EXHIBIT 10.1

TRANSDIGM INC.,

TRANSDIGM GROUP INCORPORATED,

THE GUARANTORS named herein

and

THE BANK OF NEW YORK TRUST COMPANY, N.A., as Trustee

FOURTH SUPPLEMENTAL INDENTURE

Dated as of August 10, 2007

To

Indenture Dated as of June 23, 2006

by and among

TRANSDIGM INC.,

TRANSDIGM GROUP INCORPORATED,

the GUARANTORS named therein and

THE BANK OF NEW YORK TRUST COMPANY, N.A., as Trustee

7-3/4% Senior Subordinated Notes due 2014

of TransDigm Inc.

FOURTH SUPPLEMENTAL INDENTURE (this “SUPPLEMENTAL INDENTURE”), dated as of August 10, 2007, among Bruce Aerospace, Inc., a Delaware corporation (“BRUCE AEROSPACE”), Bruce Industries, Inc., a Colorado corporation (“BRUCE INDUSTRIES” and, together with Bruce Aerospace, the “GUARANTEEING SUBSIDIARIES”), TransDigm Inc., a Delaware corporation (the “COMPANY”), TransDigm Group Incorporated, a Delaware corporation (“TD GROUP”), Adams Rite Aerospace, Inc., a California corporation (“ADAMS RITE”), MarathonNorco Aerospace, Inc., a Delaware corporation (“MARATHON”), Champion Aerospace LLC, a Delaware limited liability company and successor to Champion Aerospace Inc. (“CHAMPION”), Avionic Instruments LLC, a Delaware limited liability company and successor to Avionic Instruments Inc. (“AVIONIC”), Skurka Aerospace Inc., a Delaware corporation (“SKURKA”), CDA InterCorp LLC, a Florida limited liability company and successor to CDA Intercorp (“CDA”), Aviation Technologies, Inc., a Delaware corporation (“ATI”), Avtech Corporation, a Washington corporation (“AVTECH”), Transicoil LLC, a Delaware limited liability company and successor to Transicoil Corp. (“TRANSICOIL”), AeroControlex Group, Inc., a Delaware corporation (“AEROCONTROLEX”),  Malaysian Aerospace Services, Inc., a Delaware corporation (“MALAYSIAN” and, together with TD Group, Adams Rite, Marathon, Champion, Avionic, Skurka, CDA, ATI, Avtech, Transicoil and AeroControlex, the “EXISTING GUARANTORS”), and The Bank of New York Trust Company, N.A., as trustee under the indenture referred to below (the “TRUSTEE”).

W I T N E S S E T H

WHEREAS, the Company and the Existing Guarantors have heretofore executed and delivered to the Trustee an indenture (as supplemented by the First Supplemental Indenture thereto, dated as of November 2, 2006, the Second Supplemental Indenture, dated as of February 7, 2007 and the Third Supplemental Indenture, dated June 29, 2007, the “INDENTURE”), dated as of June 23, 2006, providing for the issuance by the Company of 7-3/4% Senior Subordinated Notes due 2014 (the “NOTES”) and the guarantees thereof by each of the Existing Guarantors;

WHEREAS, the Indenture provides that under certain circumstances described therein, newly created or acquired Domestic Restricted Subsidiaries shall execute and deliver to the Trustee a supplemental indenture to the Indenture providing for a senior subordinated guarantee of payment of the Notes by such Domestic Restricted Subsidiary (the “SUBSIDIARY GUARANTEE”);

WHEREAS, pursuant to Section 9.01(g) of the Indenture, the Trustee is authorized to execute and deliver this Supplemental Indenture without the consent of the Holders Notes; and

WHEREAS, in June 2007, the following intercompany mergers were effected: (i) West Coast Specialties, Inc. was merged with and into Avtech with Avtech continuing as the surviving corporation; (ii) Christie Electric Corp. was merged with and into Marathon with Marathon continuing as the surviving corporation; (iii) ZMP, Inc. was merged with and into Adams Rite with Adams Rite continuing as the surviving corporation; (iv) DAC Realty Corp.

was merged with and into Avionic with Avionic continuing as the surviving entity; and (v) Sweeney Engineering Corp. was merged with and into the Company with the Company continuing as the surviving corporation.

NOW THEREFORE, in consideration of the foregoing and for other good and valuable consideration, the receipt of which is hereby acknowledged, the Guaranteeing Subsidiaries covenant and agree for the equal and ratable benefit of the Holders of the Notes as follows:

1.  CAPITALIZED TERMS.  Capitalized terms used herein without definition shall have the meanings assigned to them in the Indenture.

2.  GUARANTEE, ETC.  The Guaranteeing Subsidiaries hereby agree that from and after the date hereof each shall be a Guarantor under the Indenture and be bound by the terms thereof applicable to Guarantors and shall be entitled to all of the rights and subject to all the obligations of a Guarantor thereunder.

3.  RATIFICATION OF INDENTURE; SUPPLEMENTAL INDENTURES PART OF INDENTURE.  The Indenture is in all respects ratified and confirmed and all the terms, conditions and provisions thereof shall remain in full force and effect.  This Supplemental Indenture shall form a part of the Indenture for all purposes, and every holder of Notes heretofore or hereafter authenticated and delivered shall be bound hereby.

4.  EXECUTION AND DELIVERY.  The Guaranteeing Subsidiaries agree that the Guarantee granted by each of them pursuant to the terms hereof shall remain in full force and effect notwithstanding any failure to endorse on each Note a notation of such Guarantee.

5.  NO RECOURSE AGAINST OTHERS.  No past, present or future director, officer, employee, incorporator, stockholder or agent of any of the Guaranteeing Subsidiaries (or any successor entity) (other than the Company or the Existing Guarantors), as such, shall have any liability for any obligations of the Company, TD Group, the Guaranteeing Subsidiaries or any other Guarantor under the Notes, any Guarantee, the Indenture or this Supplemental Indenture or for any claim based on, in respect of, or by reason of, such obligations or their creation.  Each Holder of the Notes by accepting a Note waives and releases all such liability.  The waiver and release are part of the consideration for issuance of the Notes.

6.  NEW YORK LAW TO GOVERN.  THE INTERNAL LAW OF THE STATE OF NEW YORK SHALL GOVERN AND BE USED TO CONSTRUE THIS SUPPLEMENTAL INDENTURE AND THE GUARANTEE GRANTED HEREUNDER WITHOUT GIVING EFFECT TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW TO THE EXTENT THAT THE APPLICATION OF THE LAWS OF ANOTHER JURISDICTION WOULD BE REQUIRED THEREBY.

7.  COUNTERPART ORIGINALS.  The parties may sign any number of copies of this Supplemental Indenture.  Each signed copy shall be an original, but all of them together represent the same agreement.

8.  EFFECT OF HEADINGS.  The Section headings have been inserted for convenience of reference only, are not to be considered part of this Supplemental Indenture and shall in no way modify or restrict any of the terms or provisions hereof.

9.  THE TRUSTEE.  The Trustee shall not be responsible in any manner whatsoever for or in respect of the validity or sufficiency of this Supplemental Indenture or for or in respect of the recitals contained herein, all of which recitals are made solely by the Guaranteeing Subsidiaries, the Existing Guarantors and the Company.

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IN WITNESS WHEREOF, the parties hereto have caused this Fourth Supplemental Indenture to be duly executed as of the date first above written.

Fourth Supplemental Indenture