TRUST SUPPLEMENT NO. 2009-1A-S

Exhibit 4.2

TRUST SUPPLEMENT NO. 2009-1A-S

dated as of October 13, 2009

between

WILMINGTON TRUST COMPANY

as Trustee,

and

UNITED AIR LINES, INC.

to

PASS THROUGH TRUST AGREEMENT

dated as of June 26, 2007

$659,107,000

United Air Lines Pass Through Trust 2009-1A-S

United Air Lines

Pass Through Certificates,

Series 2009-1A-S

Vedder Price P.C.

Chicago, Illinois

TABLE OF CONTENTS

i

THIS TRUST SUPPLEMENT NO. 2009-1A-S , dated as of October 13, 2009 (herein called the “ Trust Supplement ”), between United Air Lines, Inc., a Delaware corporation (the “ Company ”), and Wilmington Trust Company (the “ Trustee ”), to the Pass Through Trust Agreement, dated as of June 26, 2007, between the Company and the Trustee (the “ Basic Agreement ”).

W I T N E S S E T H :

WHEREAS , the Basic Agreement, unlimited as to the aggregate principal amount of Certificates (unless otherwise specified herein, capitalized terms used herein without definition having the respective meanings specified in the Basic Agreement) which may be issued thereunder, has heretofore been executed and delivered;

WHEREAS , as of the Transfer Date (as defined below), the Company will have financed the acquisition of all or a portion of such Aircraft through separate secured loan transactions, under which the Company owns such Aircraft (collectively, the “ Aircraft ”);

WHEREAS , as of the Transfer Date, in the case of each Aircraft, the Company will have issued pursuant to an Indenture, on a recourse basis, Equipment Notes to finance a portion of the purchase price of such Aircraft;

WHEREAS , as of the Transfer Date, the Related Trustee will assign, transfer and deliver all of such trustee’s right, title and interest to the trust property held by the Related Trustee to the Trustee pursuant to the Assignment and Assumption Agreement (as defined below);

WHEREAS , the Trustee, effective only, but automatically, upon execution and delivery of the Assignment and Assumption Agreement, will be deemed to have declared the creation of the United Air Lines Pass Through Trust 2009-1A-S (the “ Applicable Trust ”) for the benefit of the Applicable Certificateholders, and the initial Applicable Certificateholders as the grantors of the Applicable Trust, by their respective acceptances of the Applicable Certificates, join in the creation of the Applicable Trust with the Trustee;

WHEREAS , all Certificates deemed issued by the Applicable Trust will evidence fractional undivided interests in the Applicable Trust and will convey no rights, benefits or interests in respect of any property other than the Trust Property except for those Certificates to which an Escrow Receipt has been affixed;

WHEREAS , upon the execution and delivery of the Assignment and Assumption Agreement, all of the conditions and requirements necessary to make this Trust Supplement, when duly executed and delivered, a valid, binding and legal instrument in accordance with its terms and for the purposes herein expressed, have been done, performed and fulfilled, and the execution and delivery of this Trust Supplement in the form and with the terms hereof have been in all respects duly authorized;

WHEREAS , this Trust Supplement is subject to the provisions of the Trust Indenture Act of 1939, as amended, and shall, to the extent applicable, be governed by such provisions;

NOW THEREFORE , in consideration of the premises herein, it is agreed between the Company and the Trustee as follows:

ARTICLE I

THE CERTIFICATES

Section 1.01. The Certificates . The Applicable Certificates shall be known as “United Air Lines Pass Through Certificates, Series 2009-1A-S”. Each Applicable Certificate represents a fractional undivided interest in the Applicable Trust created hereby. The Applicable Certificates shall be the only instruments evidencing a fractional undivided interest in the Applicable Trust.

The terms and conditions applicable to the Applicable Certificates are as follows:

(a) The aggregate principal amount of the Applicable Certificates that shall be authenticated under the Agreement (except for Applicable Certificates authenticated and delivered under Sections 3.03, 3.04, 3.05 and 3.06 of the Basic Agreement) is $659,107,000.

(b) The Regular Distribution Dates with respect to any payment of Scheduled Payments means May 1 and November 1 of each year, commencing on May 1, 2010, until payment of all of the Scheduled Payments to be made under the Equipment Notes has been made.

(c) The Special Distribution Dates with respect to the Applicable Certificates means any Business Day on which a Special Payment is to be distributed pursuant to the Agreement.

(d) At the Escrow Agent’s request under the Escrow Agreement, the Trustee shall affix the corresponding Escrow Receipt to each Applicable Certificate. In any event, any transfer or exchange of any Applicable Certificate shall also effect a transfer or exchange of the related Escrow Receipt. Prior to the Final Withdrawal Date, no transfer or exchange of any Applicable Certificate shall be permitted unless the corresponding Escrow Receipt is attached thereto and also is so transferred or exchanged. By acceptance of any Applicable Certificate to which an Escrow Receipt is attached, each Holder of such an Applicable Certificate acknowledges and accepts the restrictions on transfer of the Escrow Receipt set forth herein and in the Escrow Agreement.

(e) (i) The Applicable Certificates shall be in the form attached as Exhibit A to the Related Pass Through Trust Supplement, with such appropriate insertions, omissions, substitutions and other variations as are required or permitted by the Related Pass Through Trust Agreement or the Agreement, as the case may be, or as the Trustee may deem appropriate, to reflect the fact that the Applicable Certificates are being issued under the Agreement as opposed to under the Related Pass Through Trust Agreement. Any Person acquiring or accepting an Applicable Certificate or an interest therein will, by such acquisition or acceptance, be deemed to represent and warrant to and for the benefit of the Company that either (i) the assets of an employee benefit plan subject to Title I of the Employee Retirement Income Security Act of 1974, as amended (“ ERISA ”), or of a plan subject to Section 4975 of the

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Internal Revenue Code of 1986, as amended (the “ Code ”), have not been used to purchase or hold Applicable Certificates or an interest therein or (ii) the purchase and holding of Applicable Certificates or an interest therein is exempt from the prohibited transaction restrictions of ERISA and the Code pursuant to one or more prohibited transaction statutory or administrative exemptions. In addition, the Applicable Certificates will bear a legend regarding ERISA compliance matters.

(ii) The Applicable Certificates shall be Book-Entry Certificates and shall be subject to the conditions set forth in the Letter of Representations between the Company and the Clearing Agency attached as Exhibit B to the Related Pass Through Trust Supplement.

(f) The “Participation Agreements” as defined in this Trust Supplement are the “Note Purchase Agreements” referred to in the Basic Agreement.

(g) The Applicable Certificates are subject to the Intercreditor Agreement, the Deposit Agreement and the Escrow Agreement.

(h) The Applicable Certificates are entitled to the benefits of the Liquidity Facility.

(i) The Equipment Notes to be acquired and held in the Applicable Trust, and the related Aircraft and Note Documents, are described in the Note Purchase Agreement.

ARTICLE II

DEFINITIONS

Section 2.01. Definitions . For all purposes of the Basic Agreement as supplemented by this Trust Supplement, the following capitalized terms have the following meanings (any capitalized term used herein but not defined shall have the meaning assigned to it in the Basic Agreement, and any term used herein which is defined in both this Trust Supplement and the Basic Agreement shall have the meaning assigned thereto in this Trust Supplement for purposes of the Basic Agreement as supplemented by this Trust Supplement):

Agreement : Means the Basic Agreement, as supplemented by the Trust Supplement.

Aircraft : Means each of the Aircraft in respect of which a Participation Agreement is entered into in accordance with the Note Purchase Agreement (or any substitute aircraft, including engines therefor, owned by the Company and securing one or more Equipment Notes).

Applicable Certificate : Means any of the “Applicable Certificates” issued by the Related Trust and that are “Outstanding” (as defined in the Related Pass Through Trust Agreement) as of the Transfer Date (the “ Transfer Date Certificates ”) and any Certificate issued in exchange therefor or replacement thereof pursuant to the Agreement.

Applicable Certificateholder : Means the Person in whose name an Applicable Certificate is registered on the Register for the Applicable Certificates.

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Applicable Closing Date : Has the meaning specified in Section 5.01(b) of this Trust Supplement.

Applicable Participation Agreement : Has the meaning specified in Section 5.01(b) of this Trust Supplement.

Applicable Trust : Has the meaning specified in the recitals hereto.

Assignment and Assumption Agreement : Means the assignment and assumption agreement substantially in the form of Exhibit C to the Related Pass Through Trust Supplement executed and delivered in accordance with Section 7.01 of the Related Trust Supplement.

Basic Agreement : Has the meaning specified in the first paragraph of this Trust Supplement.

Business Day : Means any day other than a Saturday, a Sunday or a day on which commercial banks are required or authorized to close in Chicago, Illinois, New York, New York, or, so long as any Applicable Certificate is outstanding, the city and state in which the Trustee, the Subordination Agent or any related Loan Trustee maintains its corporate trust office or receives and disburses funds.

Certificate : Has the meaning specified in the Intercreditor Agreement.

Certificate Buyout Event : Means that a United Bankruptcy Event has occurred and is continuing and the following events in either clause (A) or (B) have occurred: (A) (i) the 60-day period specified in Section 1110(a)(2)(A) of the U.S. Bankruptcy Code (the “ 60-Day Period ”) has expired and (ii) the Company has not entered into one or more agreements under Section 1110(a)(2)(A) of the U.S. Bankruptcy Code to perform all of its obligations under all of the Indentures or, if it has entered into such agreements, has at any time thereafter failed to cure any default under any of the Indentures in accordance with Section 1110(a)(2)(B) of the Bankruptcy Code; or (B) if prior to the expiry of the 60-Day Period, the Company shall have abandoned any Aircraft.

Class : Has the meaning specified in the Intercreditor Agreement.

Closing Notice : Has the meaning specified in the Note Purchase Agreement.

Company : Has the meaning specified in the first paragraph of this Trust Supplement.

Controlling Party : Has the meaning specified in the Intercreditor Agreement.

Cut-off Date : Means the earlier of (a) the Deposit Period Termination Date and (b) the date on which a Triggering Event occurs.

Deposit Agreement : Means the Deposit Agreement dated as of October 13, 2009 relating to the Applicable Certificates between the Depositary and the Escrow Agent, as the same may be amended, supplemented or otherwise modified from time to time in accordance with its terms.

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Deposit Period Termination Date : Has the meaning specified in the Related Pass Through Trust Supplement.

Depositary : Means JPMorgan Chase Bank, N.A., a national banking association chartered under the laws of the United States.

Deposits : Has the meaning specified in the Deposit Agreement.

Distribution Date : Means any Regular Distribution Date or Special Distribution Date as the context requires.

Escrow Agent : Means, initially, Wilmington Trust Company, and any replacement or successor therefor appointed in accordance with the Escrow Agreement.

Escrow Agreement : Means the Escrow and Paying Agent Agreement dated as of October 13, 2009 relating to the Applicable Certificates, among the Escrow Agent, the Escrow Paying Agent, the Related Trustee (and after the Transfer Date, the Trustee) and Underwriters, as the same may be amended, supplemented or otherwise modified from time to time in accordance with its terms.

Escrow Paying Agent : Means the Person acting as paying agent under the Escrow Agreement.

Escrow Receipt : Means the receipt substantially in the form annexed to the Escrow Agreement representing a fractional undivided interest in the funds held in escrow thereunder.

Final Maturity Date : Means November 1, 2016.

Final Withdrawal : Has the meaning specified in the Escrow Agreement.

Final Withdrawal Date : Has the meaning specified in the Escrow Agreement.

Final Withdrawal Notice : Has the meaning specified in Section 5.02 of this Trust Supplement.

Indenture : Means each of the separate trust indentures and mortgages relating to the Aircraft, each as specified or described in a Closing Notice delivered pursuant to the Note Purchase Agreement or the related Participation Agreement, in each case as the same may be amended, supplemented or otherwise modified from time to time in accordance with its terms.

Intercreditor Agreement : Means the Intercreditor Agreement dated as of October 13, 2009 among the Related Trustee (and after the Transfer Date, the Trustee), the Liquidity Provider and Wilmington Trust Company, as Subordination Agent and as trustee thereunder, as amended, supplemented or otherwise modified from time to time in accordance with its terms.

Investors : Means the Underwriters, together with all subsequent beneficial owners of the Applicable Certificates.

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Liquidity Facility : Means, initially, the Revolving Credit Agreement dated as of October 13, 2009 relating to the Applicable Certificates, between the Liquidity Provider and Wilmington Trust Company, as Subordination Agent, as agent and trustee for the Applicable Trust, and, from and after the replacement of such agreement pursuant to the Intercreditor Agreement, the replacement liquidity facility therefor, in each case as amended, supplemented or otherwise modified from time to time in accordance with their respective terms.

Liquidity Provider : Means, initially, Morgan Stanley Bank, N.A., a national banking association chartered under the laws of the United States, and any replacements or successors therefor appointed in accordance with the Intercreditor Agreement.

Note Documents : Means the Equipment Notes with respect to the Applicable Certificates and, with respect to any such Equipment Note, the Indenture, the Parent Guarantee and the Participation Agreement relating to such Equipment Note.

Note Purchase Agreement : Means the Note Purchase Agreement dated as of October 13, 2009 among the Related Trustee (and after the Transfer Date, the Trustee), the Company, the Escrow Agent, the Escrow Paying Agent and the Subordination Agent, providing for, among other things, the purchase of Equipment Notes by the Trustee on behalf of the Applicable Trust, as the same may be amended, supplemented or otherwise modified from time to time, in accordance with its terms.

Notice of Financing Withdrawal : Has the meaning specified in the Deposit Agreement.

Other Agreement : Means the Basic Agreement as supplemented by a trust supplement relating to the Additional Trust, if any.

Outstanding : When used with respect to Applicable Certificates, means, as of the date of determination, all Transfer Date Certificates, and all other Applicable Certificates theretofore authenticated and delivered under the Agreement, in each case except:

(i) Applicable Certificates theretofore canceled by the Registrar or delivered to the Trustee or the Registrar for cancellation;

(ii) Applicable Certificates for which money in the full amount required to make the final distribution with respect to such Applicable Certificates pursuant to Section 11.01 of the Basic Agreement has been theretofore deposited with the Trustee in trust for the Applicable Certificateholders as provided in Section 4.01 of the Basic Agreement pending distribution of such money to such Applicable Certificateholders pursuant to payment of such final distribution; and

(iii) Applicable Certificates in exchange for or in lieu of which other Applicable Certificates have been authenticated and delivered pursuant to the Agreement.

Participation Agreement : Means each Participation Agreement to be entered into, or entered into (as the case may be), by the Related Trustee pursuant to the Note Purchase Agreement, as the same may be amended, supplemented or otherwise modified in accordance with its terms.

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Pool Balance : Means, with respect to the Applicable Trust or the Applicable Certificates issued by the Applicable Trust, as of any date, (i) the original aggregate face amount of the Applicable Certificates less (ii) the aggregate amount of all payments made as of such date in respect of such Applicable Certificates or in respect of Deposits other than payments made in respect of interest or premium thereon or reimbursement of any costs and expenses incurred in connection therewith. The Pool Balance as of any date shall be computed after giving effect to any special distribution with respect to unused Deposits, the payment of principal, if any, on the Equipment Notes or payment with respect to other Trust Property and the distribution thereof to be made on that date.

Prospectus Supplement : Means the final Prospectus Supplement dated October 5, 2009 relating to the offering of the Applicable Certificates.

Ratings Confirmation : Has the meaning specified in the Intercreditor Agreement.

Related Pass Through Trust Agreement : Means the Basic Agreement as supplemented by the Trust Supplement No. 2009-1A-O, dated as of the date hereof (the “ Related Pass Through Trust Supplement ”), relating to the United Air Lines Pass Through Trust 2009-1A-O and entered into by the Company and the Related Trustee, as amended, supplemented or otherwise modified from time to time in accordance with its terms.

Related Trust : Means United Air Lines Pass Through Trust, Series 2009-1A-O, formed under the Related Pass Through Trust Agreement.

Related Trustee : Means the trustee under the Related Pass Through Trust Agreement.

Scheduled Payment : Means, with respect to any Equipment Note, (i) any payment of principal or interest on such Equipment Note (other than any such payment which is not in fact received by the Trustee or any Subordination Agent within ten (10) Business Days of the date on which such payment is scheduled to be made) or (ii) any payment of interest on the Applicable Certificates with funds drawn under the Liquidity Facility, which payment in any such case represents the installment of principal on such Equipment Note at the stated maturity of such installment, the payment of regularly scheduled interest accrued on the unpaid principal amount of such Equipment Note, or both; provided , however , that any payment of principal, premium, if any, or interest resulting from the redemption or purchase of any Equipment Note shall not constitute a Scheduled Payment.

Special Payment : Means any payment (other tha