Exhibit 4.2

                             SUPPLEMENTAL INDENTURE



          SUPPLEMENTAL INDENTURE (this "SUPPLEMENTAL INDENTURE"), dated as of
April 23, 1999 among Adams Rite Aerospace, Inc. a California corporation, ZMP,
Inc. a California corporation (collectively, the "GUARANTEEING SUBSIDIARIES,"
and, each a "GUARANTEEING SUBSIDIARY"), each a subsidiary of TransDigm Inc., a
Delaware corporation (the "COMPANY"), the Company, TransDigm Holding Company, a
Delaware corporation ("HOLDINGS") and Marathon Power Technologies Company, a
Delaware corporation ("MARATHON" and together with Holdings and the Guaranteeing
Subsidiaries, the "GUARANTORS") and State Street Bank and Trust Company, as
trustee under the indenture referred to below (the "TRUSTEE").

W I T N E S S E T H

          WHEREAS, the Company, Holdings and Marathon have heretofore executed
and delivered to the Trustee an indenture (the "INDENTURE"), dated as of
December 3, 1998 providing for the issuance of an aggregate principal amount of
up to $200.0 million of 10 3/8% Senior Subordinated Notes due 2008 (the "Notes")
and the guarantees thereof by Holdings and Marathon;

          WHEREAS, the Indenture provides that under certain circumstances newly
acquired Subsidiaries of the Company shall execute and deliver to the Trustee a
supplemental indenture pursuant to which such Subsidiaries shall unconditionally
guarantee all of the Company's Obligations under the Notes and the Indenture on
the terms and conditions set forth therein (the "GUARANTEE"); and

          WHEREAS, pursuant to Section 9.01 of the Indenture, the Trustee is
authorized to execute and deliver this Supplemental Indenture.

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

          1. CAPITALIZED TERMS. Capitalized terms used herein without definition
shall have the meanings assigned to them in the indenture.

          2. AGREEMENT TO GUARANTEE. Each Guaranteeing Subsidiary hereby agrees
as follows:

          (a)  Along with all Guarantors named in the Indenture, to jointly and
               severally Guarantee to each Holder of a Note authenticated and
               delivered by the 


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               Trustee and to the Trustee and its successors and assigns,
               irrespective of the validity and enforceability of the Indenture,
               the Notes or the obligations of the Company hereunder or
               thereunder, that:

               (i)    the principal of and interest on the Notes will be
                      promptly paid in full when due, whether at maturity, by
                      acceleration, redemption or otherwise, and interest on the
                      overdue principal of and interest on the Notes, if any, if
                      lawful, and all other obligations of the Company to the
                      Holders or the Trustee hereunder or thereunder will be
                      promptly paid in full or performed, all in accordance with
                      the terms hereof and thereof; and

               (ii)   in case of any extension of time of payment or renewal of
                      any Notes or any of such other obligations, that same will
                      be promptly paid in full when due or performed in
                      accordance with the terms of the extension or renewal,
                      whether at stated maturity, by acceleration or otherwise.
                      Failing payment when due of any amount so guaranteed or
                      any performance so guaranteed for whatever reason, the
                      Guarantors shall be jointly and severally obligated to pay
                      the same immediately.

          (b)  The obligations hereunder shall be unconditional, irrespective of
               the validity, regularity or enforceability of the Notes or the
               Indenture, the absence of any action to enforce the same, any
               waiver or consent by any Holder of the Notes with respect to any
               provisions hereof or thereof, the recovery of any judgment
               against the Company, any action to enforce the same or any other
               circumstance which might otherwise constitute a legal or
               equitable discharge or defense of a Guarantor.


          (c)  The following is hereby waived: diligence, presentment, demand of
               payment, filing of claims with a court in the event of insolvency
               or bankruptcy of the Company, any right to require a proceeding
               first against the Company, protest, notice and all demands
               whatsoever.


          (d)  This Guarantee shall not be discharged except by complete
               performance of the obligations contained in the Notes and the
               Indenture or pursuant to Section 6 hereof.


          (e)  If any Holder or the Trustee is required by any court or
               otherwise to return to the Company, the Guarantors, or any
               custodian, Trustee, liquidator or other similar official acting
               in relation to either the Company or the Guarantors, any amount
               paid by either to the Trustee or such Holder, this Guarantee, to
               the extent theretofore discharged, shall be reinstated in full
               force and effect.

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          (f)  Each Guaranteeing Subsidiary shall not be entitled to any right
               of subrogation in relation to the Holders in respect of any
               obligations guaranteed hereby until payment in full of all
               obligations guaranteed hereby.


          (g)  As between the Guarantors, on the one hand, and the Holders and
               the Trustee, on the other hand, (x) the maturity of the
               obligations guaranteed hereby may be accelerated as provided in
               Article 6 of the Indenture for the purposes of this Guarantee,
               notwithstanding any stay, injunction or other prohibition
               preventing such acceleration in respect of the obligations
               guaranteed hereby, and (y) in the event of any declaration of
               acceleration of such obligations as provided in Article 6 of the
               Indenture, such obligations (whether or not due and payable)
               shall forthwith become due and payable by the Guarantors for the
               purpose of this Guarantee.


          (h)  The Guarantors shall have the right to seek contribution from any
               non-paying Guarantor so long as the exercise of such right does
               not impair the rights of the Holders under the Guarantee.


          (i)  Pursuant to Section 11.03 of the Indenture, after giving effect
               to any maximum amount and any other contingent and fixed
               liabilities that are relevant under any applicable Bankruptcy or
               fraudulent conveyance laws, and after giving effect to any
               collections from, rights to receive contribution from or payments
               made by or on behalf of any other Guarantor in respect of the
               obligations of such other Guarantor under Article 11 of the
               Indenture, the obligations of each Guaranteeing Subsidiary shall
               be limited to the maximum amount as will result in the
               obligations of such Guarantor under its Guarantee not
               constituting a fraudulent transfer or conveyance.

          3. SUBORDINATION. The obligations of each Guaranteeing Subsidiary
under its guarantee pursuant to this Supplemental Indenture shall be junior and
subordinated to the Senior Debt of each such Guaranteeing Subsidiary on the same
basis as the Notes are junior and subordinated to the Senior Debt of the
Company. For the purposes of the foregoing sentence, the Trustee and the Holders
shall have the right to receive and/or retain payments by each Guaranteeing
Subsidiary only at such time as they may receive and/or retain payments in
respect of the notes pursuant to the indenture, including article 10 thereof.

          4. EXECUTION AND DELIVERY. Each guaranteeing subsidiary agrees that
the guarantees shall remain in full force and effect notwithstanding any failure
to endorse on each note a notation of such guarantee.

          5. GUARANTEEING SUBSIDIARY MAY CONSOLIDATE, ETC. ON CERTAIN TERMS.
Each Guaranteeing Subsidiary will be subject to Section 11.05 of the Indenture.

          6. RELEASES. In the event of a sale or other disposition of all of the
assets of any guarantor, by way of merger, consolidation or otherwise, or a sale
or other disposition of all to the 

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capital stock of any Guarantor, then such guarantor (in the event of a sale or
other disposition, by way of merger, consolidation or otherwise, of all of the
capital stock of such Guarantor) or the corporation acquiring the property (in
the event of a sale or other disposition of all or substantially all of the
assets of such Guarantor) will be released and relieved of any obligations under
its Guarantee; PROVIDED that the net proceeds of such sale or other disposition
are applied in accordance with the applicable provisions of the Indenture,
including without limitation section 4.10 of the Indenture. Upon delivery by the
Company to the Trustee of an Officers' Certificate and an Opinion of Counsel to
the effect that such sale or other disposition was made by the Company in
accordance with the provisions of the indenture, including without limitation
Section 4.10 of the Indenture, the Trustee shall execute any documents
reasonably required in order to evidence the release of any Guarantor from its
obligations under its Guarantee.

          (a)  Any Guarantor not released from its obligations under its
               Guarantee shall remain liable for the full amount of principal of
               and interest on the and for the other obligations of any
               guarantor under the indenture as provided in Article 10 of the
               indenture.

          7. NO RECOURSE AGAINST OTHERS. No past, present or future director,
officer, employee, incorporator, stockholder or agent of either Guaranteeing
Subsidiary, as such, shall have any liability for any obligations of the Company
or any Guaranteeing Subsidiary under the Notes, any Guarantees, the Indenture or
this Supplemental Indenture or for any claim based on, in respect of, or by
reason of, such obligations or their creation. Each Holder of the Notes by
accepting a Note waives and releases all such liability. The waiver and release
are part of the consideration for issuance of the Notes. Such waiver may not be
effective to waive liabilities under the federal securities laws and it is the
view of the Commission that such a waiver is against public policy.

          8. NEW YORK LAW TO GOVERN. THE INTERNAL LAW OF THE STATE OF NEW YORK
SHALL GOVERN AND BE USED TO CONSTRUE THIS SUPPLEMENTAL INDENTURE WITHOUT GIVING
EFFECT TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW TO THE EXTENT THAT THE
APPLICATION OF THE LAWS OF ANOTHER JURISDICTION WOULD BE REQUIRED THEREBY.

          9. COUNTERPARTS The parties may sign any number of copies of this
Supplemental Indenture. Each signed copy shall be an original, but all of them
together represent the same agreement.

          10. EFFECT OF HEADINGS. The Section headings herein are for
convenience only and shall not affect the construction hereof.

          11. THE TRUSTEE. The Trustee shall not be responsible in any manner
whatsoever for or in respect of the validity or sufficiency of this Supplemental
Indenture or for or in respect of the recitals contained herein, all of which
recitals are made solely by the Guaranteeing Subsidiaries and the Company.


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          IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed and attested, all as of the date first above
written.

Dated:  April 23, 1999

                                  TRANSDIGM, INC.


                                  By: /s/ Peter B. Radekevich 
                                      -----------------------------------------
                                      Name:  Peter B. Radekevich 
                                      Title: Chief Financial Officer

                                  TRANSDIGM HOLDING COMPANY


                                  By: /s/ Peter B. Radekevich 
                                      -----------------------------------------
                                      Name:  Peter B. Radekevich 
                                      Title: Chief Financial Officer

                                  MARATHON POWER TECHNOLOGIES COMPANY


                                  By: /s/ Peter B. Radekevich 
                                      -----------------------------------------
                                      Name:  Peter B. Radekevich 
                                      Title: Chief Financial Officer

                                  ADAMS RITE AEROSPACE, INC.


                                  By: /s/ Peter B. Radekevich 
                                      -----------------------------------------
                                      Name:  Peter B. Radekevich 
                                      Title: Chief Financial Officer

                                  ZMP, INC.


                                  By: /s/ Peter B. Radekevich 
                                      -----------------------------------------
                                      Name:  Peter B. Radekevich 
                                      Title: Chief Financial Officer


                                  STATE STREET BANK AND TRUST COMPANY, as 
                                  Trustee


                                  By: /s/ Michael Hopkins
                                      -----------------------------------------
                                      Name:  Michael Hopkins
                                      Title: Vice President