AD 97-24-17

This document publishes in the Federal Register an amendment adopting priority letter airworthiness directive (AD) 97-24-17, which was sent previously to all known U.S. owners and operators of Bell Helicopter Textron Canada (BHTC) Model 407 helicopters by individual letters. This AD requires inspections of components in the tail rotor drive system for scratches, cracks, fretting, corrosion, and proper torquing, lubrications of the oil cooler blower shaft hanger bearings and oil cooler hanger bearings (hanger bearings), and removal of corrosion inhibitive adhesive barrier tape (barrier tape) from the tail rotor gearbox and the tail rotor gearbox support assembly faying surfaces. This amendment is prompted by numerous reports of three problems, all of which are related to the tail rotor drive system. The actions specified by this AD are intended to: detect scratches, cracks, fretting, and corrosion in the disc pack couplings; prevent inadequate lubrication of the hanger bearingsand oil cooler blower shaft; and prevent loss of mounting torque on the tail rotor gearbox. Failure of any of these components could result in loss of power to the tail rotor and subsequent loss of control of the helicopter.

Final rule; request for comments

97-24-17 BELL HELICOPTER TEXTRON CANADA: Amendment 39-10428. Docket No. 97-SW-67-AD. Supersedes Priority Letter 97-22-15, issued October 23, 1997.

Applicability: Model 407 helicopters, certificated in any category.

NOTE 1: This AD applies to each helicopter 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 helicopters that have been modified, altered, or repaired so that the performance of the requirements of this AD is affected, the owner/operator must use the authority provided in paragraph (g) to request approval from the FAA. This approval may address either no action, if the current configuration eliminates the unsafe condition, or different actions necessary to address the unsafe condition described in this AD. Such a request should include an assessment of the effect of the changed configuration on the unsafe condition addressed by this AD. In no case does the presence of any modification, alteration, or repair remove any helicopter from the applicability of this AD.

Compliance: Required as indicated, unless accomplished previously.

(a) Tail Rotor Drive Coupling Disc Pack Inspections:

To prevent failure of a tail rotor drive coupling disc pack (disc pack coupling), part number (P/N) 406-040-340-101, loss of tail rotor drive and subsequent loss of control of the helicopter, within 25 hours time-in-service (TIS), and thereafter at intervals not to exceed 25 hours TIS, accomplish the following:

(1) Visually inspect each of the eight (8) disc pack couplings for any scratch, crack, fretting, or corrosion. This inspection can be accomplished with the disc pack couplings installed. If any scratch, crack, fretting, or corrosion is found, remove and replace the disc pack coupling with an airworthy disc pack coupling. Torque on replacement disc pack coupling nuts and bolts must be a minimum of the run-on-tare torque plus 150 inch-lbs. to a maximum of the run-on-tare torque plus 180 inch-lbs.

(2) Inspect the four nuts and bolts that attach each of the disc pack couplings to the driveshaft and tail rotor gearbox adapters for proper torque. Apply a minimum torque of 170 inch-lbs. to a maximum torque of 175 inch-lbs., which includes a 20 inch-lbs. run-on-tare torque.

NOTE 2: This torque inspection should be performed on the nuts instead of the bolt heads wherever possible.

(i) If there is no nut or bolt movement, the torque is acceptable.

(ii) If any nut or bolt moved, remove and replace the disc pack coupling with an airworthy disc pack coupling. Torque on the replacement disc pack coupling nuts and bolts must be a minimum of the run-on-tare torque plus 150 inch-lbs. to a maximum of the run-on-tare torque plus 180 inch-lbs.(b) Oil Cooler Blower Shaft (Fan Shaft) Hanger Bearing Lubrication:

To prevent failure of an oil cooler blower shaft hanger bearing (hanger bearing), P/N 406-040-339, that can result in an unsafe level of vibration, failure of the tail rotor drive system, and subsequent loss of control of the helicopter, within 25 hours TIS, and thereafter at intervals not to exceed 25 hours TIS, accomplish the following:

(1) Gain access to the oil cooler blower, P/N 206-061-432-115.
(2) Grease both oil cooler blower shaft hanger bearings.

(c) Oil Cooler Blower Hanger Bearing Inspection:
To prevent failure of the hanger bearing, P/N 406-040-339, that can result in an unsafe level of vibration, failure of the tail rotor drive system, and subsequent loss of control of the helicopter, within 25 hours TIS, and thereafter at intervals not to exceed 100 hours TIS, accomplish the following:(1) Gain access to the oil cooler blower, P/N 206-061-432-115.

(2) Remove the forward short shaft, P/N 406-040-315-111.

(3) Remove the aft short shaft, P/N 407-040-325-101.

(4) Manually rotate the oil cooler blower shaft, P/N 406-040-320-101, at various speeds and feel both the bearing hanger housings and the oil cooler blower shaft. If there is any binding or roughness indicated by feel or sound, remove the oil cooler blower shaft and replace any unairworthy hanger bearing with an airworthy hanger bearing.

(5) Grease both oil cooler blower hanger bearings.

(6) Inspect the splines on the oil cooler blower shaft and on the splined flywheel adapter, P/N 407-040-316-101, for airworthy condition.

(d) Adhesive Barrier Tape Between Tail Rotor Gearbox and Gearbox Support Assembly Removal From Helicopters Prior To Serial Number (S/N) 53225:

To prevent separation of the tail rotor gearbox from the tailboom and subsequent loss of control of the helicopter, for helicopters prior to S/N 53225, within 25 hours TIS, accomplish the following:

(1) Remove cowling and covers to expose the tail rotor gearbox (gearbox) and the gearbox support assembly, P/N 407-030-833-101.

(2) Remove the gearbox from the gearbox support assembly.

(3) Remove all corrosion inhibitive adhesive barrier tape (MIL-T-23142) between the gearbox and the gearbox support assembly faying surfaces.

(4) Reinstall the gearbox.

(i) When reinstalling the gearbox, DO NOT use barrier tape on faying surfaces.

(ii) Coat the dowel pins and the shank portion of the gearbox studs that interface with the gearbox support assembly with epoxy polyamide primer (MIL-P-23377).

(iii) Coat the gearbox support assembly mounting pads with corrosion inhibitive sealant conforming to MIL-S-81733.

(iv) Reinstall the gearbox on the gearbox support assembly and torque the nuts to the required torque within 15 minutes of primer and sealant application. Torque on the gearbox attachment nuts must be a minimum of the run-on-tare torque plus 100 inch-lbs. to a maximum of the run-on-tare torque plus 140 inch-lbs.

(e) Tail Rotor Gearbox Attachment Inspection:

To prevent separation of the tail rotor gearbox from the tailboom and subsequent loss of control of the helicopter, within 25 hours TIS, and thereafter at intervals not to exceed 25 hours TIS, accomplish the following:

(1) Remove cowling and covers to expose the tail rotor gearbox (gearbox) and gearbox support assembly, P/N 407-030-833-101.(2) Inspect the four nuts that attach the gearbox to the tailboom for proper torque. Apply a minimum torque of 120 inch-lbs. to a maximum torque of 125 inch-lbs., which includes a run-on-tare torque of 20 inch-lbs.

(i) If there is no nut or bolt movement, the torque is acceptable.

(ii) If any of the nuts or bolts move, remove the gearbox from the gearbox support assembly and accomplish the following:

(A) Inspect the tail rotor gearbox.

(1) If there is any wear on a gearbox mounting pad, replace the gearbox with an airworthy gearbox.

(2) If there is a loose, missing, or unairworthy stud or dowel pin, replace the gearbox with an airworthy gearbox.

(B) Inspect the gearbox support assembly.(1) If there is any wear on a gearbox support assembly mounting pad, remove and replace the gearbox support assembly with an airworthy gearbox support assembly.

(2) If there is a crack or elongated hole in the gearbox support assembly, remove and replace the gearbox support assembly with an airworthy gearbox support assembly.

(3) If there is any loose, cracked, or missing rivets, or cracked or corroded skin in the area of the double rivet row at the aft tailboom-to-gearbox support assembly attachment, replace all loose, cracked, or missing rivets. Repair or replace a tailboom that has cracked or corroded skin.

(C) When installing the gearbox on the gearbox support assembly:

(1) DO NOT use barrier tape on faying surfaces.(2) Coat the dowel pins and the shank portion of the gearbox studs that interface with the gearbox support assembly with epoxy polyamide primer (MIL-P-23377).

(3) Coat the gearbox support assembly mounting pads with corrosion inhibitive sealant conforming to MIL-S-81733.

(4) Torque the nuts to the required torque within 15 minutes of primer and sealant application. Torque on the gearbox attachment nuts must be a minimum of the run-on-tare torque plus 100 inch-lbs. to a maximum of the run-on-tare torque plus 140 inch-lbs.

(D) Inspect the tailboom.

(f) Report any instances of loose or undertorqued tail rotor gearbox attachment nuts, unairworthy oil cooler blower hanger bearings, unairworthy oil cooler blower shafts, unairworthy splined flywheel adapters, or disc pack couplings with more than one unairworthy disc, within 10 working days after discovery to Mr. Jurgen Priester, Aerospace Engineer, Rotorcraft Certification Office, Rotorcraft Directorate, FAA, 2601 Meacham Blvd., Fort Worth, Texas 76137-4298, telephone (817) 222-5159, fax (817) 222-5783. Reporting requirements have been approved by the Office of Management and Budget and assigned OMB control number 2120-0056.

(g) 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, Rotorcraft Certification Office. Operators shall submit their requests through an FAA Principal Maintenance Inspector, who may concur or comment and then send it to the Manager, Rotorcraft Certification Office.

NOTE 3: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Rotorcraft Certification Office.(h) Special flight permits may be issued in accordance with sections 21.197 and 21.199 of the Federal Aviation Regulations (14 CFR 21.197 and 21.199) to operate the helicopter to a location where the requirements of this AD can be accomplished.

NOTE 4: The subjects of this AD are addressed in Transport Canada AD CF-97-19, dated September 30, 1997, and AD CF-97-20, dated October 17, 1997.

(i) This amendment becomes effective on April 16, 1998, to all persons except those persons to whom it was made immediately effective by Priority Letter AD 97-24-17, issued November 20, 1997, which contained the requirements of this amendment.

Transport Canada, which is the airworthiness authority for Canada, recently notified the FAA that an unsafe condition may exist on the BHTC Model 407 helicopter. Transport Canada advises that some operators have reported a number of cracked disc pack couplings in Thomas disc coupling packs, part number (P/N) 406-040-340-101, and a few reports of cracks and breaks in the oil cooler blower and oil tank support brackets and associated airframe components. Transport Canada issued AD CF-97-19, dated September 30, 1997, to require a one-time inspection of the disc pack couplings, inspection of the oil cooler blower and oil tank support brackets for cracks, and general condition of the tail rotor assembly, tail rotor gearbox, tail rotor drive system, and tailboom. Later, Transport Canada also issued AD CF-97-20, dated October 17, 1997, to require repetitive inspections of the disc pack couplings every 25 hours time-in-service (TIS).

This helicopter model is manufactured in Canada and is type certificated for operation in the United States under the provisions of section 21.29 of the Federal Aviation Regulations (14 CFR 21.29) and the applicable bilateral airworthiness agreement. Pursuant to this bilateral airworthiness agreement, Transport Canada has kept the FAA informed about the situation described above. The FAA has examined the findings of Transport Canada, reviewed all available information including the information contained in the FAA service difficulty data base, and determined that AD action is necessary for products of this type design that are certificated for operation in the United States. After reviewing the information received from Transport Canada, the reports from operators of service difficulties, and discussions with the manufacturer, the FAA further determined that AD actions relating to other tail rotor drive system components was necessary.

On November 20, 1997, the FAA issued priority letter AD 97-24-17, applicable to BHTC Model 407 helicopters, which requires visually inspecting each disc pack coupling for scratches, cracks, fretting, or corrosion and for proper torque of the disc pack coupling retaining nuts and bolts; lubricating the oil cooler blower shaft hanger bearings; listening and feeling for binding roughness of the hanger bearings; inspecting the splines on the oil cooler blower shaft and splined flywheel adapter; removing the adhesive barrier tape from between the tail rotor gearbox (gearbox) and the gearbox support assembly; inspecting the gearbox, gearbox support assembly, and gearbox mounting pads for wear, cracks, or elongated holes; inspecting the nuts that secure the gearbox to the tailboom for proper torquing; and inspecting the skin around the area of these components for corrosion or loose, cracked, or missing rivets. Priority Letter AD 97-24-17 superseded priority letter AD 97-22-15, Docket No. 97-SW-56-AD, issued October 23, 1997, which required a portion of the same AD actions as are currently required by this AD. Those actions were prompted by numerous reports of problems related to the tail rotor drive system.

There have been several reports of disc cracks in some disc pack couplings after as few as 35 hours TIS. A crack in the disc pack coupling can result in failure of the disc pack coupling, loss of tail rotor drive, and subsequent loss of control of the helicopter.

There have also been several reports of hanger bearing roughness due to insufficient lubrication. The cause of the insufficient lubrication has not been determined. There have also been at least two reports of bearing cages and balls separating from the hanger bearing due to the lack of lubrication. Failure of a hanger bearing can result in an unsafe level of vibration, failure of the tail rotor drive system, and subsequent loss of control of the helicopter.

Finally, there have been at least ten (10) reports of undertorqued tail rotor gearbox attachment nuts. In one case, a foreign operator reported that the gearbox attachment nuts were properly torqued during an inspection at 119 hours TIS. A subsequent inspection at 300 hours TIS revealed that the gearbox attachment nuts were loose. Further inspection revealed a separated dowel pin, damaged threads on all four studs, and elongated gearbox attachment holes on the tailboom. The pilot reported feeling some vibration prior to the inspection. Another operator reported that all four gearbox attachment nuts were determined to be undertorqued after only 27.5 hours TIS since manufacture. There have also been several reports of excessive tail rotor drive system vibration from other operators. These vibrations may indicate improperly torqued tail rotor gearbox attachment nuts. There is concern that the thickness of the corrosion inhibitive adhesive barrier MIL-T-23142 tape, which was installed at the factory between the gearbox and gearbox support assembly, is reduced when the gearbox attachment nuts are torqued to the required torque value. This reduction in tape thickness results in a lower clamping force, which allows relative motion between the gearbox and the gearbox support assembly due to loss of torque on the gearbox attachment nuts and studs. The helicopter manufacturer has already incorporated a design change that eliminates the barrier tape, starting with helicopter serial number (S/N) 53225. Loss of torque on the gearbox attachment nuts could result in separation of the tail rotor gearbox from the tailboom and subsequent loss of control of the helicopter.

Since the unsafe condition described is likely to exist or develop on other BHTC Model 407 helicopters of the same type design, the FAA issued superseding priority letter AD 97-24-17. The AD requires visually inspecting each disc pack coupling for scratches, cracks, fretting, or corrosion and for proper torque of the disc pack coupling retaining nuts and bolts; lubricating the oil cooler blower shaft hanger bearings; listening and feeling for binding or roughness of the oil cooler blower shaft hanger bearings; inspecting the splines on the oil cooler blower shaft and splined flywheel adapter; removing the adhesive barrier tape from between the tail rotor gearbox (gearbox) and the gearbox support assembly; inspecting the gearbox, gearbox support assembly, and gearbox mounting pads for wear, cracks, or elongated holes; inspecting the nuts that secure the gearbox to the tailboom for proper torquing; and inspecting the tailboom skin around the area of these components for corrosion or loose, cracked, or missing rivets. The tail rotor drive system provides the power to the tail rotor to permit the operator to offset the torque effects of the main rotor system during flight. Due to the criticality of these tail rotor drive system components to the continued safe flight of this model helicopter and the short times before compliance is required, this AD must be issued immediately.

Since it was found that immediate corrective action was required, notice and opportunity for prior public comment thereon were impracticable and contrary to the public interest, and good cause existed to make the AD effective immediately by individual letters issued on November 20, 1997 to all known U.S. owners and operators of BHTC Model 407 helicopters. These conditions still exist, and the AD is hereby published in the Federal Register as an amendment to section 39.13 of the Federal Aviation Regulations (14 CFR 39.13) to make it effective to all persons.

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 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 theircomments submitted in response to this rule must submit a self-addressed, stamped postcard on which the following statement is made: "Comments to Docket No. 97-SW-67-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 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, 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 a newairworthiness directive to read as follows: