AD 97-09-06

The FAA has received several reports of incidents in which the tubes of the fire extinguishing and smoke detection systems of the aft cargo compartment chafed against the stiffener rings and support brackets of the pneumatic duct of the auxiliary power unit (APU). These incidents occurred on Boeing Model 757 series airplanes. Investigation revealed that thermal growth of the pneumatic duct of the APU caused the stiffener rings to contact the tubes. Such thermal growth also can cause a stiffener ring to contact a support bracket of the pneumatic duct of the APU, which could eventually break the support bracket. A broken support bracket could chafe the tubes of the fire extinguishing and smoke detection systems of the aft cargo compartment. Chafing of the subject tubes could eventually create a hole in the tubes.

Unsafe Conditions

A hole in the tubes of the fire extinguishing system, if not detected and corrected, could prevent the proper distribution of the fire extinguishing agent within the aft cargo compartment in the event of a fire. In addition, such a hole could release the fire extinguishing agent outside of the aft cargo compartment, which could migrate through the return air grilles in the passenger cabin. Localized hazardous concentrations of a fire extinguishing agent could cause deleterious physiological effects on the passengers and crew.

Additionally, a hole in the tubes of the smoke detection system, if not detected and corrected, could result in the delay of detection of a fire in the aft cargo compartment.

Explanation of Relevant Service Information

The FAA has reviewed and approved Boeing Alert Service Bulletin 757-26A0040, dated March 27, 1997. The alert service bulletin describes procedures for repetitive detailed visual inspections to detect damage of the tubes of the fire extinguishing and smoke detection systems, and support brackets of the pneumatic duct of the APU. The alert service bulletin also describes procedures for replacing, repairing, or splicing (as applicable) any damaged part.

Explanation of the Requirements of the Rule

Since an unsafe condition has been identified that is likely to exist or develop on other Boeing Model 757 series airplanes of the same type design, this AD is being issued to detect and correct chafing of the tubes of the fire extinguishing and smoke detection systems. Such chafing could result in a hole in a tube of the fire extinguishing system, and consequently prevent the proper distribution of the fire extinguishing agent within the aft cargo compartment in the event of a fire. Such chafing also could result in a hole in a tube of the smoke detection system, which could result in the delay of detection of a fire in the aft cargo compartment. This AD requires repetitive detailed visual inspections to detect damage of the tubes of the fire extinguishing and smoke detection systems, and support brackets of the pneumatic ductof the APU; and corrective actions, if necessary. The actions are required to be accomplished in accordance with the alert service bulletin described previously.

Interim Action

This is considered to be interim action until final action is identified, at which time the FAA may consider further rulemaking.

Determination of Rule's Effective Date

Since a situation exists that requires the immediate adoption of this regulation, it is found that notice and opportunity for prior public comment hereon are impracticable, and that good cause exists for making this amendment effective in less than 30 days.

Comments Invited

Although this action is in the form of a final rule that involves requirements affecting flight safety and, thus, was not preceded by notice and an opportunity for public comment, comments are invited on this rule. Interested persons are invited to comment on this 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 under the caption "ADDRESSES." All communications received on or before the closing date for comments will be considered, and this rule may be amended in light of the comments received. Factual information that supports the commenter's ideas and suggestions is extremely helpful in evaluating the effectiveness of the AD action and determining whether additional rulemaking action would be needed.

Comments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the rule that might suggest a need to modify the 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 that summarizes each FAA-public contact concerned with the substance of this AD will be filed in the Rules Docket.

Commenters wishing the FAA to acknowledge receipt of their comments submitted in response to this rule must submit a self-addressed, stamped postcard on which the following statement is made: "Comments to Docket Number 97-NM-73-AD." The postcard will be date stamped and returned to the commenter.

Regulatory Impact

The regulations adopted herein will 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 final rule does not have sufficient federalism implications to warrant the preparation of a Federalism Assessment.

The FAA has determined that this regulation is an emergency regulation that must be issued immediately to correct an unsafe condition in aircraft, and that it is not a "significant regulatory action" under Executive Order 12866. It has been determined further that this action involves an emergency regulation under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979). If it is determined that this emergency regulation otherwise would be significant under DOT Regulatory Policies and Procedures, a final regulatory evaluation will be prepared and placed in the Rules Docket. A copy of it, if filed, may be obtained from the Rules Docket at the location provided under the caption "ADDRESSES."

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.

Adoption of the Amendment

Accordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration amends 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. § 39.13 - (Amended) 2. Section 39.13 is amended by adding the following new airworthiness directive: