May 13, 1977, or Boeing Service Bulletin 3183, Revision 2, dated January 28, 1988, or 

Boeing Service Bulletin 3183, Revision 4, dated July 8, 1992. This incorporation by reference was approved previously by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51 as of October 8, 

1992 (57 FR 43890, September 23, 1992). 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.

(i) This amendment becomes effective on July 6, 1993.

Issued in Renton, Washington, on May 27, 1993.

Bill R. Boxwell,

Acting Manager, Transport Airplane Directorate, Aircraft Certification Service.

58 FR 31650-01, 1993