Airworthiness Directives; AlliedSignal Inc. 36-300(A), 36-280(B), and 36-280(D) Series Auxiliary Power Units

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 should 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-NE-34-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, New England Region, Office of the Regional Counsel, Attention: Rules Docket No. 99-NE-34-AD, 12 New England Executive Park, Burlington, MA 01803-5299.

Discussion

The Federal Aviation Administration (FAA) has received reports of load compressor impeller cracks on AlliedSignal Inc. 36-300(A), 36- 280(B), and 36-280(D) series Auxiliary Power Units (APUs). In three incidents, the load compressor impellers separated, resulting in uncontained APU failures and debris entering the APU compartment. Uncontained APU failures potentially could damage wiring, control and fluid lines, and airplane structure. Investigation revealed that the outboard rim of the load compressor impeller can crack at the damper ring groove location. Cracks propagate circumferentially, leading to loss of sections of the rim from the impeller. The load compressor impeller was designed with a damper ring. The damper ring retention groove was machined into the impeller with a tight radius at the corners. The resulting high stress concentrations caused cracking which progresses circumferentially allowing pieces of the rim to fail radially outward. The condition is most acute on impellers that were originally manufactured with a 0.005 inch radius. Some of these parts were subsequently modified to 0.035 inch radius and carry a 3822270-4 part number (P/N) designation. All of the parts that have failed in service accumulated a portion of their operating time with the 0.005 inch radius condition. The P/N 3822270-5 configuration was originally manufactured with the 0.035 inch radius. Although none of the -5 parts have failed in service, the stress concentration at the 0.035 inch radius is sufficiently high to initiate low cycle fatigue cracking at higher service times. Four -5 configuration parts have been tested to failure by the manufacturer confirming the identical failure modes with the -4 parts, the difference being initiation time taking longer on the -5 part. This condition, if not corrected, could result in an uncontained APU failure and damage to the airplane. The FAA has reviewed and approved the technical contents of AlliedSignal Inc. Service Bulletins (SBs) No. GTCP36-49-7471, dated April 20, 1999, GTCP36-49-7472, dated March 31, 1999, and GTCP36-49- 7473, dated March 31, 1999, that describe procedures for installation of an external load compressor containment shield. Since an unsafe condition has been identified that is likely to exist or develop on other products of the same design, the proposed AD would require installation of an external load compressor containment shield at the next shop visit, or 6 months after the effective date of this AD, whichever occurs first. The 6 month time frame is based upon engineering assessment of the risk of operating without containment. An additional compliance option would be installation of a load compressor impeller, P/N 3822270-5, to extend cyclic service life to 26,000 cycles-since-new (CSN) before mandatory installation of the containment shield. Operators cannot operate with a load compressor installed, P/N 3822270-5, past 26,000 CSN unless they have installed an external containment shield. The actions would be required to be accomplished in accordance with the SBs described previously. There are approximately 1,044 APUs of the affected design in the worldwide fleet. The FAA estimates that 465 APUs installed on airplanes of US registry would be affected by this proposed AD, that it would take approximately 6 work hours per Model 36-300(A) APU (85 units) to accomplish the proposed actions, and 8 work hours per Model 36-280(D) APU (380 units), and that the average labor rate is $60 per work hour. Required parts would cost approximately $3,103 per APU. Fifteen installations on domestic Boeing 737 aircraft (Model 36-280(B)) would require a tube assembly kit, which would cost approximately $1,042. The manufacturer has informed the FAA that it may offset some of these costs thereby lowering the total cost to operators. Based on these figures, the total cost impact of the proposed AD on US operators is estimated to be $1,725,270. 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 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:

AlliedSignal Inc.: Docket No. 99-NE-34-AD.

Applicability: AlliedSignal Inc. 36-300(A), 36-280(B), and 36- 280(D) series Auxiliary Power Units (APUs), installed on but not limited to Airbus Industrie A319, A320, and A321 series; Boeing 737- 300, -400, -500 series; and McDonnell Douglas MD-80 series airplanes.

Note 1: This airworthiness directive (AD) applies to each APU 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 APUs 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 an uncontained APU failure and damage to the airplane, accomplish the following: (a) For APUs with load compressor impellers, part number (P/N) 3822270-4, at the next shop visit, or within 6 months after the effective date of this AD, whichever occurs first, accomplish either of the following: (1) Install an external load compressor containment shield in accordance with AlliedSignal Inc. Service Bulletins (SBs) No. GTCP36-49-7471, dated April 20, 1999, GTCP36-49-7472, dated March 31, 1999, and GTCP36-49-7473, dated March 31, 1999, as applicable; or (2) Install load compressor impeller, P/N 3822270-5. (b) For APUs with load compressor impellers, P/N 3822270-5, install an external load compressor containment shield within 6 months after the effective date of this AD, or prior to exceeding 26,000 cycles-since-new (CSN), whichever occurs later, in accordance with AlliedSignal Inc. SBs No. GTCP36-49-7471, dated April 20, 1999, GTCP36-49-7472, dated March 31, 1999, and GTCP36-49-7473, dated March 31, 1999, as applicable. (c) Operators cannot operate with a load compressor, P/N 3822270-5, installed, past 26,000 cycles unless they have installed an improved external containment shield. (d) For the purpose of this AD, a shop visit is defined as when the APU is inducted into a shop for any reason. (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, Los Angeles Aircraft Certification Office. Operators shall submit their request through an appropriate FAA Principal Maintenance Inspector, who may add comments and then send it to the Manager, Los Angeles Aircraft Certification Office.

Note 2: Information concerning the existence of approved alternative methods of compliance with this airworthiness directive, if any, may be obtained from the Los Angeles Aircraft Certification Office.

(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 Burlington, Massachusetts, on September 1, 1999. Jay J. Pardee, Manager, Engine and Propeller Directorate, Aircraft Certification Service. [FR Doc. 99-23284 Filed 9-7-99; 8:45 am] BILLING CODE 4910-13-U