AD 96-24-09

On October 11, 1996, the Federal Aviation Administration (FAA) issued priority letter airworthiness directive (AD) 96-21-12, applicable to Bell Helicopter Textron, A Division of Textron Canada Ltd. (BHTC) Model 407 helicopters, which prohibits further flight. That action was prompted by reports of uncommanded inflight engine shutdowns on Allison Engine Company Model 250-C47B turboshaft engines installed in those helicopters. In each case, the harness failed and caused the electronic control unit (ECU) to go into a fail fixed fuel flow condition. Subsequent pilot action (reduction in collective), caused the engine to reach the overspeed trip point, with resultant default to zero fuel flow and engine shutdown. That condition, if not corrected, could result in uncommanded inflight engine shutdowns, which can result in autorotation, forced landing, and possible loss of the helicopter.

Since the issuance of that priority letter AD, the investigation revealed that the cause of the uncommanded inflight engine shutdowns was an ECU hard fault to a fail fixed fuel flow condition, and subsequent main rotor and power turbine overspeed limit exceedances coincident with pilot collective input. These overspeed conditions activated the analog overspeed trip, which results in default to a zero fuel flow condition. The ECU fault resulted from a manufacturing defect in the engine main electrical harness assembly.

Additionally, in a related incident involving a not yet certificated Allison Engine Company engine, an ECU hard fault to fail fixed fuel flow was attributed to the electrical noise characteristics of the ECU internal PW10 voltage, as affected by certain ECU power modulator subcomponents. This same power modulator Part Number (P/N) is currently in use on the Allison Engine Company Model 250-C47B engine application. The noted ECU power modulator problem can also lead to the overspeed condition and uncommanded engine shutdown described above, and is therefore addressed in this AD action.

The FAA has reviewed and approved the technical contents of Allison Engine Company Alert Commercial Engine Bulletin (CEB) No. CEB-A-73-6010, dated October 15, 1996, that describes procedures for replacing the engine main electrical harness assembly with an improved assembly; CEB-A-73-6011, dated October 31, 1996, that describes procedures for disabling the overspeed solenoid (thereby deactivating the engine overspeed protection system); and CEB-A-73-6012, dated October 31, 1996, that describes procedures for inspecting the ECU internal PW10 voltage to determine electrical noise characteristics.

Since an unsafe condition has been identified that is likely to exist or develop on other engines of this same type design, this AD supersedes priority letter AD 96-21-12, applicable to BHTC Model 407 helicopters, to require the following actions: replacing the engine main electrical harness assembly with an improved assembly, disabling the overspeed solenoid (thereby deactivating the engine overspeed protection system), inspecting the ECU internal PW10 voltage to determine electrical noise characteristics, and replacing units not considered serviceable due to excessive electrical noise. In addition, this AD requires adding a helicopter instrument panel placard notifying the pilot that the overspeed protection system is disabled; removes the placard required by AD 96-21-12 which prohibited further flight; and revises the BHTC Model 407 Rotorcraft Flight Manual (RFM) to clarify emergency flight procedures and to require maintenance actions to clear Full Authority Digital Engine Control (FADEC) fault annunciations prior to each flight. Accomplishment of these actions will enable operators to resume flight operations on an interim basis. Additional rulemaking may reactivate the engine overspeed protection system in conjunction with raising the overspeed trip speed, and require additional control system modification of going to minimum fuel flow as a terminating action. The actions are required to be accomplished in accordance with the CEBs described previously.

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 should 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 commentswill 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 notice must submit a self-addressed, stamped postcard on which the following statement is made: "Comments to Docket Number 96-ANE-41." The postcard will be date stamped and returned to the commenter.

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 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 USC 106(g), 40113, 44701.

39.13 - [AMENDED] 2. Section 39.13 is amended by adding the following new airworthiness directive: