<PAGE>
 
                                                                 EXHIBIT 4.8(c)



                              HOWMET CORPORATION,

                                   as Issuer

                                      and

                              MARINE MIDLAND BANK,

                                   as Trustee

                             ____________________

                          FIRST SUPPLEMENTAL INDENTURE
                         Dated as of December 15, 1997

                         Supplementing Indenture Dated
                             as of December 7, 1995

                             ____________________

                                  $125,000,000

                     10% Senior Subordinated Notes Due 2003

                                       
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                                      -2-



          FIRST SUPPLEMENTAL INDENTURE, dated as of December 15, 1997 (this
"Supplemental Indenture"), between HOWMET CORPORATION, a Delaware corporation
and successor to Howmet Acquisition Corp., as issuer (the "Company"), and MARINE
MIDLAND BANK, as trustee (the "Trustee").

          WHEREAS, the Company has heretofore executed and delivered to the
Trustee an Indenture, dated as of December 7, 1995 (the "Indenture"), providing
for the creation and issuance by the Company of its 10% Senior Subordinated
Notes due 2003 (the "Notes", and together with the Exchange Notes, in each case,
issued, authenticated and delivered under the Indenture, in each case, as
amended or supplemented from time to time pursuant to the terms of the
Indenture, the "Securities");

          WHEREAS, Section 9.02 of the Indenture provides that the Company and
the Trustee may amend or supplement the Indenture or the Securities with the
written consent of the Holders of at least a majority in aggregate principal
amount of the Securities then outstanding, subject to certain exceptions
specified in Section 9.02 of the Indenture;

          WHEREAS, the parties hereto are entering into this Supplemental
Indenture to, among other things, (i) eliminate certain of the definitions
contained in Section 1.01 of the Indenture, (ii) eliminate certain restrictive
covenants contained in Article Four of the Indenture, (iii) amend certain
provisions contained in Articles Four, Five, Six and Eight of the Indenture; and
(iv) eliminate all references in the Indenture to sections and definitions to be
deleted in accordance with the preceding clauses (i), (ii) and (iii)
(collectively, the "Proposed Amendments");

          WHEREAS, the Holders of at least a majority in aggregate principal
amount of the Securities have duly consented to the Proposed Amendments in the
manner provided in Section 9.02 of the Indenture; and

          WHEREAS, all conditions and requirements necessary to make this
Supplemental Indenture a valid, binding and legal instrument enforceable in
accordance with its terms have been performed and fulfilled by the parties
hereto and the execution and delivery of this Supplemental Indenture have been
in all respects duly authorized by the parties hereto.

          NOW, THEREFORE, in consideration of the above premises, each party
hereto agrees, for the benefit of the other party and for the equal and ratable
benefit of the Holders of the Securities, as follows:
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                                      -3-

          SECTION 1.   Definitions.
                       ----------- 

          For all purposes of this Supplemental Indenture, except as otherwise
expressly provided or unless the context otherwise requires, the terms used
herein shall have the respective meanings assigned to them in the Indenture.

          SECTION 2.   Elimination of Definitions from the Indenture.
                       ----------------------------------------------

          The definitions of the following terms and any references thereto are
hereby deleted in their entireties from Section 1.01 of the Indenture and from
the other sections of the Indenture where such terms or definitions are
referenced:

             (i)       Acquired Indebtedness;
             (ii)      Acquisition Date;
             (iii)     Affiliate Transaction;
             (iv)      Asset Sales;
             (v)       Capital Stock;
             (vi)      Change of Control;
             (vii)     Change of Control Date;
             (viii)    Change of Control Offer;
             (ix)      Change of Control Payment Date;
             (x)       Collateral;
             (xi)      Collateral Account;
             (xii)     Collateral Funds;
             (xiii)    Consolidated EBITDA;
             (xiv)     Consolidated Net Income;
             (xv)      Excess Net Proceeds;
             (xvi)     Holdings Intercompany Note;
             (xvii)    Independent;
             (xviii)   Independent Financial Advisor;
             (xix)     Initial Amount;
             (xx)      Initial Distribution;
             (xxi)     Investment;
             (xxii)    Management Agreements;
             (xxiii)   Money Market Account;
             (xxiv)    Net Cash Proceeds;
             (xxv)     Net Offering Proceeds;
             (xxvi)   Net Proceeds Offer;
             (xxvii)   Operating Coverage Ratio;
             (xxviii)  Permitted Investment;
             (xxix)    Permitted Liens;
             (xxx)     Preferred Stock;
             (xxxi)    Proceeds Purchase Date;
             (xxxii)   Purchase Money Note;
             (xxxiii)  Qualified Receivables Transaction;
             (xxxiv)   Receivables Subsidiary;
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                                      -4-

              (xxxv)     Reference Date;
              (xxxvi)    Refinancing Indebtedness;
              (xxxvii)   Restricted Payment;
              (xxxviii)  Restricted Subsidiary;
              (xxxix)    Significant Stockholder;
              (xl)       Significant Subsidiary;
              (xli)      Special Redemption Amount;
              (xlii)     Standard Securitization Undertakings;
              (xliii)    Subordinated Obligations;
              (xliv)     Tax Sharing Agreement;
              (xlv)      Unrestricted Subsidiary; and
              (xlvi)     Weighted Average Life to Maturity.

          SECTION 3.   Elimination or Amendment of Certain Provisions of the
                                                           -----------------
                       Indenture.
                       ----------

          (a)  Section 4.08 of the Indenture is hereby amended to state, in its
entirety, the following:

          "SECTION 4.08.  Reports.
                          ------- 

          (a) The Company shall comply with the provisions of TIA (S) 314(a).

          (b)  [Intentionally Omitted].

          (c)  [Intentionally Omitted]."

          (b)  Section 4.10 of the Indenture entitled "Limitation on Restricted
Payments" is hereby deleted in its entirety, together with any references
thereto in the Indenture, and replaced with the words:  "SECTION 4.10.
[Intentionally Omitted]."

          (c)  Section 4.11 of the Indenture entitled "Limitation on
Transactions with Affiliates" is hereby deleted in its entirety, together with
any references thereto in the Indenture, and replaced with the words:  "SECTION
4.11.  [Intentionally Omitted]."

          (d)  Section 4.12 of the Indenture entitled "Limitation on
Indebtedness" is hereby deleted in its entirety, together with any references
thereto in the Indenture, and replaced with the words:  "SECTION 4.12.
[Intentionally Omitted]."

          (e)  Section 4.13 of the Indenture entitled "Limitation on Payment
Restrictions Affecting Subsidiaries" is hereby deleted in its entirety, together
with any references 
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                                      -5-

thereto in the Indenture, and replaced with the words:
"SECTION 4.13.  [Intentionally Omitted]."

          (f)  Section 4.14 of the Indenture entitled "Limitation on Additional
Senior Subordinated Indebtedness" is hereby deleted in its entirety, together
with any references thereto in the Indenture, and replaced with the words:
"SECTION 4.14.  [Intentionally Omitted]."

          (g)   Section 4.15 of the Indenture entitled "Limitation on Change of
Control" is hereby deleted in its entirety, together with any references thereto
in the Indenture, and replaced with the words:  "SECTION 4.15.  [Intentionally
Omitted]."

          (h)   Section 4.16 of the Indenture entitled "Limitation on Asset
Sales" is hereby deleted in its entirety, together with any references thereto
in the Indenture, and replaced with the words:  "SECTION 4.16.  [Intentionally
Omitted]."

          (i)   Section 4.17 of the Indenture entitled "Limitation on Capital
Stock of Restricted Subsidiaries" is hereby deleted in its entirety, together
with any references thereto in the Indenture, and replaced with the words:
"SECTION 4.17.  [Intentionally Omitted]."

          (j)   Section 4.18 of the Indenture entitled "Limitation on Liens" is
hereby deleted in its entirety, together with any references thereto in the
Indenture, and replaced with the words:  "SECTION 4.18.  [Intentionally
Omitted]."

          (k)   Section 4.19 of the Indenture entitled "Limitation on Transfer
of Assets to Certain Subsidiaries" is hereby deleted in its entirety, together
with any references thereto in the Indenture, and replaced with the words:  "
SECTION 4.19.  [Intentionally Omitted]."

          (l)   Section 4.20 of the Indenture entitled "Deposit of Proceeds
with Trustee Pending Consummation of the Acquisition" is hereby deleted in its
entirety, together with any references thereto in the Indenture, and replaced
with the words:  "SECTION 4.20.  [Intentionally Omitted]."

          (m)   Section 5.01 of the Indenture is hereby amended to state, in
its entirety, the following:
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                                      -6-

          "SECTION 5.01.  Merger, Consolidation
                          and Sale of Assets.
                          ---------------------

          (a) The Company shall not in a single transaction or through a series
     of related transactions consolidate with or merge with or into any other
     Person, or transfer (by lease, assignment, sale or otherwise) all or
     substantially all of its Properties and assets unless:

               (1) either the Company shall be the continuing Person, or the
          Person (if other than the Company) formed by such consolidation or
          into which the Company is merged or to which all or substantially all
          of the properties and assets of the Company are transferred (the
          Company or such other Person being hereinafter referred to as the
          "Surviving Person") shall be a corporation organized and validly
           ----------------                                               
          existing under the laws of the United States, any State thereof or the
          District of Columbia, and shall expressly assume, by an indenture
          supplemental hereto, executed and delivered to the Trustee, in form
          satisfactory to the Trustee, all of the Obligations of the Company
          under the Notes and this Indenture.

               (2)  [Intentionally Omitted];

               (3) immediately before or immediately after and giving effect to
          such transaction no Default or Event of Default shall have occurred or
          be continuing; and

               (4) the Company has delivered to the Trustee an Officers'
          Certificate and an Opinion of Counsel, each stating that such
          consolidation, merger, transfer or adoption and such supplemental
          indenture comply with this Article Five, that the Surviving Person
          agrees to be bound hereby, and that all conditions precedent herein
          provided (which, in the case of the Opinion of Counsel, may be limited
          to the condition specified in clause (1) of this Section 5.01(a))
          relating to such transaction have been satisfied.

          (b) For purposes of the foregoing, the transfer (by lease, assignment,
     sale or otherwise, in a single 
<PAGE>
 
                                      -7-

     transaction or series of transactions) of all or substantially all of the
     Properties and assets of one or more Subsidiaries the Capital Stock of
     which constitutes all or substantially all of the Properties and assets of
     the Company shall be deemed to be the transfer of all or substantially all
     of the properties and assets of the Company.

          (c)  [Intentionally Omitted]."

          (n)  Section 6.01 of the Indenture is hereby amended to state, in
its entirety, the following:

          "SECTION 6.01.  Events of Default.
                          ----------------- 

          An "Event of Default" occurs if:

               (1) the Company defaults in the payment of any installment of
          interest on any Notes as and when the same becomes due and payable and
          the Default continues for a period of 30 days (whether or not
          prohibited by the subordination provisions of this Indenture);

               (2) the Company defaults in payment of all or any part of the
          principal on any Notes when the same becomes due and payable at
          maturity, upon any redemption, by declaration or otherwise (whether or
          not prohibited by the subordination provisions of this Indenture);

               (3) the Company or any Subsidiary Guarantor fails duly to observe
          or comply with any of its other covenants or agreements contained in
          the Notes or this Indenture and the Default continues for a 30-day
          period and after the notice specified below;

               (4)  [Intentionally Omitted];

               (5) the Company (A) commences a voluntary case or proceeding
          under any Bankruptcy Law with respect to itself; (B) consents to the
          entry of a judgment, decree or order for relief against it in an
          involuntary case or proceeding under any Bankruptcy Law, (C) consents
          to the appointment of a Custodian of it or for substantially all of
          its property, (D) consents to or acquiesces in the institution of a
<PAGE>
 
                                      -8-

          bankruptcy or an insolvency proceeding against it, (E) makes a general
          assignment for the benefit of its creditors; or (F) takes any
          corporate action to authorize or effect any of the foregoing;

               (6) a court of competent jurisdiction enters a judgment, decree
          or order for relief in respect of the Company in an involuntary case
          or proceeding under any Bankruptcy Law, which shall (A) approve as
          properly filed a petition seeking reorganization, arrangement,
          adjustment or composition in respect of the Company, (B) appoint a
          Custodian of the Company or for substantially all of its property or
          (C) order the winding-up or liquidation of its affairs; and such
          judgment, decree or order shall remain unstayed and in effect for a
          period of 60 consecutive days;

               (7)  [Intentionally Omitted]; or

               (8)  [Intentionally Omitted].

          A Default under clause (3) above is not an Event of Default until the
     Trustee notifies the Company, or the Holders of at least 50% in principal
     amount of the then outstanding Notes notify the Company and the Trustee, of
     the Default, and the Company does not cure the Default within 30 days after
     receipt of the notice.  The notice must specify the Default, demand that it
     be remedied and state that the notice is a "Notice of Default."  Such
     notice shall be given by the Trustee if so requested by the Holders of at
     least 50% in principal amount of the Notes then outstanding.  Such notice
     shall be given by registered or certified mail, return receipt requested."

          (o)  Section 6.02 of the Indenture is hereby amended to state, in
its entirety, the following:
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                                      -9-


          "SECTION 6.02.  Acceleration.
                          ------------ 

          If an Event of Default occurs and is continuing and has not been
     waived pursuant to Section 6.04, then the Trustee or the Holders of at
     least 50% in principal amount of outstanding Notes may declare the
     principal of and accrued interest on all the Notes to be due and payable by
     notice in writing to the Company and the Trustee specifying the respective
     Event of Default and that it is a "notice of acceleration" (the
     "Acceleration Notice"), and the same (i) shall become immediately due and
     --------------------                                                     
     payable or (ii) if there are any amounts outstanding under the New Bank
     Credit Facility, shall become immediately due and payable upon the first to
     occur of an acceleration under the New Bank Credit Facility or 5 Business
     Days after receipt by the Company and the Representative under the New Bank
     Credit Facility of such Acceleration Notice unless all Events of Default
     specified in such Acceleration Notice (other than any Event of Default
     described in clause (2) of Section 6.01) shall have been cured.  If an
     Event of Default specified in Section 6.01(5) or (6) occurs with respect to
     the Company, all unpaid principal and accrued interest on the Notes then
     outstanding shall ipso facto become and be immediately due and payable
                       ----------                                          
     without any declaration or other act on the part of the Trustee or any
     Noteholder.

          The declaration of acceleration is subject to the condition that if,
     at any time after the principal of the Notes shall have been so declared
     due and payable, and before any judgment or decree for the payment of the
     moneys due shall have been obtained or entered as hereinafter provided, the
     Company shall pay or shall deposit with the Trustee a sum sufficient to pay
     all matured installments of interest upon all the Notes and the principal
     of any and all Notes which shall have become due otherwise than by
     acceleration (with interest upon such principal and, to the extent that
     payment of such interest is enforceable under applicable law, on overdue
     installments of interest, at the same rate as the rate of interest
     specified in the Notes, to the date of such payment or deposit) and such
     amount as shall be sufficient to cover reasonable compensation to the
     Trustee and each predecessor Trustee, their respective agents, attorneys
     and counsel, and all other expenses and liabilities incurred, and all
<PAGE>
 
                                      -10-

     advances made, by the Trustee and each predecessor Trustee except as a
     result of negligence or bad faith, and if any and all Events of Default,
     other than the non-payment of the principal of Notes which shall have
     become due by acceleration, shall have been cured, waived or otherwise
     remedied as provided herein, then and in every such case the holders of a
     majority in aggregate principal amount of the Notes then outstanding, by
     written notice to the Company and to the Trustee, may waive all defaults
     and rescind and annul such declaration and its consequences, but no such
     waiver or rescission and annulment shall extend to or shall affect any
     subsequent default or shall impair any right consequent thereon."

          (p)   Section 8.02 of the Indenture is hereby amended to state, in
its entirety, the following:

          "SECTION 8.02.  Defeasance and Discharge
                          of Indenture.
                          ------------------------

          The Company shall be deemed to have paid and discharged the entire
     Indebtedness on all the outstanding Notes on the 91st day after the date of
     the deposit referred to in subparagraph (a) hereof, and the provisions of
     this Indenture, as it relates to such outstanding Notes, shall no longer be
     in effect (and the Trustee, at the expense of the Company, shall execute
     proper instruments acknowledging the same), except as to:  (1) rights of
     registration of transfer and exchange, and the Company's right of optional
     redemption, (2) substitution of mutilated, defaced, destroyed, lost or
     stolen Notes, (3) rights of Holders to receive payments of principal
     thereof and interest thereon, (4) the rights, obligations and immunities of
     the Trustee hereunder and (5) the rights of the Noteholders as
     beneficiaries hereof with respect to the property so deposited with the
     Trustee payable to all or any of them; provided that all of the following
                                            --------                          
     conditions shall have been satisfied:

               (a) the Company has deposited or caused to be irrevocably
          deposited with the Trustee (or another trustee satisfying the
          requirements of Section 7.10 who shall agree to comply with the
          provisions of this Article) as trust funds in trust, specifically
          pledged as security for, and 
<PAGE>
 
                                      -11-

          dedicated solely to, the benefit of the Holders of the Notes, (i) U.S.
          Legal Tender in an amount, or (ii) U.S. Government Obligations which
          through the payment of interest and principal in respect thereof in
          accordance with their terms will provide not later than one business
          Day before the due date of any payment referred to below U.S. Legal
          Tender in an amount, or (iii) a combination thereof, sufficient, in
          the opinion of a nationally recognized firm of independent public
          accountants expressed in a written certification thereof delivered to
          the Trustee, to pay and discharge, without consideration of the
          reinvestment of such interest and after payment of all federal, state
          and local taxes or other charges and assessments in respect thereof
          payable by the Trustee, the principal of and each installment of
          principal and interest on the outstanding Notes as of the maturity
          date of such principal or installment of interest;

               (b) such deposit shall not cause the Trustee to have a
          conflicting interest as defined in the TIA;

               (c) such deposit shall not result in a breach or violation of, or
          constitute a default under, this Indenture or any other agreement or
          instrument to which the Company or any Subsidiary Guarantor, if any,
          is a party or by which it is bound;

               (d) no Default or Event of Default shall have occurred and be
          continuing on the date of such deposit or during the period ending on
          the 91st day after such date;

               (e) the Company has delivered to the Trustee an Opinion of
          Counsel to the effect that (i) the Holders of the Notes shall not
          recognize income, gain or loss for Federal income tax purposes as a
          result of such deposit, defeasance and discharge and will be subject
          to Federal income tax on the same amount and in the same manner and at
          the same times as would have been the case if such deposit, defeasance
          and discharge had not occurred, and (ii) the 
<PAGE>
 
                                      -12-

          creation of the trust will not violate the Investment Company Act of
          1940, as amended; and

               (f) the Company has delivered to the Trustee an Officers'
          Certificate and an Opinion of Counsel, each stating that all
          conditions precedent provided for relating to the defeasance
          contemplated by this provision have been complied with."

          SECTION 4.   Operation of Supplemental Indenture.
                       ----------------------------------- 

          This Supplemental Indenture shall become effective upon execution by
the parties hereto and the Proposed Amendments shall become operative upon the
delivery to the Trustee of an Officers' Certificate certifying that the Company
has purchased, by accepting for payment, all Securities that have been validly
tendered (and not withdrawn) pursuant to the Company's offer to purchase
Securities on the terms and conditions set forth in the Company's Offer to
Purchase and Consent Solicitation Statement, dated November 24, 1997, as it may
be supplemented from time to time, and the related Consent and Letter of
Transmittal.

          SECTION 5.   Concerning the Trustee.
                       ---------------------- 

          The Trustee accepts the trusts of the Indenture, as supplemented by
this Supplemental Indenture, and agrees to perform the same, but only upon the
terms and conditions set forth in the Indenture, as supplemented by this
Supplemental Indenture, to which the parties hereto and the Holders from time to
time of the Securities agree and, except as expressly set forth in the
Indenture, as supplemented by this Supplemental Indenture, shall incur no
liability or responsibility in respect thereof.  The Trustee makes no
representation and shall have no responsibility as to the validity or
sufficiency of this Supplemental Indenture, or for or in respect to the recitals
contained herein, all of which recitals are made solely by the Company.

          SECTION 6.   Miscellaneous.
                       ------------- 

          (a) Except as hereby expressly amended, the Indenture is in all
respects ratified and confirmed and all the terms, provisions and conditions
thereof shall be and remain in full force and effect.

          (b) All agreements of the Company in this Supplemental Indenture
shall bind the Company's successors.  
<PAGE>
 
                                      -13-

All agreements of the Trustee in this Supplemental Indenture shall bind its
successors.

          (c) THIS SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY AND CONSTRUED IN
ACORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, AS APPLIED TO CONTRACTS MADE
AND PERFORMED WITHIN THE STATE OF NEW YORK, WITHOUT REGARD TO PRINCIPLES OF
CONFLICT OF LAWS.  EACH OF THE PARTIES HERETO AGREES TO SUBMIT TO THE
JURISDICTION OF THE COURTS OF THE STATE OF NEW YORK IN ANY ACTION OR PROCEEDING
ARISING OUT OF OR RELATING TO THIS SUPPLEMENTAL INDENTURE.

          (d) If and to the extent that any provision of this Supplemental
Indenture limits, qualifies or conflicts with another provision that is required
to be included in this Supplemental Indenture or in the Indenture by the TIA,
the required provision shall control.

          (e) The titles and headings of the sections of this Supplemental
Indenture have been inserted for convenience of reference only, and are not to
be considered a part hereof and shall in no way modify or restrict any of the
terms or provisions hereof.

          (f) This Supplemental Indenture may be executed in any number of
counterparts, each of which shall be deemed an original, but all of which
together shall represent one and the same agreement.

          (g) In case any provision of this Supplemental Indenture shall be
determined to be invalid, illegal or unenforceable, the validity, legality and
enforceability of the remaining provisions hereof or of the Indenture shall not
in any way be affected or impaired thereby.

                            [Signature page follows]
<PAGE>
 
                                      -14-

                                   SIGNATURES

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

                              HOWMET CORPORATION,
                              as Issuer

                              By:    /s/ Roland A. Paul
                                     ------------------
                              Name:  Roland A. Paul
                              Title: Vice President


                              MARINE MIDLAND BANK,
                              as Trustee

                              By:    /s/ Robert A. Conrad
                                     --------------------
                              Name:  Robert A. Conrad
                              Title: Vice President