Document:

<PAGE>

                                                                    EXHIBIT 4.21

              AMERICAN AIRLINES, INC. PASS THROUGH TRUST AGREEMENT
                                  March 21,2002

INSTRUMENT OF RESIGNATION, APPOINTMENT AND ACCEPTANCE, dated as of June 30, 2003
among U.S. Bank National Association ("Resigning Trustee"), American Airlines,
Inc., a Delaware corporation (the "Company"), and U.S. Bank Trust National
Association (the "Successor Trustee").

                                    RECITALS

         A.       The Resigning Trustee acts as successor Trustee under the Pass
                  Through Trust Agreement, dated as of March 21, 2002 (the
                  "Basic Agreement"), as supplemented by those certain separate
                  Trust Supplements as described on Schedule 1 hereto (the
                  "Trust Supplements"), between the Company and the Resigning
                  Trustee. Capitalized terms used herein and not defined shall
                  have the meanings ascribed to them in or by reference in the
                  Basic Agreement and the Trust Supplements.

         B.       The Resigning Trustee wishes to resign as Trustee under the
                  Basic Agreement and each of the Trust Supplements; the Company
                  wishes to appoint the Successor Trustee to succeed the
                  Resigning Trustee as Trustee under the Basic Agreement and the
                  Trust Supplements and the Successor Trustee wishes to accept
                  such appointments.

         NOW, THEREFORE, the Company, the Resigning Trustee and the Successor
Trustee agree as follows:

                                   ARTICLE ONE
                              THE RESIGNING TRUSTEE

Section 101. Pursuant to Section 7.09(b) of the Basic Agreement, the Resigning
Trustee hereby resigns as Trustee under the Basic Agreement, the Trust
Supplements, the Intercreditor Agreements, the Participation Agreements (as
described in the Trust Supplements) and all such other related transaction
documents or instruments to which the Resigning Trustee is a party
(collectively, the "Operative Documents"). The Resigning Trustee agrees to
execute and deliver such agreements and other instruments as may be necessary or
desirable to effectuate the succession of the Successor Trustee as Trustee under
the Basic Agreement, the Trust Supplements and the other Operative Documents.

Section 102. The Resigning Trustee hereby represents and warrants to the
Successor Trustee that:

         (a)      To the knowledge of the Resigning Trustee, no Events of
                  Default have occurred or are continuing under the Trust
                  Supplements or the other Operative Documents.

<PAGE>

                                       -2-

         (b)      There is no action, suit or proceeding pending or, to the best
                  of the knowledge of the Responsible Officers of the Resigning
                  Trustee assigned to its Corporate Trust Department, threatened
                  against the Resigning Trustee before any court or governmental
                  authority arising out of any action or omission by the
                  Resigning Trustee as Trustee under the Basic Agreement or the
                  other Operative Documents.

         (c)      It assumes continued responsibility for its actions or
                  omissions during its term as Trustee under the Basic
                  Agreement, each Trust Supplement and the other Operative
                  Documents.

                                   ARTICLE TWO
                                   THE COMPANY

Section 201. The Company hereby (a) appoints the Successor Trustee as Trustee
under the Basic Agreement and each Trust Supplement.

Section 202. The Company hereby confirms to the Successor Trustee all the
rights, trusts and powers under the Operative Documents hereby assigned,
transferred, delivered and confirmed to the Successor Trustee.

                                  ARTICLE THREE
                              THE SUCCESSOR TRUSTEE

Section 301. The Successor Trustee hereby represents and warrants to the Company
and the Secretary that the Successor Trustee is qualified and eligible under the
provisions of Article VII of the Basic Agreement.

Section 302. The Successor Trustee hereby accepts its appointment as Trustee
under the Basic Agreement and the Trust Supplements and shall hereby be vested
with all the rights, powers, trusts and duties of the Trustee under the Basic
Agreement, the Trust Supplements and the other Operative Documents. The
Successor Trustee agrees to execute and deliver such agreements and other
instruments as may be necessary or desirable to effectuate the succession of the
Successor Trustee as Trustee under the Basic Agreement, the Trust Supplements
and the other Operative Documents.

                                  ARTICLE FOUR
                                  MISCELLANEOUS

Section 401. This Instrument and the resignation, appointment and acceptance
effected hereby shall be effective as of the opening of business on the date
first above written upon the execution and delivery hereof by each of the
parties hereto.

Section 402. The Instrument shall be governed by and constructed in accordance
with the laws of the jurisdiction which govern the Basic Agreement and the Trust
Supplements and their construction.

<PAGE>

                                       -3-

Section 403. This instrument may be executed in any number of counterparts each
of which shall be an original, but such counterparts shall together constitute
but one and the same instrument.

               [SIGNATURES OF THE PARTIES APPEAR ON THE NEXT PAGE]

<PAGE>

         IN WITNESS WHEREOF, the parties hereby have caused this Instrument of
Resignation, Appointment and Acceptance to be executed by their duly authorized
officers all as of the day and year first above written.

                                          AMERICAN AIRLINES, INC.
                                          (the "Company")

                                          By: /s/ Michael P. Thomas
                                             -----------------------------------
                                                  Michael P. Thomas
                                                  Managing Director,
                                                  Corporate Finance and Banking

<PAGE>

                                       -2-

                                          U.S. BANK NATIONAL ASSOCIATION
                                          (the "Resigning Trustee")

                                          By: /s/ Alison D.B. Nadeau
                                             -----------------------------------
                                          Name: Alison D.B. Nadeau
                                          Title: Vice President

                                          U.S. BANK TRUST NATIONAL ASSOCIATION
                                          (the "Successor Trustee")

                                          By: /s/ Alison D.B. Nadeau
                                             -----------------------------------
                                          Name: Alison D.B. Nadeau
                                          Title: Vice President

<PAGE>

                                   SCHEDULE 1

Trust Supplement No. 2002-1G, dated as of September 24, 2002, between the
Company and the Resigning Trustee in respect of American Airlines Pass Through
Trust 2002-1G.

Trust Supplement No. 2002-1C, dated as of September 24, 2002, between the
Company and the Resigning Trustee in respect of American Airlines Pass Through
Trust 2002-1C.

Trust Supplement No. 2002-1D, dated as of September 24, 2002, between the
Company and the Resigning Trustee in respect of American Airlines Pass Through
Trust 2002-1D.<PAGE>
                                                                    Exhibit 10.1

                                SECOND AMENDMENT
                                      TO
                           LOAN AND SECURITY AGREEMENT

         SECOND AMENDMENT, dated as of November 18, 2003 (the "Amendment"), to
the LOAN AND SECURITY AGREEMENT, dated as of December 26, 2001 (as amended by
the First Amendment to the Loan and Security Agreement, dated as of December 31,
2002, and as further modified, amended and/or restated from time to time, the
"Loan Agreement"), among AMTROL HOLDINGS, INC., a Delaware corporation
("Holdings" or "Guarantor"), AMTROL INC., a Rhode Island corporation ("AMTROL"),
WATER SOFT INC., a Rhode Island corporation ("Water Soft"), AMTROL CANADA LTD.,
an Ontario corporation (each individually, a "Borrower" and collectively the
"Borrowers"), CYPRESS MERCHANT BANKING PARTNERS, L.P. ("CMBP") and CYPRESS
OFFSHORE PARTNERS, L.P. ("COP"; together with CMBP, the "Lenders").

                                    Preamble

         Pursuant to the Loan Agreement, the Lenders made (i) term loans to the
Borrowers on the Closing Date (as defined in the Loan Agreement) and (ii)
Tranche B Loans to the Borrowers from time to time.

         The Borrowers request that the Lenders agree to amend the Loan
Agreement to permit the Borrowers to borrow additional Tranche B Loans from the
Lenders from time to time in an aggregate principal amount not to exceed
$6,300,000 to be used to repurchase Senior Subordinated Notes (as defined in the
Loan Agreement).

         Based upon the terms and conditions set forth herein, the Foothill
Agent and the Lenders have agreed to such amendments. Accordingly, the
Borrowers, Holdings and the Lenders hereby agree as follows:

         1. Definitions. All capitalized terms used herein which are defined in
the Loan Agreement and not otherwise defined herein are used herein as defined
therein.

         2. Amendments to Section 2 of the Loan Agreement (The Loans and the
Notes).

         (a) Section 2 of the Loan Agreement is hereby amended by deleting
clause (b) thereof in its entirety and substituting in lieu therefor the
following new clause (b):

         "(b) Tranche B Loans. At any time and from time to time after the
         Closing Date, each Lender agrees to make term loans (individually, a
         "Tranche B Loan" and collectively, the "Tranche B Loans"; together with
         the Tranche A Loans, the "Loans") to AMTROL in an aggregate principal
         amount not to exceed $7,022,880.00 for CMBP and $400,400.00 for COP.
         The proceeds of the Tranche B Loans will be used to repurchase Senior
         Subordinated Notes."
<PAGE>
         (b) Clause (c) of Section 2 of the Loan Agreement is hereby amended by
deleting subsections (B), (C), (D) and (E) thereof in their entirety and
substituting in lieu therefor the following new subsections (B), (C) and (D) and
relettering subsection (F) to be subsection (E):

         "(B) the proposed Borrowing Date is on or prior to December 31, 2003;

         (C) after giving effect to the requested Tranche B Loan to be made on
         such Borrowing Date the aggregate amount of Tranche B Loans is less
         than or equal to $7,423,280.00;

         (D) the proceeds of the Tranche B Loans shall be used to repurchase
         Senior Subordinated Notes.

         3. Amendment to Section 6 of the Loan Agreement (Representations and
Warranties). Section 6(v) of the Loan Agreement is hereby amended by deleting it
in its entirety and substituting in lieu therefor the following new Section
6(v):

         "(v) Senior Subordinated Notes. The subordination provisions contained
in the Senior Subordinated Notes are enforceable by the Lenders against the
Borrowers and the holders of such Senior Subordinated Notes, subject to the
effects of bankruptcy, insolvency, reorganization, moratorium and other similar
laws relating to or affecting creditors' rights generally, general equitable
principles (regardless of whether enforcement is sought in equity or at law) and
an implied covenant of good faith and fair dealing and all Cypress Obligations
of Borrowers are or will be within the definition of (and are hereby designated
as) "Designated Senior Indebtedness" included in such provisions of the
Indenture and Senior Subordinated Notes."

         4. Representations and Warranties. Borrowers and Guarantor hereby
represent and warrant to Lenders that:

            (a) Borrowers and Guarantor is duly organized and existing and in
good standing under the laws of its respective jurisdiction of formation and is
duly qualified to do business in every jurisdiction where the failure to be so
qualified reasonably could be expected to have a Material Adverse Change;

            (b) Borrowers and Guarantor each have all requisite power and
authority necessary to enter into this Second Amendment, to amend the Cypress
Mortgages ("Cypress Mortgage Amendments"), to enter into the First Amendment and
Waiver, dated as of November 18, 2003, to the Foothill Loan Agreement (the
"Foothill First Amendment"), to amend the mortgages securing the Foothill
Obligations (collectively, the "Related Foothill Amendments) and to perform its
respective obligations under this Second Amendment, the Cypress Mortgage
Amendments, the Foothill First Amendment and the Related Foothill Amendments;

            (c) Borrowers and Guarantor each have taken all corporate action
necessary to be taken by it to authorize the execution and delivery of this
Second Amendment, the Cypress Mortgage Amendments, the Foothill First Amendment
and the Related Foothill

                                      -2-
<PAGE>
Amendments. This Second Amendment, the Cypress Mortgage Amendments, the Foothill
First Amendment and the Related Foothill Amendments have been duly executed and
delivered by Borrowers and Guarantor and constitute legal, valid and binding
obligations of Borrowers and Guarantor, enforceable against Borrowers and
Guarantor in accordance with their respective terms;

            (d) After giving effect to the amendments and waivers herein, no
event has occurred and no condition exists which constitutes a Default or an
Event of Default under the Loan Agreement or the other Loan Documents; and

            (e) The Loan Agreement and all other Loan Documents and all
representations, warranties, terms and conditions therein remain in full force
and effect, each Borrower and Guarantor hereby (i) confirms and agrees that each
Loan Document to which it is a party is, and shall continue to be, in full force
and effect and is hereby ratified and confirmed in all respects except that on
and after the date hereof and (ii) confirms and agrees that to the extent that
any such Loan Document purports to grant a security interest in or Lien on, any
collateral as security for the obligations of the Borrowers or the Guarantor
from time to time existing in respect of the Loan Agreement and the other Loan
Documents, such pledge, assignment and/or grant of the security interest or Lien
is hereby ratified and confirmed in all respects.

         5. Conditions to Effectiveness. This Amendment shall become effective
only upon satisfaction in full of the following conditions precedent (the first
date upon which all such conditions have been satisfied being herein referred to
as the "Effective Date"):

            (a) immediately prior to and after giving effect to this Amendment,
the representations and warranties contained in this Amendment and in Section 6
of the Loan Agreement and each other Loan Document shall be correct in all
material respects on and as of the Effective Date as though made on and as of
such date (except where such representations and warranties relate to an earlier
date in which case such representations and warranties shall be true and correct
as of such earlier date);

            (b) no Default or Event of Default shall have occurred and be
continuing on the Effective Date or result from this Amendment or the Foothill
First Amendment becoming effective in accordance with its terms;

            (c) the Lenders shall have received all counterparts of this
Amendment, duly executed by the Lenders, the Borrowers and the Guarantors and
duly acknowledged and agreed upon by the Foothill Agent;

            (d) the Lenders shall have received a copy of the Foothill First
Amendment and any other agreement, document or instrument related thereto, each
certified by an Authorized Person of AMTROL as true, complete and correct and in
full force and effect on the Effective Date;

            (e) pursuant to Section 14(b) of the Intercreditor Agreement, dated
as of December 26, 2001 (as hereafter modified, amended and/or restated from
time to time, the "Intercreditor Agreement"), among the Foothill Agent, the
Lenders, the Borrowers, Holdings and

                                      -3-
<PAGE>
AMTROL International Investments Inc., the Foothill Agent must have provided its
written consent of this Amendment to the Lenders; and

            (f) all legal matters incident to this Amendment shall be reasonably
satisfactory to the Lenders and their counsel.

         6. Covenant. No later than 30 days after the Effective Date, AMTROL
shall execute and deliver to the Lenders amendments and other documents
reasonably requested by the Lenders necessary to amend the Cypress Mortgages,
each in form and substance satisfactory to the Lenders.

         7. Continued Effectiveness of the Loan Agreement and the other Loan
Documents. Each Borrower and Guarantor hereby (i) confirms and agrees that each
Loan Document to which it is a party is, and shall continue to be, in full force
and effect and is hereby ratified and confirmed in all respects except that on
and after the Effective Date (x) all references in any such Loan Document to
"the Loan and Security Agreement", the "Loan Agreement", the "Agreement",
"thereto", "thereof", "therein", "thereunder", "hereunder", "herein", "hereof"
or words of like import referring to the Loan Agreement shall mean the Loan
Agreement as modified by this Amendment and (y) all references in any such Loan
Document to "the Intercreditor Agreement", the "Agreement", "thereto",
"thereof", "therein", "thereunder", "hereunder", "herein", "hereof" or words of
like import referring to the Intercreditor Agreement shall mean the
Intercreditor Agreement as modified by this Amendment, and (ii) confirms and
agrees that to the extent that any such Loan Document purports to assign or
pledge to the Lenders, or to grant a security interest in or Lien on, any
collateral as security for the obligations of the Borrowers or the Guarantors
from time to time existing in respect of the Loan Agreement and the other Loan
Documents, such pledge, assignment and/or grant of the security interest or Lien
is hereby ratified and confirmed in all respects.

         8. Miscellaneous.

            (a) This Amendment may be executed in any number of counterparts and
by different parties hereto in separate counterparts, each of which shall be
deemed to be an original but all of which taken together shall constitute one
and the same agreement. Delivery of an executed counterpart of this Amendment by
telefacsimile shall be equally as effective as delivery of an original executed
counterpart of this Amendment. Any party delivering an executed counterpart of
this Amendment by telefacsimile also shall deliver an original executed
counterpart of this Amendment, but the failure to deliver an original executed
counterpart shall not affect the validity, enforceability and binding effect of
this Amendment.

            (b) Section and paragraph headings herein are included for
convenience of reference only and shall not constitute a part of this Amendment
for any other purpose.

            (c) This Amendment shall be governed by and construed in accordance
with, the laws of the State of New York.

            (d) Borrowers will pay on demand all reasonable fees, costs and
expenses of the Lenders in connection with the preparation, execution and
delivery of this

                                      -4-
<PAGE>
Amendment including, without limitation, reasonable fees disbursements and other
charges of Simpson Thacher & Bartlett LLP, counsel to the Lenders.

                                      -5-
<PAGE>
         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
executed and delivered as of the date set forth on the first page hereof.

                                     CYPRESS MERCHANT BANKING PARTNERS, L.P.

                                            By: CYPRESS ASSOCIATES, L.P.,
                                                 its General Partner

                                              By:  CYPRESS GROUP, L.L.C.,
                                                  its General Partner

                                     By:  /s/ David P. Spalding
                                          --------------------------------------
                                          Name:
                                          Title:

                                     CYPRESS OFFSHORE PARTNERS, L.P.

                                            By: CYPRESS ASSOCIATES, L.P.,
                                                 its General Partner

                                                 By:  CYPRESS GROUP, L.L.C.,
                                                        its General Partner

                                     By:  /s/ David P. Spalding
                                          --------------------------------------
                                          Name:
                                          Title:

                                     AMTROL INC.,
                                     as a Borrower

                                     By:  /s/ Larry T. Guillemette
                                          --------------------------------------
                                          Name: Larry T. Guillemette
                                          Title: Executive Vice President and
                                                  Chief Financial Officer
<PAGE>
                                     WATER SOFT INC.,
                                     as a Borrower

                                     By:  /s/ Larry T. Guillemette
                                          --------------------------------------
                                          Name:  Larry T. Guillemette
                                          Title: Chief Executive Officer
                                                 and President

                                     AMTROL CANADA LTD.,
                                     as a Borrower

                                     By:  /s/ Larry T. Guillemette
                                          --------------------------------------
                                          Name:  Larry T. Guillemette
                                          Title: President

                                     AMTROL HOLDINGS, INC.,
                                     as a Guarantor

                                     By:  /s/ Larry T. Guillemette
                                          --------------------------------------
                                          Name:  Larry T. Guillemette
                                          Title: Treasurer
<PAGE>
                           ACKNOWLEDGEMENT AND CONSENT

                  The undersigned does hereby (a) consent, acknowledge and agree
to the transactions described in the foregoing Second Amendment and (b) after
giving effect to such Second Amendment, (i) ratifies and confirms each of the
Cypress Guaranty, Cypress Stock Pledge Agreement and Cypress Contribution
Agreement (together, the "Security Documents") and (ii) confirms and agrees that
each such Security Document is, and shall continue to be, in full force and
effect, with the Collateral described therein securing, and continuing to
secure, the payment of all obligations of the undersigned referred to therein;
provided that each reference to the Loan Agreement therein and in each of the
other Loan Documents shall be deemed to be a reference to the Loan Agreement
after giving effect to such Second Amendment.

                                     AMTROL INTERNATIONAL INVESTMENTS, INC.,
                                                  as Guarantor

                                     By:  /s/ Larry T. Guillemette
                                          --------------------------------------
                                          Name:  Larry T. Guillemette
                                          Title: Chief Executive Officer
                                                 and President

Dated as of November __, 2003

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