TRANSDIGM UK HOLDINGS PLC, TRANSDIGM INC., TRANSDIGM GROUP INCORPORATED, THE GUARANTORS NAMED HEREIN, AND THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A., as Trustee THIRD SUPPLEMENTAL INDENTURE Dated as of March 29, 2019 to Indenture Dated as of May...

Exhibit 4.56

TRANSDIGM UK HOLDINGS PLC,

TRANSDIGM INC.,

TRANSDIGM GROUP INCORPORATED,

THE GUARANTORS NAMED HEREIN,

AND

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

as Trustee

THIRD SUPPLEMENTAL INDENTURE

Dated as of March 29, 2019

to

Indenture

Dated as of May 8, 2018

by and among

TRANSDIGM UK HOLDINGS PLC,

TRANSDIGM INC.,

TRANSDIGM GROUP INCORPORATED,

THE GUARANTORS NAMED THEREIN,

AND

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

as Trustee

6.875% Senior Subordinated Notes due 2026

of TransDigm UK Holdings plc

This THIRD SUPPLEMENTAL INDENTURE (this “Supplemental Indenture”), dated as of March 29, 2019, is entered into by and among ▇▇▇▇▇▇▇▇▇ Technologies Corporation, a Delaware corporation, Souriau USA, Inc., a Delaware corporation, Esterline International Company, a Delaware corporation, ▇▇▇▇▇ Holding Corporation, a Delaware corporation, ▇▇▇▇▇ International Corporation, a Delaware corporation, ▇▇▇▇▇ Technology Group, Inc., a Delaware corporation, TA Aerospace Co., a California corporation, Esterline US LLC, a Delaware Limited Liability Company, CMC Electronics Aurora LLC, a Delaware Limited Liability Company, Advanced Input Devices, Inc., a Delaware corporation, Esterline Europe Company LLC, a Delaware Limited Liability Company, Esterline Georgia US LLC, a Delaware Limited Liability Company, Esterline Federal LLC, a Delaware Limited Liability Company, Angus Electronics Co., a Delaware corporation, Avista Incorporated, a Wisconsin corporation, Esterline Sensors Services Americas, Inc., a Delaware corporation, ▇▇▇▇▇▇▇▇▇ Technologies SGIP LLC, a Delaware Limited Liability Company, Hytek Finishes Co., a Delaware corporation, ▇▇▇▇▇ Corporation, a California corporation, ▇▇▇▇▇ Electric Co., a Delaware corporation, NMC Group Inc., a California corporation, Norwich Aero Products, Inc., a New York corporation, Palomar Products, Inc., a Delaware corporation, ▇▇▇▇▇ ▇▇▇▇▇▇▇▇▇ ▇▇▇▇ LLC, a Delaware Limited Liability Company, Korry Electronics Co., a Delaware corporation, Memtron Technologies Co., a Delaware corporation, Sunbank Family of Companies LLC, a California Limited Liability Company, Joslyn Sunbank Company, LLC, a California Limited Liability Company, Armtec Defense Products Co., a Delaware corporation, Armtec Countermeasures Co., a Delaware corporation, Armtec Countermeasures TNO Co., a Delaware corporation, Racal Acoustics, Inc., a Delaware corporation, and Gamesman Inc., a Nevada corporation (collectively, the “Guaranteeing Subsidiaries”), TransDigm UK Holdings plc, a United Kingdom public limited company (the “Issuer”) TransDigm Inc., a Delaware corporation (the “Company”), TransDigm Group Incorporated, a Delaware corporation (“TD Group”), ▇▇▇▇▇ Rite Aerospace, Inc., a California corporation (“▇▇▇▇▇ 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”), ▇▇▇▇▇▇ Aerospace Inc., a Delaware corporation (“▇▇▇▇▇▇”), 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”), ▇▇▇▇▇ Aerospace Inc., a Delaware corporation (“▇▇▇▇▇ Aerospace”), CEF Industries, LLC, a Delaware limited liability company (“CEF”), Acme Aerospace, Inc., a Delaware corporation (“Acme”), ▇▇▇▇▇ Aerospace, Inc., a Delaware corporation (“▇▇▇▇▇”), Semco Instruments, Inc., a Delaware corporation, (“Semco”), ▇▇▇▇▇▇▇▇ Corporation, a California corporation (“▇▇▇▇▇▇▇▇”), ▇▇▇▇▇▇▇▇▇ Aerospace DE, Inc., a Delaware corporation (“▇▇▇▇▇▇▇▇▇ Aerospace DE”), ▇▇▇▇▇▇▇▇▇ Aerospace Holdings, Inc., a Delaware corporation (“▇▇▇▇▇▇▇▇▇ Aerospace Holdings”), ▇▇▇▇▇▇▇▇▇ Aerospace US LLC, a Delaware limited liability company (“▇▇▇▇▇▇▇▇▇ Aerospace US”), Texas Rotronics, Inc., a Texas corporation (“Rotronics”), Electromech Technologies LLC (formerly Western Sky Industries, LLC), a Delaware limited liability company (“Electromech”), ▇▇▇▇▇▇▇▇▇ LLC, a Delaware limited liability company (“▇▇▇▇▇▇▇▇▇”), HarcoSemco LLC, a Connecticut limited liability company (“HarcoSemco”), AmSafe Global Holdings, Inc., a Delaware corporation (“AmSafe Global”), Bridport Holdings, Inc., a Delaware corporation (“Bridport Holdings”), AmSafe, Inc., a Delaware corporation (“AmSafe Inc.”), Shield Restraint Systems, Inc. (formerly AmSafe Commercial Products, Inc.), a Delaware corporation (“Shield”), Bridport-Air Carrier, Inc., a Washington corporation (“Bridport-Air”), Bridport Erie Aviation, Inc., a Delaware corporation (“Bridport Erie”), Arkwin Industries, Inc., a New York corporation (“Arkwin”), Whippany Actuation Systems, LLC, a Delaware limited liability company (“Whippany”), 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”), Airborne Systems North America of NJ Inc., a New Jersey corporation (“Airborne Systems North America NJ”), Telair US LLC, a Delaware limited liability company (“Telair US”), Telair International LLC, a Delaware limited liability company (“Telair International”), Pexco Aerospace, Inc., a Delaware corporation (“Pexco Aerospace”), PneuDraulics, Inc., a California corporation (“PneuDraulics”), Breeze-Eastern LLC, a Delaware limited liability company (“Breeze-Eastern”), ILC Holdings, Inc., a Delaware corporation (“ILC Holdings”), Data Device Corporation, a Delaware corporation (“DDC”), Beta Transformer Technology Corporation, a New York corporation (“Beta Corporation”), Beta Transformer Technology LLC, a Delaware limited liability company (“Beta LLC”), Young & Franklin Inc., a New York corporation (“Young & Franklin”), Tactair Fluid Controls, Inc., a New York corporation (“Tactair”), ▇▇▇▇▇▇▇ Liverpool LLC, a Delaware limited liability company (“▇▇▇▇▇▇▇ Liverpool”), North Hills Signal

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Processing Corp., a Delaware corporation (“North Hills”), North Hills Signal Processing Overseas Corp., a Delaware corporation (“North Hills Overseas”), Kirkhill, Inc., a Delaware corporation (“Kirkhill”), Extant Components Group Holdings, Inc., a Delaware corporation (“Extant”), Extant Components Group Intermediate, Inc., a Delaware corporation (“Extant Intermediate”), Symetrics Industries, LLC, a Florida limited liability company (“Symetrics Industries”), Symetrics Technology Group, LLC, a Florida limited liability company (“Symetrics Technology”), TEAC Aerospace Holdings, Inc., a Delaware corporation (“TEAC Holdings”), TEAC Aerospace Technologies, Inc., a Delaware corporation (“TEAC Technologies”), and Skandia, Inc., an Illinois corporation (collectively with the Company, TD Group, ▇▇▇▇▇ Rite, Marathon, Champion, Avionic, ▇▇▇▇▇▇, CDA, ATI, Avtech, Transicoil, AeroControlex, ▇▇▇▇▇ Aerospace, CEF, Acme, ▇▇▇▇▇, Semco, ▇▇▇▇▇▇▇▇, ▇▇▇▇▇▇▇▇▇ Aerospace DE, ▇▇▇▇▇▇▇▇▇ Aerospace Holdings, ▇▇▇▇▇▇▇▇▇ Aerospace US, Rotronics, Electromech, ▇▇▇▇▇▇▇▇▇, HarcoSemco, AmSafe Global, Bridport Holdings, AmSafe Inc., Shield, Bridport-Air, Bridport Erie, Arkwin, Whippany, Aerosonic, Avionics Specialties, Airborne Global, Airborne Holdings, Airborne Acquisitions, Airborne Systems NA, Airborne Systems North America, Airborne Systems North America CA, Airborne Systems North America NJ, Telair US, Telair International, Pexco Aerospace, PneuDraulics, Breeze-Eastern, ILC Holdings, DDC, Beta Corporation, Beta LLC, Young & Franklin, Tactair, ▇▇▇▇▇▇▇ Liverpool, North Hills, North Hills Overseas, Kirkhill, Extant, Extant Intermediate, Symetrics Industries, Symetrics Technology, TEAC Holdings and TEAC Technologies, the “Existing Guarantors”), and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Trustee”) under the Indenture referred to below.

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WITNESSETH:

WHEREAS, the Issuer and the Existing Guarantors have heretofore executed and delivered to the Trustee an indenture, dated as of May 8, 2018 (as supplemented by the First Supplemental Indenture thereto, dated as of May 22, 2018, and the Second Supplemental Indenture thereto, dated as of July 31, 2018, the “Indenture”), providing for the issuance by the Issuer of 6.875% Senior Subordinated Notes due 2026 (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 Subsidiaries (the “Guarantee”);

WHEREAS, all things necessary to make this Supplemental Indenture the legal, valid and binding obligation of the Issuer, 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, the Guaranteeing Subsidiaries covenant and agree for the equal and ratable benefit of the Holders of the Notes as follows:

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[Signature page 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.

[Signature page to the Third Supplemental Indenture – TD UK Notes]

[Signature page to the Third Supplemental Indenture – TD UK Notes]

[Signature page to the Third Supplemental Indenture – TD UK Notes]

[Signature page to the Third Supplemental Indenture – TD UK Notes]