[Federal Register Volume 68, Number 219 (Thursday, November 13, 2003)]
[Proposed Rules]
[Pages 64283-64286]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 03-28399]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 2001-NM-366-AD]
RIN 2120-AA64


Airworthiness Directives; Learjet Model 31, 31A, 35, 35A (C-21A), 
36, and 36A 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 certain Learjet Model 31, 31A, 35, 
35A (C-21A), 36, and 36A airplanes. This proposal would require 
modification of the drag angles of the fuselage and engine pylons to 
gain access to the shear webs of the forward engine beams; repetitive 
inspections of the shear webs of the forward engine beams for cracks; 
follow-on actions; and modification/repair of the shear webs of the 
forward engine beams, as necessary, which would terminate the 
repetitive inspections. This action is necessary to prevent significant 
structural damage to the engine pylons, possible separation of the 
engines from the fuselage, and consequent reduced controllability of 
the airplane. This action is intended to address the identified unsafe 
condition.

DATES: Comments must be received by December 29, 2003.

ADDRESSES: Submit comments in triplicate to the Federal Aviation 
Administration (FAA), Transport Airplane Directorate, ANM-114, 
Attention: Rules Docket No. 2001-NM-366-AD, 1601 Lind Avenue, SW., 
Renton, Washington 98055-4056. Comments may be inspected at this 
location between 9 a.m. and 3 p.m., Monday through Friday, except 
Federal holidays. Comments may be submitted via fax to (425) 227-1232. 
Comments may also be sent via the Internet using the following address: 
[email protected]. Comments sent via fax or the Internet must 
contain ``Docket No. 2001-NM-366-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 or 2000 or 
ASCII text.
    The service information referenced in the proposed rule may be 
obtained from Learjet, Inc., One Learjet Way, Wichita, Kansas 67209-
2942. This information may be examined at the FAA, Transport Airplane 
Directorate, 1601 Lind Avenue, SW., Renton, Washington; or at the FAA, 
Wichita Aircraft Certification Office, 1801 Airport Road, Room 100, 
Mid-Continent Airport, Wichita, Kansas.

FOR FURTHER INFORMATION CONTACT: Steven Litke, Aerospace Engineer, 
Airframe Branch, ACE-118W, FAA, Wichita Aircraft Certification Office, 
1801 Airport Road, Room 100, Mid-Continent Airport, Wichita, Kansas 
67209; telephone (316) 946-4127; fax (316) 946-4107.

SUPPLEMENTARY INFORMATION:

[[Page 64284]]

Comments Invited

    Interested persons are invited to participate in the making of the 
proposed rule by submitting such written data, views, or arguments as 
they may desire. Communications shall identify the Rules Docket number 
and be submitted in triplicate to the address specified above. All 
communications received on or before the closing date for comments, 
specified above, will be considered before taking action on the 
proposed rule. The proposals contained in this action may be changed in 
light of the comments received.
    Submit comments using the following format:
    [sbull] Organize comments issue-by-issue. For example, discuss a 
request to change the compliance time and a request to change the 
service bulletin reference as two separate issues.
    [sbull] For each issue, state what specific change to the proposed 
AD is being requested.
    [sbull] Include justification (e.g., reasons or data) for each 
request.
    Comments are specifically invited on the overall regulatory, 
economic, environmental, and energy aspects of the proposed rule. All 
comments submitted will be available, both before and after the closing 
date for comments, in the Rules Docket for examination by interested 
persons. A report summarizing each FAA-public contact concerned with 
the substance of this proposal will be filed in the Rules Docket.
    Commenters wishing the FAA to acknowledge receipt of their comments 
submitted in response to this action must submit a self-addressed, 
stamped postcard on which the following statement is made: ``Comments 
to Docket Number 2001-NM-366-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. 2001-NM-366-AD, 1601 Lind Avenue, SW., Renton, Washington 
98055-4056.

Discussion

    The FAA has received reports that cracks in the shear webs of the 
forward engine beams were discovered on certain Learjet Model 31, 31A, 
35, 35A (C-21A), 36, and 36A airplanes, which experienced damage while 
landing. Further investigation revealed that the cracks were caused by 
repetitive loading of the engine beams during airplane operation (i.e., 
flight, maneuver, taxi, and landing). The engine beams are the primary 
structural elements of the pylon support for each engine. Such 
cracking, if not corrected, could result in significant structural 
damage to the engine pylons, possible separation of the engines from 
the fuselage, and consequent reduced controllability of the airplane.

Explanation of Relevant Service Information

    We have reviewed and approved Bombardier Service Bulletin 31-51-2 
(for Model 31 airplanes) and Bombardier Service Bulletin 35/36-51-3 
(for Model 35 and 36 airplanes); both dated February 1, 2001. These 
service bulletins include procedures for modifying the drag angles of 
the fuselage and engine pylons to gain access to the shear webs of the 
forward engine beams. The modification includes including removing the 
upper forward drag angles, trimming the slots in the fuselage and pylon 
skins, creating slots in the drag angles to match slots in the fuselage 
and pylon skins, grit blasting the radius of the engine shear webs, re-
identifying the drag angles, installing new nutplates on the pylon 
skins, re-installing the upper forward drag angles, fillet sealing the 
drag angles to the fuselage and engine pylon skins, installing covers 
on the drag angles, fay surfacing the covers to the drag angles, and 
fillet sealing the cover edges.
    The service bulletins also include procedures for repetitive 
detailed inspections (using a probe) and general visual inspections of 
the shear webs of the forward engine beams for cracking. The probe 
inspection includes grit blasting areas of the shear webs of the 
forward engine beams, calibrating a micro-ohmmeter, attaching a test 
probe to the micro-ohmmeter, applying the test probe to the base of the 
forward and aft shear webs, applying the test probe to the radius of 
the forward and aft shear webs, inspecting the forward and aft shear 
webs for visible cracks, applying primer to the grit blasted areas of 
the shear webs of the forward engine beam, and determining if the 
resistance values of the probe inspection are within the acceptable 
limits specified in the service bulletins.
    We have also reviewed and approved Bombardier Service Bulletin 31-
51-3, Revision 1 (for Model 31 airplanes) and Bombardier Service 
Bulletin 35/36-51-4, Revision 1 (for Model 35 and 36 airplanes), both 
dated August 2, 2001. These service bulletins describe procedures for 
modifying/repairing the shear webs of the forward engine beams. The 
modification/repair procedures include trimming the upper and lower 
flanges of frame 24, measuring the lengths and dimensions of existing 
cracks in the shear webs of the forward engine beams and reporting the 
results to the manufacturer, installing the appropriate parts kits, 
fabricating certain parts, and installing new hardware and a jumper on 
the conduit located between stringers 6 and 7 on the left side of the 
airplane. Modification/repair of the shear webs eliminates the need for 
the repetitive detailed inspections (using a probe) and general visual 
inspections of the shear webs of the forward engine beams for cracking.
    Accomplishment of the actions specified in these service bulletins 
is 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 bulletins described previously, except as discussed under 
``Differences Between the Proposed Rule and the Service Bulletins.''

Flight With Cracks

    Operators should note that, while it is not the FAA's normal policy 
to allow flight with known cracks, this proposed AD does permit further 
flight with cracking within certain limits. If the crack size limits 
are strictly observed and if repetitive inspections are performed at 
the required intervals, cracks that grow beyond the limits will be 
detected, and corrective action taken, before they can grow to a size 
that would create an unacceptable risk of structural failure.
    This proposed AD allows flight with crack openings less than 0.03 
inch, provided that (1) the crack is not part of multi-site damage, (2) 
the crack growth is easily detectable, and (3) the established 
inspection procedures would detect cracked structure at intervals that 
would permit repairs to be accomplished before the structure's strength 
falls below ultimate load carrying capability.

Differences Between the Proposed Rule and the Service Bulletins

    Although the service bulletins either do not reference repair 
instructions or specify that operators may contact the manufacturer for 
disposition of certain repair conditions, this proposed AD would 
require operators to repair those conditions per a method approved by 
the FAA.

[[Page 64285]]

    The service bulletins also specify to submit information to the 
manufacturer; however, this proposed AD does not include such a 
requirement.

Clarification of Compliance Times

    The follow-on actions and compliance times for the general visual 
inspection required by paragraph (a) of this AD are not clearly stated 
in the service bulletins. We have specified the compliance time and 
follow-on actions in paragraph (d) of this proposed AD.

Cost Impact

    There are approximately 893 airplanes of the affected design in the 
worldwide fleet. The FAA estimates that 673 airplanes of U.S. registry 
would be affected by this proposed AD.
    It would take between 2 and 3 work hours per airplane to accomplish 
the proposed modification, at an average labor rate of $65 per work 
hour. Required parts would cost approximately $243 per airplane. Based 
on these figures, the cost impact of the proposed modification on U.S. 
operators is estimated to be between $251,029 and $294,774, or between 
$373 and $438 per airplane.
    We estimate that it would take 3 work hours to perform the proposed 
inspection, and that the average labor rate is $65 per work hour. Based 
on this figure, the cost impact of the proposed inspections on U.S. 
operators is estimated to be $131,235, or $195 per airplane, per 
inspection cycle.
    The cost impact figures discussed above are based on assumptions 
that no operator has yet accomplished any of the proposed requirements 
of this AD action, and that no operator would accomplish those actions 
in the future if this proposed 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. The manufacturer may 
cover the cost of replacement parts associated with this proposed AD, 
subject to warranty conditions. Manufacturer warranty remedies may also 
be available for labor costs associated with this proposed AD. As a 
result, the costs attributable to the proposed AD may be less than 
stated above.

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.


Sec.  39.13  [Amended]

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

Learjet: Docket 2001-NM-366-AD.

    Applicability: The following airplanes, certificated in any 
category, as applicable:

                                             Table 1.--Applicability
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                 Model                                   As listed in Bombardier service bulletin
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31 and 31A Airplanes...................  31-51-2, dated February 1, 2001; and 31-51-3, Revision 1, dated August
                                          2, 2001.
35, 35A (C-21A), 36 and 36A Airplanes..  35/36-51-3, dated February 1, 2001; and 35/36-51-4, Revision 1, dated
                                          August 2, 2001.
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    Compliance: Required as indicated, unless accomplished 
previously.
    To prevent significant structural damage to the engine pylons, 
possible separation of the engines from the fuselage, and consequent 
reduced controllability of the airplane, accomplish the following:

Inspections

    (a) At the later of the times specified in paragraphs (a)(1) and 
(a)(2) of this AD: Do a detailed inspection (using a probe) and a 
general visual inspection of the shear webs of the forward engine 
beams (including modification of the drag angles) for cracking in 
accordance with the Accomplishment Instructions of Bombardier 
Service Bulletin 31-51-2 (for Model 31 airplanes) or 35/36-51-3 (for 
Model 35 and 36 airplanes), both dated February 1, 2001; as 
applicable.
    (1) Prior to the accumulation of 3,000 total flight hours; or
    (2) Within 1,200 flight hours or 1 year after the effective date 
of this AD, whichever occurs first.

    Note 1: 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 from within 
touching distance unless otherwise specified. A mirror may be 
necessary to enhance visual access to all exposed surfaces in the 
inspection area. This level of inspection is made under normally 
available lighting conditions such as daylight, hangar lighting, 
flashlight, or droplight 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 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.''

Detailed Probe Inspection Follow-on Actions

    (b) Following the detailed probe inspection required by 
paragraph (a) of this AD, do the follow-on actions specified in 
paragraphs (b)(1), (b)(2), or (b)(3) of this AD, as

[[Page 64286]]

applicable, in accordance with the Accomplishment Instructions of 
Bombardier Service Bulletin 31-51-2 or 35/36-51-3, both dated 
February 1, 2001; as applicable.
    (1) If the resistance measured during the inspection is less 
than 0.110 milliohm: Repeat the inspections required by paragraph 
(a) of this AD thereafter at intervals not to exceed 1,200 flight 
hours.
    (2) If the resistance measured during the inspection is 0.110 
milliohm or more, but less than 0.150 milliohm: Within the next 
1,200 flight hours, repair and modify the forward engine beam shear 
web in accordance with the Accomplishment Instructions of Bombardier 
Service Bulletin 31-51-3, Revision 1 (for Model 31 airplanes) or 35/
36-51-4, Revision 1 (for Model 35 and 36 airplanes), both dated 
August 2, 2001; as applicable.
    (3) If the resistance measured during the inspection is 0.150 
milliohm or more: Before further flight, repair and modify the 
forward engine beam shear web in accordance with the Accomplishment 
Instructions of Bombardier Service Bulletin 31-51-3, Revision 1, or 
35/36-51-4, Revision 1; as applicable.

General Visual Inspection Follow-On Actions

    (c) Following the general visual inspection required by 
paragraph (a) of this AD, do all of the applicable follow-on actions 
at the times specified in the Accomplishment Instructions of 
Bombardier Service Bulletin 31-51-2 or 35/36-51-3, both dated 
February 1, 2001; as applicable; except as specified in paragraph 
(d) of this AD.
    (d) If any crack opening is found that is more than 0.03 inch 
during the general visual inspection required by paragraph (a) of 
this AD: Before further flight, do the actions specified in 
paragraphs 2.C.(16)(a) and 2.C.(16)(b) of Bombardier Service 
Bulletin 31-51-2 or 35/36-51-3, both dated February 1, 2001; as 
applicable; repair per a method approved by the Manager, Wichita 
Aircraft Certification Office (ACO), FAA; and do the terminating 
action specified in paragraph (e) of this AD.

Terminating Action

    (e) Modification of the shear webs by accomplishing all of the 
actions specified in the Accomplishment Instructions of Bombardier 
Service Bulletin 31-51-3, Revision 1, or 35/36-51-4, Revision 1, 
both dated August 2, 2001; as applicable; terminates the initial 
inspections required by paragraph (a) and the repetitive inspections 
required by paragraph (b)(1) of this AD.

Repair Approval

    (f) Where any service bulletin identified in this AD specifies 
that the manufacturer may be contacted for disposition of certain 
repair conditions, repair per a method approved by the Manager, 
Wichita ACO, FAA.

Submission of Inspection Results Not Required

    (g) Although the service bulletins identified in this AD specify 
to submit information to the manufacturer, this AD does not include 
such a requirement.

Alternative Methods of Compliance

    (h) In accordance with 14 CFR 39.19, the Manager, Wichita ACO, 
is authorized to approve alternative methods of compliance for this 
AD.

    Issued in Renton, Washington, on November 6, 2003.
Ali Bahrami,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 03-28399 Filed 11-12-03; 8:45 am]
BILLING CODE 4910-13-P