Exhibit 10.4

EXECUTION VERSION

JOINDER AGREEMENT

THIS JOINDER AGREEMENT (this “Agreement”), dated as of September 3, 2010, is
entered into between SEMCO INSTRUMENTS, INC., a Delaware corporation (the “New
Subsidiary”), and CREDIT SUISSE AG, as Agent, under that certain Credit
Agreement, dated as of June 23, 2006 (as the same may be amended, supplemented
or otherwise modified from time to time, the “Credit Agreement”), among
TransDigm Inc., a Delaware corporation (the “Borrower”), TransDigm Group
Incorporated, a Delaware corporation, the Subsidiaries of the Borrower from time
to time party thereto, the Lenders from time to time party thereto and the
Agent, as amended by (i) that certain Amendment No. 1, Consent and Agreement
dated as of January 25, 2007 and (ii) that certain Assumption Agreement dated as
of February 7, 2007. All capitalized terms used herein and not otherwise defined
shall have the meanings set forth in the Credit Agreement.

The New Subsidiary and the Agent, for the benefit of the Lenders, hereby agree
as follows:

1. The New Subsidiary hereby acknowledges, agrees and confirms that, by its
execution of this Agreement, the New Subsidiary will be deemed to be a Loan
Party under the Credit Agreement and a Loan Guarantor for all purposes of the
Credit Agreement and shall have all of the obligations of a Loan Party and a
Loan Guarantor thereunder as if it had executed the Credit Agreement. The New
Subsidiary hereby ratifies, as of the date hereof, and agrees to be bound by,
all of the terms, provisions and conditions contained in the Credit Agreement,
including without limitation (a) all of the representations and warranties of
the Loan Parties set forth in Article III of the Credit Agreement (to the extent
made or deemed made on or after the effective date hereof), (b) all of the
covenants set forth in Articles V and VI of the Credit Agreement and (c) all of
the guaranty obligations set forth in the Guarantee and Collateral Agreement.
Without limiting the generality of the foregoing terms of this paragraph 1, the
New Subsidiary, subject to the limitations set forth in the Guarantee and
Collateral Agreement, hereby absolutely and unconditionally guarantees, jointly
and severally with the other Loan Guarantors, to the Agent and the Lenders, the
prompt payment of the Secured Obligations in full when due (whether at stated
maturity, upon acceleration or otherwise) to the extent of and in accordance
with the Guarantee and Collateral Agreement.

2. If required, the New Subsidiary is, simultaneously with the execution of this
Agreement, executing and delivering such Collateral Documents (and such other
documents and instruments) as requested by the Agent in accordance with the
Credit Agreement, except for items listed on Schedule I hereto which will be
executed and delivered as soon as practical.

3. The New Subsidiary hereby waives acceptance by the Agent and the Lenders of
the guaranty by the New Subsidiary upon the execution of this Agreement by the
New Subsidiary.

4. This Agreement may be executed in any number of counterparts, each of which
when so executed and delivered shall be an original, but all of which shall

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constitute one and the same instrument. Delivery of an executed signature page
to this Agreement by facsimile transmission or electronic transmission
(including in pdf format) shall be as effective as delivery of a manually signed
counterpart of this Agreement.

5. THIS AGREEMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES HEREUNDER SHALL
BE CONSTRUED IN ACCORDANCE WITH AND GOVERNED BY THE LAWS OF THE STATE OF NEW
YORK.

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2

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IN WITNESS WHEREOF, the New Subsidiary has caused this Agreement to be duly
executed by its authorized officer, and the Agent, for the benefit of the
Lenders, has caused the same to be accepted by its authorized officer, as of the
day and year first above written.

 

SEMCO INSTRUMENTS, INC.

  by  

/s/ Gregory Rufus

    Name:   Gregory Rufus     Title:   Secretary and Treasurer

Semco Instruments, Inc. Joinder Agreement

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Acknowledged and accepted:

CREDIT SUISSE AG, CAYMAN ISLANDS BRANCH, as Agent

  by  

/s/ Ari Bruger

    Name:   Ari Bruger     Title:   Vice President   by  

/s/ Kevin Buddhdew

    Name:   Kevin Buddhdew     Title:   Associate

Semco Instruments, Inc. Joinder Agreement

 

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SCHEDULE I

ITEMS TO BE EXECUTED AND DELIVERED

None.

 

Schedule I-1