TRANSDIGM INC., TRANSDIGM GROUP INCORPORATED, THE GUARANTORS NAMED HEREIN, AND THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A., as Trustee FIRST SUPPLEMENTAL INDENTURE Dated as of December 19, 2013 to Indenture Dated as of July 1, 2013 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 7.50% Senior Subordinated Notes due 2021 of TransDigm Inc.

Exhibit 4.3

TRANSDIGM INC.,

TRANSDIGM GROUP INCORPORATED,

THE GUARANTORS NAMED HEREIN,

AND

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

as Trustee

FIRST SUPPLEMENTAL INDENTURE

Dated as of December 19, 2013

to

Indenture

Dated as of July 1, 2013

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

7.50% Senior Subordinated Notes due 2021

of TransDigm Inc.

This FIRST SUPPLEMENTAL INDENTURE (this “Supplemental Indenture”), dated as of December 19, 2013, by and among Aerosonic LLC, a Delaware limited liability company (“Aerosonic”), Avionics Specialties, Inc., a Virginia corporation (“Avionics Specialties”), Airborne Global, Inc., a Delaware corporation (“Airborne Global”), Airborne Holdings, Inc., a Delaware Corporation (“Airborne Holdings”), Airborne Acquisition, Inc., a Delaware corporation (“Airborne Acquisitions”), Airborne Systems NA Inc., a Delaware corporation (“Airborne Systems NA”), Airborne Systems North America Inc., a Delaware corporation (“Airborne Systems North America”), Airborne Systems North America of CA Inc., a Delaware corporation (“Airborne Systems North America CA”), and Airborne Systems North America of NJ Inc., a New Jersey corporation (collectively with Aerosonic, Avionics Specialties, Airborne Global, Airborne Holdings, Airborne Acquisition, Airborne Systems NA, Airborne Systems North America and Airborne Systems North America CA, 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 (“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”), AvtechTyee, Inc., 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”), 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 Commercial Products, Inc., a Delaware corporation (“AmSafe Commercial”), Bridport-Air Carrier, Inc., a Washington corporation (“Bridport-Air”), Bridport Erie Aviation, Inc., a Delaware corporation (“Bridport Erie”), AmSafe – C Safe, Inc., a Delaware corporation (“AmSafe – C Safe”), Arkwin Industries, Inc., a New York corporation (“Arkwin”) and Whippany Actuation Systems, LLC, a Delaware limited liability company (collectively with TD Group, Adams Rite, Marathon, Champion, Avionic, Skurka, CDA, ATI, Avtech, Transicoil, AeroControlex, Malaysian, Bruce Aerospace, Bruce Industries, CEF, 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 Commercial, Bridport-Air, Bridport Erie, AmSafe – C Safe and Arkwin, 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 July 1, 2013 (the “Indenture”), providing for the issuance by the Company of 7.50% Senior Subordinated Notes due 2021 (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 Subsidiaries 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, each of the Guaranteeing Subsidiaries covenants and agrees for the equal and ratable benefit of the Holders of the Notes as follows:

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