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###### Department of Transportation

###### Federal Aviation Administration

          - 14 CFR Part 39
          - [Docket No. FAA-2005-22169; Directorate Identifier 2005-NM-094-AD]
          - RIN 2120-AA64

# AGENCY:

Federal Aviation Administration (FAA), Department of Transportation (DOT).

# ACTION:

Notice of proposed rulemaking (NPRM).

# SUMMARY:

The FAA proposes to adopt a new airworthiness directive (AD) for certain Learjet Model 23, 24, 24A, 24B, 24B-A, 24C, 24D, 24D-A, 24E, 24F, 24F-A, 25, 25A, 25B, 25C, 25D, and 25F airplanes. This proposed AD would require replacement of the spherical accumulator for the main hydraulic system with a new cylindrical accumulator. For certain airplanes, this proposed AD would also require modification of the accumulator pressure gauge. This proposed AD is prompted by reports of the failure of two thrust reverser accumulators (which are similar to the main hydraulic system's spherical accumulator) and fatigue cracks found on four thrust reverser accumulators. We are proposing this AD to prevent failure of the spherical accumulator for the main hydraulic system, due to fatigue cracking on the threads, which could result in the loss of hydraulic power, damage to the surrounding airplane structure, and loss of airplane control. The failure of the accumulator could also result in injury to any persons in the surrounding area. The loss of hydraulic fluid could also leak onto a potential source of ignition and result in a consequent fire.

# DATES:

We must receive comments on this proposed AD by October 7, 2005.

      ( print page 49211)    

# ADDRESSES:

Use one of the following addresses to submit comments on this proposed AD.
      - DOT Docket Web site: Go to http://dms.dot.gov                          and follow the instructions for sending your comments electronically.                    
      - Government-wide rulemaking Web site: Go to http://www.regulations.gov                          and follow the instructions for sending your comments electronically.                    
      - Mail: Docket Management Facility, U.S. Department of Transportation, 400 Seventh Street SW., Nassif Building, room PL-401, Washington, DC 20590.
      - By fax: (202) 493-2251.
      - Hand Delivery: Room PL-401 on the plaza level of the Nassif Building, 400 Seventh Street SW., Washington, DC, between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays.

For service information identified in this proposed AD, contact Learjet, Inc., One Learjet Way, Wichita, Kansas 67209-2942.

                        You can examine the contents of this AD docket on the Internet at                         *http://dms.dot.gov*, or in person at the Docket Management Facility, U.S. Department of Transportation, 400 Seventh Street SW., room PL-401, on the plaza level of the Nassif Building, Washington, DC. This docket number is FAA-2005-22169; the directorate identifier for this docket is 2005-NM-094-AD.                    

# FOR FURTHER INFORMATION CONTACT:

Robert Busto, Aerospace Engineer, Systems and Propulsion Branch, ACE-116W, FAA, Wichita Aircraft Certification Office, 1801 Airport Road, Room 100, Mid-Continent Airport, Wichita, Kansas 67209; telephone (316) 946-4157; fax (316) 946-4107.

# SUPPLEMENTARY INFORMATION:

## Comments Invited

                    We invite you to submit any relevant written data, views, or arguments regarding this proposed AD. Send your comments to an address listed under                     **ADDRESSES**                    . Include “Docket No. FAA-2005-22169; Directorate Identifier 2005-NM-094-AD” in the subject line of your comments. We specifically invite comments on the overall regulatory, economic, environmental, and energy aspects of the proposed AD. We will consider all comments submitted by the closing date and may amend the proposed AD in light of those comments.                

                    We will post all comments we receive, without change, to                     *http://dms.dot.gov*, including any personal information you provide. We will also post a report summarizing each substantive verbal contact with FAA personnel concerning this proposed AD. Using the search function of that website, anyone can find and read the comments in any of our dockets, including the name of the individual who sent the comment (or signed the comment on behalf of an association, business, labor union, etc.). You can review DOT's complete Privacy Act Statement in the                     **Federal Register**                     published on April 11, 2000 (65 FR 19477-78), or you can visit                     *http://dms.dot.gov*.                

## Examining the Docket

                    You can examine the AD docket on the Internet at                     *http://dms.dot.gov*, or in person at the Docket Management Facility office between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. The Docket Management Facility office (telephone (800) 647-5227) is located on the plaza level of the Nassif Building at the DOT street address stated in the                     **ADDRESSES**                     section. Comments will be available in the AD docket shortly after the Docket Management System (DMS) receives them.                

## Discussion

We have received a report indicating that two failures of thrust reverser accumulators occurred, and inspections have found four thrust reverser accumulators with fatigue cracks. One failure occurred during flight on a Learjet Model 25B airplane and led to an emergency landing. The second failure occurred during bench testing and resulted in injury to a person. Detailed inspections of four thrust reverser accumulators found fatigue cracks on the inner threads that hold the two halves of the accumulator together. The spherical accumulator used for the main airplane hydraulic system is similar to the spherical accumulator used for the thrust reverser hydraulic system. Therefore, these spherical accumulators may be subject to the same unsafe condition. This fatigue cracking, if not corrected, could result in the failure of the accumulator, which could cause loss of hydraulic fluid and the hydraulic systems, resulting in a loss of airplane control. The failure of the accumulator could also result in damage to the surrounding airplane structure and injury to any persons in the surrounding area. The loss of hydraulic fluid could also leak onto a potential source of ignition and result in a consequent fire.

## Other Relevant Rulemaking

                    We have published a notice of proposed rulemaking (NPRM) in the                     **Federal Register**                     on April 14, 2005 (70 FR 19718), applicable to Learjet Model 23, 24, 24A, 24B, 24B-A, 24D, 24D-A, 24E, 24F, 25, 25A, 25B, 25C, 25D, and 25F airplanes modified by Supplemental Type Certificate SA1731SW, SA1669SW, or SA1670SW; equipped with certain Nordam thrust reversers. That NPRM proposed to require removing the thrust reverser accumulator, and making the thrust reverser hydraulic system and the thrust reversers inoperable. The actions proposed in that NPRM are intended to prevent failure of the thrust reverser accumulators, due to fatigue cracking on the female threads, which could result in the loss of hydraulic power and damage to the surrounding airplane structure.                

## Relevant Service Information

We have reviewed Bombardier Alert Service Bulletin A23/24/25-29-4, Revision 1, dated January 17, 2005. The service bulletin describes procedures for replacing certain spherical accumulators with new cylindrical accumulators, and reporting accomplishment of the service bulletin to the manufacturer. Replacing the accumulators may involve replacing the supports and attachment hardware.

For certain airplanes, Bombardier Alert Service Bulletin A23/24/25-29-4 recommends prior or concurrent accomplishment of Learjet Service Kit SK23-215, dated April 4, 1966. The concurrent service bulletin describes procedures for relocating the accumulator pressure gauge.

Accomplishing the actions specified in the service information is intended to adequately address the unsafe condition.

## FAA's Determination and Requirements of the Proposed AD

We have evaluated all pertinent information and identified an unsafe condition that is likely to exist or develop on other airplanes of this same type design. Therefore, we are proposing this AD, which would require accomplishing the actions specified in the service information described previously, except as discussed under “Differences Between the Proposed AD and the Service Bulletin.”

## Differences Between the Proposed AD and the Service Bulletin

                    Although Bombardier Alert Service Bulletin A23/24/25-29-4, Revision 1, dated January 17, 2005, recommends replacing the spherical accumulator for the main hydraulic system within 25 flight hours after the receipt of the service bulletin, this proposed AD specifies a compliance time of 60 days after the effective date of the AD. In developing an appropriate compliance                           ( print page 49212)                        time for this AD, we considered the manufacturer's recommendation, the degree of urgency associated with the subject unsafe condition, the average utilization of the affected fleet, and the time necessary to perform the actions required by the proposed AD. In light of all of these factors, we find that a longer compliance time represents an appropriate interval of time for affected airplanes to continue to operate without compromising safety. This difference has been coordinated with Learjet.                

Operators should also note that, although the Accomplishment Instructions of the referenced service bulletin describe procedures for submitting a comment sheet related to service bulletin quality and a sheet recording compliance with the service bulletin, this proposed AD would not require those actions. We do not need this information from operators.

## Costs of Compliance

There are about 434 airplanes of the affected design in the worldwide fleet. This proposed AD would affect about 242 airplanes of U.S. registry. The proposed actions would take about 9 to 13 work hours per airplane, at an average labor rate of $65 per work hour. Required parts would cost about $1,336 to $1,363 per airplane. Based on these figures, the estimated cost of the proposed AD for U.S. operators is $464,882 to $534,336, or $1,921 to $2,208 per airplane.

## Authority for This Rulemaking

Title 49 of the United States Code specifies the FAA's authority to issue rules on aviation safety. Subtitle I, Section 106, describes the authority of the FAA Administrator. Subtitle VII, Aviation Programs, describes in more detail the scope of the Agency's authority.

We are issuing this rulemaking under the authority described in subtitle VII, part A, subpart III, section 44701, “General requirements.” Under that section, Congress charges the FAA with promoting safe flight of civil aircraft in air commerce by prescribing regulations for practices, methods, and procedures the Administrator finds necessary for safety in air commerce. This regulation is within the scope of that authority because it addresses an unsafe condition that is likely to exist or develop on products identified in this rulemaking action.

## Regulatory Findings

We have determined that this proposed AD would not have federalism implications under Executive Order 13132. This proposed AD 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.

For the reasons discussed above, I certify that the 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. 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.

                    We prepared a regulatory evaluation of the estimated costs to comply with this proposed AD. See the                     **ADDRESSES**                     section for a location to examine the regulatory evaluation.                

# List of Subjects in 14 CFR Part 39

      - Air transportation
      - Aircraft
      - Aviation safety
      - Safety

## The Proposed Amendment

Accordingly, under the authority delegated to me by the Administrator, the FAA proposes to amend 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.

§ 39.13 

2. The FAA amends § 39.13 by adding the following new airworthiness directive (AD):

**Learjet:**                                 Docket No. FAA-2005-22169; Directorate Identifier 2005-NM-094-AD.                            

## Comments Due Date

(a) The Federal Aviation Administration (FAA) must receive comments on this AD action by October 7, 2005.

## Affected ADs

(b) None.

## Applicability

(c) This AD applies to Learjet Model 23, 24, 24A, 24B, 24B-A, 24C, 24D, 24D-A, 24E, 24F, 24F-A, 25, 25A, 25B, 25C, 25D, and 25F airplanes, certificated in any category; having serial numbers 23-003 through 23-099 inclusive, 24-100 through 24-284 inclusive, and 25-003 through 25-153 inclusive.

## Unsafe Condition

(d) This AD was prompted by reports of the failure of two thrust reverser accumulators (which are similar to the main hydraulic system's accumulator) and fatigue cracks found on four thrust reverser accumulators. We are issuing this AD to prevent failure of the spherical accumulator for the main hydraulic system, due to fatigue cracking on the threads, which could result in the loss of hydraulic power, damage to the surrounding airplane structure, and loss of airplane control. The failure of the accumulator could also result in injury to any persons in the surrounding area. The loss of hydraulic fluid could also leak onto a potential source of ignition and result in a consequent fire.

## Compliance

(e) You are responsible for having the actions required by this AD performed within the compliance times specified, unless the actions have already been done.

## Replacement

(f) Within 60 days after the effective date of this AD, replace the spherical accumulator having part number (P/N) 2380025-( ) or P/N 2380167-( ) with a new cylindrical accumulator having P/N 2497202-801 in accordance with the Accomplishment Instructions of Bombardier Alert Service Bulletin A23/24/25-29-4, Revision 1, dated January 17, 2005.

## Concurrent Action

(g) For airplanes having serial numbers 23-003 through 23-014 inclusive: Prior to or concurrently with the actions in Bombardier Alert Service Bulletin A23/24/25-29-4, Revision 1, dated January 17, 2005, relocate the accumulator pressure gauge in accordance with Learjet Service Kit SK23-215, dated April 4, 1966.

## Parts Installation

(h) As of the effective date, no spherical accumulator having P/N 2380025-() or P/N 2380167-() may be installed on any airplane.

## Previous Actions

(i) Replacements done before the effective date of this AD in accordance with Bombardier Alert Service Bulletin A23/24/25-29-4, dated August 20, 2004, are acceptable for compliance with the requirements of paragraph (f) of this AD.

## No Reporting Required

(j) Although the service bulletin referenced in this AD specifies to submit certain information to the manufacturer, this AD does not include that requirement.

## Alternative Methods of Compliance (AMOCs)

(k) The Manager, Wichita Aircraft Certification Office, FAA, has the authority to approve AMOCs for this AD, if requested in accordance with the procedures found in 14 CFR 39.19.

Issued in Renton, Washington, on August 12, 2005.

Ali Bahrami,

Manager, Transport Airplane Directorate, Aircraft Certification Service.

[FR Doc. 05-16752 Filed 8-22-05; 8:45 am]

BILLING CODE 4910-13-P