[Federal Register Volume 64, Number 161 (Friday, August 20, 1999)]
[Proposed Rules]
[Pages 45477-45481]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-21687]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 99-NM-81-AD]
RIN 2120-AA64


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

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: This document proposes the adoption of a new airworthiness 
directive (AD) that is applicable to all Boeing Model 737-100, -200, -
200C, -300, -400, and -500 series airplanes. This proposal would 
require repetitive inspections to detect cracking of the lower corners 
of the door frame and cross beam of the forward cargo door, and 
corrective actions, if necessary. This proposal also would require 
eventual modification of the outboard radius of

[[Page 45478]]

the lower corners of the door frame and reinforcement of the cross beam 
of the forward cargo door, which would constitute terminating action 
for the repetitive inspections. This proposal is prompted by reports 
indicating that fatigue cracks have been detected in the lower corners 
of the door frame and cross beam of the forward cargo door. The actions 
specified by the proposed AD are intended to prevent fatigue cracking 
of the lower corners of the door frame and cross beam of the forward 
cargo door, which could result in rapid depressurization of the 
airplane.

DATES: Comments must be received by October 4, 1999.

ADDRESSES: Submit comments in triplicate to the Federal Aviation 
Administration (FAA), Transport Airplane Directorate, ANM-114, 
Attention: Rules Docket No. 99-NM-81-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.
    The service information referenced in the proposed rule may be 
obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, 
Washington 98134-2207. This information may be examined at the FAA, 
Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, 
Washington.

FOR FURTHER INFORMATION CONTACT: Rick Kawaguchi, Aerospace Engineer, 
Airframe Branch, ANM-120S, FAA, Transport Airplane Directorate, Seattle 
Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, 
Washington 98055-4056; telephone (425) 227-1153; fax (425) 227-1181.

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 99-NM-81-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-81-AD, 1601 Lind Avenue, SW., Renton, Washington 
98055-4056.

Discussion

    The FAA has received several reports indicating that fatigue cracks 
have been detected in the lower corners of the door frame and cross 
beam of the forward cargo door on Boeing Model 737-100, -200, -200C, -
300, -400, and -500 series airplanes. Such fatigue cracking results 
from cabin pressurization cycles. The sizes of the cracks ranged from 
3/8 inch to 17.5 inches in length, and were found on airplanes that had 
accumulated between 13,500 and 53,100 total flight hours and between 
15,700 and 49,800 total flight cycles.
    The manufacturer subsequently redesigned the door frame of the 
forward cargo door to be less susceptible to fatigue cracking. However, 
investigation has revealed that such cracking has been detected on 
airplanes equipped with the redesigned door frame of the forward cargo 
door. Recently, two operators reported finding cracks on the lower 
corner radius of the aft door frame of the forward cargo door. The 
first operator reported finding a 10-inch crack on the aft door frame 
and an undisclosed sized crack on the cross beam on an airplane that 
had accumulated 23,000 total flight cycles. The second operator 
reported finding a 14.5-inch crack on the aft door frame and an 
undisclosed sized crack on the cross beam on an airplane that had 
accumulated 29,000 total flight cycles. Such cracking, if not detected 
and corrected, could result in rapid depressurization of the airplane.

Other Relevant Rulemaking

    The FAA previously has issued AD 90-06-02, amendment 39-6489 (55 FR 
8372, March 7, 1990), applicable to certain Boeing Model 737 series 
airplanes. AD 90-06-02 requires accomplishment of certain structural 
modifications. That amendment was prompted by reports of incidents 
involving fatigue cracking and corrosion in transport category 
airplanes that are approaching or have exceeded their design life goal. 
For airplanes that have those modifications installed, this proposed AD 
would require additional modifications of the aft lower corner of the 
door frame of the forward cargo door.
    Additionally, the FAA has issued AD 98-25-06, amendment 39-10931 
(55 FR 67769, December 9, 1998), applicable to certain Boeing Model 
737-200, -200C, -300, and -400 series airplanes. AD 98-25-06 requires 
repetitive inspections to detect cracking of the corners of the door 
frame and the cross beams of the aft cargo door, and corrective 
actions, if necessary. This proposed AD would not affect the 
requirements of that AD.

Explanation of Relevant Service Information

    The FAA has reviewed and approved Boeing 737 Nondestructive Test 
Manual, Part 6, Section 51-00-00, Figure 4 and Figure 23, which 
describes procedures for performing high frequency eddy current 
inspections.
    The FAA has also reviewed and approved Boeing Service Bulletin 737-
52-1100, Revision 2, dated March 31, 1994, which describes procedures 
for, among other things, repetitive close visual inspections to detect 
cracking of the lower corners (forward and aft) of the door frame and 
cross beam (i.e., upper and lower chord and web sections) of the 
forward cargo door, and corrective actions, if necessary. The 
corrective actions involve replacement of the damaged door frame of the 
forward cargo door with a new door frame and reinforcement modification 
of the upper chord and web sections of the cross beam of the forward 
cargo door. The corrective actions also involve installation of a cross 
beam repair (if necessary), and preventative modification of the 
outboard radius of the lower corners of the door frame. This 
modification involves installing a reinforcement angle along the full 
length of the lower corners (forward and aft) over the outboard radius 
of the lower end of the door frame. For certain airplanes, installation 
of the preventative modification of the outboard radius of the lower 
corner of the door frame and reinforcement of the cross beam of the 
forward cargo door will eliminate the need for the repetitive 
inspections.
    Accomplishment of the actions specified in the service documents is

[[Page 45479]]

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

Differences Between Proposed Rule and Service Bulletin

     Operators should note that, whereas the Boeing service 
bulletin specifies close visual inspections only, this proposed AD 
would require a high frequency eddy current inspection to detect 
cracking of the door frame and a detailed visual inspection to detect 
cracking of the cross beam of the forward cargo door. The FAA has 
determined that, because of the safety implications and consequences 
associated with such fatigue cracking, close visual inspection methods 
alone may be inadequate in detecting cracks.
     While the Boeing service bulletin does not provide 
corrective actions for cracking detected on the lower chord of the 
cross beam, this proposed AD would require that the repair of the lower 
chord of the cross beam be accomplished in accordance with a method 
approved by the FAA.
     Operators should further note that, unlike the procedures 
described in the Boeing service bulletin, this proposed AD would not 
permit the alternative of replacing the door frame of the forward cargo 
door in the event that cracks are detected on the cross beam. This 
proposed AD would require installation of a cross beam repair (if 
necessary) and preventative modification of the outboard radius of the 
lower corners (forward and aft) of the door frame, and installation of 
a reinforcement modification of the cross beam of the forward cargo 
door. Additionally, this proposed AD would require additional actions 
in the event that cracks are detected in the door frame of the forward 
cargo door. This proposed AD would require replacement of the door 
frame with a new door frame, reinforcement of the cross beam, and 
installation of the preventative modification of the outboard radius of 
the lower corners of the door frame. The FAA finds that, in view of the 
reports that cracking has occurred on the redesigned door frames, and 
because of the safety implications and consequences associated with 
such cracking, replacing the door frame without further modification 
will not safely address the unsafe condition.
     This AD also proposes to mandate, within 4 years, the 
preventative modification of the outboard radius of the lower corners 
(forward and aft) of the door frame and the reinforcement modification 
of the cross beam of the forward cargo door as described in the Boeing 
service bulletin (previously described). The modification would be 
accomplished, for certain airplanes, in accordance with the Boeing 
service bulletin and for certain other airplanes, in accordance with a 
method approved by the FAA, and would eliminate the need for the 
repetitive inspections.
    The 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. 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 continual inspections, has 
led the FAA to consider placing less emphasis on inspections and more 
emphasis on design improvements. The proposed modification requirements 
are in consonance with these conditions.
     The effectivity of the Boeing service bulletin includes 
Boeing Model 737 series airplanes having line numbers 0001 through 1231 
inclusive. This proposed AD would be applicable to Boeing Model 737 
series airplanes having line numbers 0001 through 1231 inclusive, and 
also would include Boeing Model 737 series airplanes having line 
numbers 1232 and on. The FAA is aware that a design change to the door 
frame of the forward cargo door was implemented on the production line 
at line number 1232. The FAA finds that, in view of the reports 
indicating that cracking has occurred on airplanes having later line 
numbers (1251 and 1790) with the change incorporated, the applicability 
specified in this proposed AD is appropriate.
     Further, operators should note that, although the Boeing 
service bulletin recommends that the initial inspection be performed 
within 4,500 flight cycles after an airplane has accumulated 12,000 
total flight cycles, this proposed AD would require that the initial 
inspection be performed within 1 year or 4,500 flight cycles after the 
effective date of this AD, whichever occurs later. The FAA has 
determined that the number of total flight cycles for an airplane may 
not be a good indicator of the total cycle count for the subject 
forward cargo door, because a door may have been removed from an 
airplane that had accumulated many total flight cycles and reinstalled 
on an airplane that had accumulated relatively fewer total flight 
cycles. Due to the limited ability to accurately track the total flight 
cycles of the subject forward cargo door, the initial compliance time 
specified by this proposed AD is appropriate.

Cost Impact

    There are approximately 3,100 airplanes of the affected design in 
the worldwide fleet. The FAA estimates that 1,400 airplanes of U.S. 
registry would be affected by this proposed AD, that it would take 
approximately 1 work hour per airplane to accomplish the proposed 
inspections, and that the average labor rate is $60 per work hour. 
Based on these figures, the cost impact of the inspections proposed by 
this AD on U.S. operators is estimated to be $84,000, or $60 per 
airplane, per inspection cycle.
    It would take approximately 38 work hours per airplane to 
accomplish the proposed terminating modifications at an average labor 
rate of $60 per work hour. Required parts would cost $1,865 per 
airplane. Based on these figures, the cost impact of the terminating 
modifications proposed by this AD on U.S. operators is estimated to be 
$5,803,000, or $4,145 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.

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

[[Page 45480]]

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 99-NM-81-AD.

    Applicability: All Model 737-100, -200, -200C, -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 fatigue cracking of the lower corners of the door 
frame and cross beam of the forward cargo door, which could result 
in rapid depressurization of the airplane, accomplish the following:

High Frequency Eddy Current Initial/Repetitive Inspections

    (a) Within 1 year or 4,500 flight cycles after the effective 
date of this AD, whichever occurs later, perform a high frequency 
eddy current (HFEC) inspection to detect cracking of the lower 
corners (forward and aft) of the door frame of the forward cargo 
door in accordance with Boeing 737 Nondestructive Test Manual, Part 
6, Section 51-00-00, Figure 4 or Figure 23.
    (1) If no cracking is detected, repeat the HFEC inspection 
thereafter at intervals not to exceed 4,500 flight cycles, until the 
requirements of paragraph (c) of this AD have been accomplished.
    (2) If any cracking is detected during any inspection required 
by paragraph (a) of this AD, prior to further flight, accomplish the 
requirements of paragraphs (a)(2)(i) and (a)(2)(ii) of this AD, 
which constitute terminating action for the repetitive inspections 
required by paragraph (a)(1) of this AD.
    (i) Replace the door frame of the forward cargo door with a new 
door frame; install a cross beam repair and reinforcement 
modification of the cross beam in accordance with Boeing Service 
Bulletin 737-52-1100, Revision 2, dated March 31, 1994; and
    (ii) Modify the replacement door frame of the forward cargo door 
in accordance with a method approved by the Manager, Seattle 
Aircraft Certification Office (ACO), FAA, Transport Airplane 
Directorate 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 or 
modification method to be approved by the Manager, Seattle ACO, as 
required by this paragraph and paragraphs (b)(2), (b)(3)(ii), and 
(c)(2), the Manager's approval letter must specifically reference 
this AD.

Detailed Visual Initial/Repetitive Inspections

    (b) Within 1 year or 4,500 flight cycles after the effective 
date of this AD, whichever occurs later, perform a detailed visual 
inspection to detect cracking of the cross beam (i.e., upper and 
lower chord and web sections) of the forward cargo door in 
accordance with Boeing Service Bulletin 737-52-1100, Revision 2, 
dated March 31, 1994.

    Note 2: For the purposes of this AD, a detailed 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.''

    (1) If no cracking is detected, repeat the inspection thereafter 
at intervals not to exceed 4,500 flight cycles until the 
requirements of paragraph (c) of this AD have been accomplished.
    (2) If any cracking is detected on the lower chord section of 
the cross beam during any inspection required by paragraph (b) of 
this AD, prior to further flight, repair in accordance with a method 
approved by the Manager, Seattle ACO, 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.
    (3) If any cracking is detected on any area excluding the lower 
chord section of the cross beam (i.e., upper chord and web section) 
during any inspection required by paragraph (b) of this AD, prior to 
further flight, accomplish the requirements of paragraph (b)(3)(i) 
or (b)(3)(ii), as applicable, of this AD, which constitute 
terminating action for the repetitive inspections required by 
paragraph (b)(1) of this AD.
    (i) For airplanes with line numbers 1 through 1231: Install a 
cross beam repair and preventative modification of the outboard 
radius of the lower corners (forward and aft) of the door frame in 
accordance with Boeing Service Bulletin 737-52-1100, Revision 2, 
dated March 31, 1994.

    Note 3: Due to implications and consequences associated with 
cracking, this AD does not allow the option of replacing the door 
frame as an alternative method of compliance to installing the 
preventative modification.

    (ii) For airplanes with line numbers 1232 and subsequent: 
Install a cross beam repair and preventative modification of the 
outboard radius of the lower corners (forward and aft) of the door 
frame in accordance with a method approved by the Manager, Seattle 
ACO 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.

Terminating Action

    (c) Within 4 years after the effective date of this AD: Install 
the preventative modification of the outboard radius of the lower 
corners (forward and aft) of the door frame and the reinforcement 
modification of the cross beam of the forward cargo door in 
accordance with paragraph (c)(1) or (c)(2) of this AD, as 
applicable. Accomplishment of paragraph (c)(1) or (c)(2) of this AD, 
as applicable, constitutes terminating action for the repetitive 
inspections required by paragraphs (a)(1) and (b)(1) of this AD.
    (1) For airplanes with line numbers 1 through 1231: Accomplish 
the preventative modification and the reinforcement modification in 
accordance with Boeing Service Bulletin 737-52-1100, Revision 2, 
dated March 31, 1994.
    (2) For airplanes with line numbers 1232 and subsequent: 
Accomplish the preventative modification and the reinforcement 
modification in accordance with a method approved by the Manager, 
Seattle ACO 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.

Modifications Previously Accomplished

    (d) For all airplanes on which modifications of the forward 
lower corner of the door frame and the cross beam of the forward 
cargo door were accomplished in accordance with Boeing Service 
Bulletin 737-52-1100, dated August 25, 1988, or Revision 1, dated 
July 20, 1989, or in accordance with the requirements of AD 90-06-
02, amendment 39-6489: Within 4 years after the effective date of 
this AD, install the reinforcement modification of the aft corner of 
the door frame of the forward cargo door in accordance with Boeing 
Service Bulletin 737-52-1100, Revision 2, dated March 31, 1994. 
Accomplishment of such modification constitutes terminating action 
for the repetitive inspections required by this AD.


[[Page 45481]]


    Note 4: Accomplishment of Boeing Service Bulletin 737-52-1100, 
Revision 2, dated March 31, 1994, does not supersede the 
requirements of AD 90-06-02, amendment 39-6489.

Alternative Methods of Compliance

    (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 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 5: 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

    (f) Special flight permits may be issued in accordance with 
Secs. 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 16, 1999.
D. L. Riggin,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 99-21687 Filed 8-19-99; 8:45 am]
BILLING CODE 4910-13-P