TRANSDIGM INC., TRANSDIGM GROUP INCORPORATED, THE GUARANTORS NAMED HEREIN, AND THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A., as Trustee SECONDSUPPLEMENTAL INDENTURE Dated as of December 9, 2011 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% SeniorSubordinated Notes due 2018 of TransDigm Inc.

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

SECOND SUPPLEMENTAL INDENTURE

Dated as of December 9, 2011

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 SECOND SUPPLEMENTAL INDENTURE (this “Supplemental Indenture”), dated as of December 9, 2011, by and among Harco Laboratories, Incorporated, a Connecticut corporation (“Harco” or 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”) and Schneller International Sales Corp., an Ohio corporation (“Schneller International” and, 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 and Schneller, 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 “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

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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