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

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

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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]

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