Exhibit 4.1

TRANSDIGM INC.,

TRANSDIGM GROUP INCORPORATED,

THE GUARANTORS NAMED HEREIN,

AND

THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A.,

as Trustee

FOURTH SUPPLEMENTAL INDENTURE

Dated as of September 14, 2012

to

Indenture

Dated as of December 14, 2010

by and among

TRANSDIGM INC.,

TRANSDIGM GROUP INCORPORATED,

THE GUARANTORS NAMED THEREIN,

AND

THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A.,

as Trustee

7.75% Senior Subordinated Notes due 2018

of TransDigm Inc.

This FOURTH SUPPLEMENTAL INDENTURE (this “Supplemental Indenture”), dated as of September 14, 2012, by and among Aero-Instruments Co., LLC, an Ohio limited liability company (the “Guaranteeing Subsidiary”), 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 (“Champion”), Avionic Instruments LLC, a Delaware limited liability company (“Avionic”), Skurka Aerospace Inc., a Delaware corporation (“Skurka”), CDA InterCorp LLC, a Florida limited liability company (“CDA”), Aviation Technologies, Inc., a Delaware corporation (“ATI”), Avtech Corporation, a Washington corporation (“Avtech”), Transicoil LLC, a Delaware limited liability company (“Transicoil”), AeroControlex Group, Inc., a Delaware corporation (“AeroControlex”), Malaysian Aerospace Services, Inc., a Delaware corporation (“Malaysian”), Bruce Aerospace Inc., a Delaware corporation (“Bruce Aerospace”), Bruce Industries, Inc., a Colorado corporation (“Bruce Industries”), CEF Industries, LLC, a Delaware limited liability company (“CEF”), Aircraft Parts Corporation, a New York corporation (“APC”), Acme Aerospace, Inc., a Delaware corporation (“Acme”), Dukes Aerospace, Inc., a Delaware corporation (“Dukes”), Semco Instruments, Inc., a Delaware corporation, (“Semco”), Hartwell Corporation, a California corporation (“Hartwell”), McKechnie Aerospace DE, Inc., a Delaware corporation (“McKechnie Aerospace DE”), McKechnie Aerospace Holdings, Inc., a Delaware corporation (“McKechnie Aerospace Holdings”), McKechnie Aerospace Investments, Inc., a Delaware corporation (“McKechnie Aerospace Investments”), McKechnie Aerospace US LLC, a Delaware limited liability company (“McKechnie Aerospace US”), Texas Rotronics, Inc., a Texas corporation (“Rotronics”), Western Sky Industries, LLC, a Delaware limited liability company (“Western”), Schneller Holdings LLC, a Delaware limited liability company (“Schneller Holdings”), Schneller LLC, a Delaware limited liability company (“Schneller”), Schneller International Sales Corp., an Ohio corporation (“Schneller International”), Harco Laboratories, Incorporated, a Connecticut corporation (“Harco”), AmSafe Global Holdings, Inc., a Delaware corporation (“AmSafe Global”), AP Global Holdings, Inc., a Delaware corporation (“AP Global Holdings”), AP Global Acquisition Corp., a Delaware corporation (“AP Global Acquisition”), AmSafe Industries, Inc., a Delaware corporation (“AmSafe Industries”), Bridport Holdings, Inc., a Delaware corporation (“Bridport Holdings”), AmSafe, Inc., a Delaware corporation (“AmSafe Inc.”), AmSafe Aviation, Inc., a Georgia corporation (“AmSafe Aviation”), AmSafe Commercial Products, Inc., a Delaware corporation (“AmSafe Commercial”), AmSafe Bridport, Inc., a Delaware corporation (“AmSafe Bridport”), Bridport-Air Carrier, Inc., a Washington corporation (“Bridport-Air”), Bridport Erie Aviation, Inc., a Delaware corporation (“Bridport Erie”), and AmSafe – C Safe, Inc., a Delaware corporation (together with TD Group, Adams Rite, Marathon, Champion, Avionic, Skurka, CDA, ATI, Avtech, Transicoil, AeroControlex, Malaysian, Bruce Aerospace, Bruce Industries, CEF, APC, Acme, Dukes, Semco, Hartwell, McKechnie Aerospace DE, McKechnie Aerospace Holdings, McKechnie Aerospace Investments, McKechnie Aerospace US, Rotronics, Western, Schneller Holdings, Schneller, Schneller International, Harco, AmSafe Global, AP Global Holdings, AP Global Acquisition, AmSafe Industries, Bridport Holdings, AmSafe Inc., AmSafe Aviation, AmSafe Commercial, AmSafe Bridport, Bridport-Air and Bridport Erie, the “Existing Guarantors”), and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Trustee”) under the Indenture referred to below. Capitalized terms used herein and not otherwise defined shall have the meaning assigned to them in the Indenture.

WITNESSETH:

WHEREAS, the Company and the Existing Guarantors have heretofore executed and delivered to the Trustee an indenture, dated as of December 14, 2010 (as supplemented by the First Supplemental Indenture thereto, dated as of September 22, 2011, the Second Supplemental Indenture thereto, dated as of December 9, 2011, and the Third Supplemental Indenture, dated as of February 15, 2012, the “Indenture”), providing for the issuance by the Company of 7.75% Senior Subordinated Notes due 2018 (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 “Guarantee”);

WHEREAS, all things necessary to make this Supplemental Indenture the legal, valid and binding obligation of the Company, the Existing Guarantors and the Guaranteeing Subsidiary have been done; and

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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 of the Notes.

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

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

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