AD 97-21-09 SUPERSEDED

This amendment supersedes existing airworthiness directive (AD) 96-24-09, applicable to Allison Engine Company Model 250-C47B turboshaft engines, that currently requires replacing the engine main electrical harness assembly with an improved assembly, disabling the overspeed solenoid, inspecting the engine control unit (ECU) internal PW10 voltage to determine electrical noise characteristics, and replacing units not considered serviceable. In addition, the existing AD requires adding a placard to the helicopter instrument panel notifying the pilot that the overspeed protection system is disabled and removes a placard which was required by priority letter AD 96-21-12; revises the Bell Helicopter Textron, A Division of Textron Canada Ltd. (BHTC) Model 407 Rotorcraft Flight Manual (RFM); and requires maintenance actions to clear the ECU of faults prior to each flight. This amendment continues to require replacing the engine main electrical harness assembly with an improved assembly, but adds the requirements to install a new hydromechanical unit (HMU) and ECU, removing the placard notifying the pilot that the overspeed protection system is disabled, and revises the BHTC Model 407 RFM. This amendment is prompted by the development of overspeed protection system modifications to reactivate the overspeed solenoid in conjunction with raising the power turbine overspeed trip point and revising the overspeed system to default to a minimum fuel flow in the event of its activation. The actions specified by this AD are intended to prevent uncommanded inflight engine shutdowns, which can result in autorotation, forced landing, and possible loss of the helicopter.

Final rule; request for comments.

97-21-09 Allison Engine Company: Amendment 39-10162. Docket 97-ANE-40-AD. Supersedes AD 96-24-09, Amendment 39-9834.

Applicability: Allison Engine Company Model 250-C47B turboshaft engines, installed on but not limited to Bell Helicopter Textron, A Division of Textron Canada Ltd. (BHTC) Model 407 helicopters.

Note 1: This airworthiness directive (AD) applies to each engine 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 engines 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 (d) 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 uncommanded inflight engine shutdowns, which can result in autorotation, forced landing, and possible loss of the helicopter, accomplish the following:

(a) Prior to further flight, replace the engine main electrical harness assembly, Part Number (P/N) 23062796, with an improved assembly, P/N 23065805, in accordance with Allison Engine Company Alert Commercial Engine Bulletin (CEB) CEB-A-73-6010, dated October 15, 1996.

(b) Prior to January 31, 1998, install a new hydromechanical unit (HMU) and engine control unit (ECU) in accordance with Allison Engine Company Alert CEB-A-73-6015, Revision 1, dated July 30, 1997, or Revision 2, dated October 31, 1997.

(c) After completing the requirements of paragraph (b) of this AD, and then prior to further flight:

(1) Remove the "OVRSPD SYSTEM INOP" placard required by paragraph (d) of AD 96-24-09, and

(2) Revise the FAA-approved Rotorcraft Flight Manual (RFM) by removing the pages added by paragraph (f) of AD 96-24-09, and incorporate BHTC Flight Manual BHT-407-FM-1, Revision 5, dated June 24, 1997.

(d) 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, Chicago Aircraft Certification Office. Operators shall submit their requests through an appropriate FAA Principal Maintenance Inspector, who may add comments and then send it to the Manager, Chicago 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 Chicago Aircraft Certification Office.

(e) Special flight permits may be issued in accordance with sections 21.197 and 21.199 of the Federal Aviation Regulations (14 CFR 21.197 and21.199) to operate the aircraft to a location where the requirements of this AD can be accomplished.

(f) The actions required by this AD shall be done in accordance with the following service documents:

Document No.
Pages
Revision
Date
Allison Engine Company Alert CEB-A-73-6010
Total Pages: 7.
1-7
Original
October 15, 1996
Allison Engine Company Alert CEB-A-73-6015
Total Pages: 4
1-4
1
July 30, 1997
Allison Engine Company Alert CEB-A-73-6015
Total pages: 4.
1-4
2
October 31, 1997
BHTC Flight Manual BHT-407-FM-1
Cover
5
June 24, 1997

NP
3
July 30, 1996

A,B
5
June 24, 1997

C/D
5
June 24, 1997

1-3
5
June 24, 1997

1-4
4
November 4, 1996

1-7,
1-8

5

June 24, 1997

1-13
4
November 4, 1996

1-14
5
June 24, 1997

1-14A/
14B

5

June 24, 1997

1-19/
1-20

5

June 24, 1997

2-3
5
June 24, 1997

2-4
1
March 8, 1996

2-7 -
2-10

5

June 24, 1997

2-13,
2-14

5June 24, 1997

3-3 -
3-5

5

June 24, 1997

3-6
2
May 9, 1996

3-7,
3-8

5

June 24, 1997

3-15
5
June 24, 1997

3-16
2
May 9, 1996

3-17 -
3-22

5

June 24, 1997

4-5,
4-6

5

June 24, 1997

4-9
Original
February 9, 1996

4-10 -
4-12

5

June 24, 1997
Total pages: 40

This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from Allison Engine Company, P.O. Box 420, Speed Code P-40A, Indianapolis, IN 46206-0420; telephone (317) 230-2720, fax (317) 230-3381. Copies may be inspected at the FAA, New England Region, Office of the Assistant Chief Counsel, 12 New England Executive Park, Burlington, MA; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.

(g) This amendment becomes effective on December 3, 1997.

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 prohibited 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.

Following issuance of priority letter AD 96-21-12, the investigation revealed that the cause of the uncommanded inflight engine shutdowns was an ECU 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 resulted in a command to zero fuel flow and engine flameout. The ECU fault resulted from a manufacturing defect in the engine main electrical harness assembly.

Additionally, in a related incident involving another Allison Engine Company engine model, an ECU 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) was in use on the Allison Engine Company Model 250-C47B engine application. The noted ECU power modulator problem could have also led to the overspeed condition and uncommanded engine shutdown described above, and was therefore addressed in the following AD action.

On November 15, 1996, the FAA issued AD 96-24-09, Amendment 39-9834 (61 FR 59828, November 25, 1996), applicable to Allison Engine Company Model 250-C47B turboshaft engines, to require 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, AD 96-24-09 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. That action was prompted by investigation into the causes of the inflight engine shutdowns.

The original decision to disconnect the overspeed protection system was arrived at by comparing the safety of operating with the system enabled to the safety of operating with the system disabled. By operating with the system enabled, there was a likely consequence of other FADEC failures leading to an overspeed trip and resulting in an engine shutdown as had happened previously. Disabling the overspeed protection system eliminated the possibility of inadvertent activation and engine shutdown, but introduced new risks associated with engine power turbine shaft failure or rotorcraft drive train failure leading to uncontained blade shedding and hazards to the aircraft. Based on service data for comparable applications, and the estimated length of time for a final fix to be designed and tested, the FAA considered operations with the overspeed protection disabled acceptable. The redesign of the system has taken longer than estimated, which now puts in question the original analysis concerning operations with the overspeed protection disabled. The FAA has determined that immediate action is necessary to prevent possible turbine shaft failure, or rotorcraft drive train failure, from leading to an uncontained blade shedding due to the lack of overspeed protection. For these reasons, the FAA has determined that implementation of the redesigned overspeed system must proceed without further delay.

Since the issuance of AD 96-24-09, Allison Engine Company has developed certain modifications that raise the power turbine overspeed trip point, and revise the overspeed system to default to a minimum fuel flow in the event of its activation. With these changes incorporated, the overspeed solenoid can be reactivated. Additionally, a capacitor was added to the ECU PW10 circuit thereby eliminating the problematic electrical noise characteristic.

The FAA has reviewed and approved the technical contents of: Allison Engine Company Alert Commercial Engine Bulletin (CEB) A-73-6015, Revision 1, dated July 30, 1997, and Revision 2, dated October 31, 1997, that describe procedures for installing a new hydromechanical unit (HMU) and ECU; Allison Engine Company Alert CEB-A-73-6010, dated October 15, 1996, that describes replacing the engine main electrical harness assembly with an improved assembly; and BHTC RFM BHT-407-FM-1, Revision 5, dated June 24, 1997, that adds new instructions for the revised overspeed system.

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 AD 96-24-09 to continue to require replacing the engine main electrical harness assembly with an improved assembly, and adds the requirements to install a new HMU and ECU, to remove the placard notifying the pilot that the overspeed protection system is disabled, and to revise the BHTC Model 407 RFM. Installation of the new HMU and ECU will reactivate the overspeed solenoid in conjunction with raising the power turbine overspeed trip point and revising the overspeed system to default to a minimum fuel flow in the event of its activation. These actions must be completed by January 31, 1998. This calendar end-date was determined based upon parts availability. The requirements of paragraph (c) of this AD have been coordinated with the Rotorcraft Directorate. The actions are required to be accomplished in accordance with the service information 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 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 andafter 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 97-ANE-40-AD." 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 U.S.C. 106(g), 40113, 44701.

39.13 [Amended]
2. Section 39.13 is amended by removing Amendment 39-9834, (61 FR 59828, November 25, 1996), and by adding a new airworthiness directive, Amendment 39-10162, to read as follows:

Submit comments in triplicate to the Federal Aviation Administration (FAA), New England Region, Office of the Assistant Chief Counsel, Attention: Rules Docket No. 97-ANE-40-AD, 12 New England Executive Park, Burlington, MA 01803-5299. Comments may also be sent via the Internet using the following address: "9-ad-engineprop@faa.dot.gov". Comments sent via the Internet must contain the docket number in the subject line.

The service information referenced in this AD may be obtained from Allison Engine Company, P.O. Box 420, Speed Code P-40A, Indianapolis, IN 46206-0420; telephone (317) 230-2720, fax (317) 230-3381. This information may be examined at the FAA, New England Region, Office of the Assistant Chief Counsel, Burlington, MA; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.

Patricia Bonnen, Aerospace Engineer, Chicago Aircraft Certification Office, FAA, Small Airplane Directorate, 2300 East Devon Ave., Des Plaines, IL 60018; telephone (847) 294-7134, fax (847) 294-7834.