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Proposed Rule

Airworthiness Directives; Learjet Model 31, 31A, 35, 35A (C-21A), 36, and 36A Airplanes

Document Details

Information about this document as published in the Federal Register.

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AGENCY:

Federal Aviation Administration, DOT.

ACTION:

Notice of proposed rulemaking (NPRM).

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: 9-anm-nprmcomment@faa.gov. 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.

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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.

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SUPPLEMENTARY INFORMATION:Start Printed 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:

  • 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.
  • For each issue, state what specific change to the proposed AD is being requested.
  • 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. Start Printed 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.

Start List of Subjects

List of Subjects in 14 CFR Part 39

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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:

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PART 39—AIRWORTHINESS DIRECTIVES

1. The authority citation for part 39 continues to read as follows:

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Authority: 49 U.S.C. 106(g), 40113, 44701.

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[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

ModelAs listed in Bombardier service bulletin
31 and 31A Airplanes31-51-2, dated February 1, 2001; and 31-51-3, Revision 1, dated August 2, 2001.
35, 35A (C-21A), 36 and 36A Airplanes35/36-51-3, dated February 1, 2001; and 35/36-51-4, Revision 1, dated August 2, 2001.

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 Start Printed Page 64286applicable, 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.

Start Signature

Issued in Renton, Washington, on November 6, 2003.

Ali Bahrami,

Acting Manager, Transport Airplane Directorate, Aircraft Certification Service.

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[FR Doc. 03-28399 Filed 11-12-03; 8:45 am]

BILLING CODE 4910-13-P