Document:

<PAGE>

                                                                     Exhibit 4.8

                        TRUST SUPPLEMENT NO. 2001-1A-2
                                      TO
                         PASS THROUGH TRUST AGREEMENT

                          dated as of August 22, 2001

                                    between

                            UNITED AIR LINES, INC.

                                      and

                      STATE STREET BANK AND TRUST COMPANY

                     OF CONNECTICUT, NATIONAL ASSOCIATION,

                                  as Trustee

                                 $263,614,000

     United Airlines Enhanced Pass Through Certificates, Series 2001-1A-2

<PAGE>

                               TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                                   Page
<S>            <C>                                                                 <C>
Article I     Declaration of Trust...............................................    1
              Section 1.01  Declaration of Trust.................................    1
Article II    The Certificates...................................................    2
              Section 2.01  The Certificates.....................................    2
Article III   Definitions........................................................    3
              Section 3.01  Definitions..........................................    3
Article IV    The Trustee........................................................    3
              Section 4.01  The Trustee..........................................    3
Article V     Miscellaneous Provisions...........................................    3
              Section 5.01  Basic Agreement Ratified.............................    3
              Section 5.02  Governing Law........................................    3
              Section 5.03  Execution in Counterparts............................    4

Exhibit A     Form of Certificate
Exhibit B     DTC Letter of Representations
Exhibit C     Scheduled Payments of Principal on Class A-2 Certificates
Exhibit D     Notes, Principal Amounts and Maturities
Exhibit E     Aircraft, Registration Numbers and Maturities
Exhibit F     Note Documents
</TABLE>

                        TRUST SUPPLEMENT NO.  2001-1A-2

                          Dated as of August 22, 2001

             Enhanced Pass Through Certificates, Series 2001-1A-2

                                       i
<PAGE>

                        TRUST SUPPLEMENT NO. 2001-1A-2

     THIS TRUST SUPPLEMENT NO. 2001-1A-2, dated as of August 22, 2001 (herein
called the "Trust Supplement"), between UNITED AIR LINES, INC., a Delaware
corporation (the "Company"), and STATE STREET BANK AND TRUST COMPANY OF
CONNECTICUT, NATIONAL ASSOCIATION, a national banking association (the
"Trustee"), to the Pass Through Trust Agreement, dated as of August 22, 2001
(the "Basic Agreement"), between the Company and the Trustee.

     WHEREAS, the Basic Agreement, which is unlimited as to the aggregate
principal amount of Certificates that may be issued thereunder, has heretofore
or concurrently herewith been executed and delivered;

     WHEREAS, pursuant to each Indenture with respect to the Aircraft, United
proposes to issue on a recourse basis the Notes to refinance its current
indebtedness originally incurred to finance the Aircraft;

     WHEREAS, the Trustee intends to declare the creation of this 2001-1A-2
Trust (the "Class A-2 Trust") for the benefit of the Certificateholders of the
Class A-2 Certificates to be issued in respect of such Class A-2 Trust, and the
initial Certificateholders of the Class A-2 Certificates, as grantors of such
Class A-2 Trust, by their respective acceptances of the Class A-2 Certificates,
shall join in the creation of the Class A-2 Trust with the Trustee;

     WHEREAS, all Certificates to be issued by the Class A-2 Trust will evidence
fractional undivided interests in the Class A-2 Trust created hereby and will
convey no rights, benefits or interests in respect of any property other than
the Trust Property of the Class A-2 Trust;

     WHEREAS, pursuant to the terms and conditions of the Basic Agreement, as
supplemented by this Trust Supplement, and the Note Purchase Agreement, the
Trustee on behalf of the Class A-2 Trust shall purchase the Notes issued by the
Company, in each case having the identical interest rate as, and final maturity
dates not later than the final Regular Distribution Date of, the Class A-2
Certificates issued hereunder and shall hold such Notes in trust for the benefit
of the Class A-2 Certificateholders;

     WHEREAS, 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;

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

                                   ARTICLE I
                             DECLARATION OF TRUST

     SECTION 1.01 Declaration of Trust. The Trustee hereby declares the creation
                  --------------------
of this Class A-2 Trust for the benefit of the Certificateholders of the
Certificates issued hereunder, and
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

the initial Certificateholders, as the grantors of the Class A-2 Trust, by their
respective acceptances of the Certificates issued hereunder, join in the
creation of this Class A-2 Trust with the Trustee.

                                  ARTICLE II
                               THE CERTIFICATES

     Section 2.01 The Certificates. There is hereby created a class of
                  ----------------
Certificates to be issued under the Agreement, designated and to be
distinguished and known as "United Airlines Enhanced Pass Through Certificates,
Series 2001-1A-2" (the "Class A-2 Certificates"). Each Certificate represents a
Fractional Undivided Interest in the Class A-2 Trust created hereby. The terms
and conditions applicable to the Class A-2 Certificates are as follows:

     1.  The maximum aggregate principal amount of the Class A-2 Certificates
that shall be authenticated under the Agreement (except for Class A-2
Certificates authenticated and delivered pursuant to Sections 3.04, 3.07 and
3.10 of the Basic Agreement) upon their initial issuance is $263,614,000.

     2.  The Cut-off Date is August 22, 2001.

     3.  The Regular Distribution Dates with respect to any payment of Scheduled
Payments means each March 1 and September 1, commencing on March 1, 2002, until
payment of all of the Scheduled Payments to be made under the Notes described in
Exhibit D.
---------

     4.  The Special Distribution Dates with respect to the Class A-2
Certificates are any Business Day on which a Special Payment is to be
distributed pursuant to the Agreement.

     5.  The Class A-2 Certificates shall be in the form attached hereto as
Exhibit A, shall be Global Certificates and shall be subject to the conditions
---------
set forth in the Letter of Representations between the Company and The
Depository Trust Company, as initial Clearing Agency, attached hereto as
Exhibit B.
---------

     6.  The Scheduled Payments of principal shall be as set forth in Exhibit C.
                                                                      ---------

     7.  The proceeds of the Class A-2 Certificates issued under the Class A-2
Trust shall be used to purchase the Notes described in Exhibit D.
                                                       ---------

     8.  The Notes described in paragraph 7 relate to the Aircraft listed in
Exhibit E.
---------

     9.  The related Note Documents are listed in Exhibit F.
                                                  ---------

     10. The Class A-2 Certificates shall bear the following legend:

     EITHER: (A) THE HOLDER IS NOT ACQUIRING THIS CERTIFICATE OR AN INTEREST
HEREIN WITH PLAN ASSETS OF ANY PLAN OR AN INDIVIDUAL RETIREMENT PLAN SUBJECT TO
TITLE I OF THE EMPLOYEE RETIREMENT INCOME

                                       2
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

SECURITY ACT OF 1974, AS AMENDED ("ERISA"), OR SECTION 4975 OF THE INTERNAL
REVENUE CODE OF 1986, AS AMENDED (THE "CODE"); OR (B) THE HOLDER'S PURCHASE AND
HOLDING OF THIS CERTIFICATE OR AN INTEREST HEREIN ARE EXEMPT FROM THE PROHIBITED
TRANSACTION RESTRICTIONS OF SECTION 406(A) OF ERISA AND SECTION 4975 OF THE CODE
BY AN ADMINISTRATIVE CLASS PROHIBITED TRANSACTION EXEMPTION GRANTED BY THE
DEPARTMENT OF LABOR.

          11. The Class A-2 Certificates shall have the benefit of the Class A-2
Liquidity Facility.

                                  ARTICLE III
                                  DEFINITIONS

     Section 3.01 Definitions. Unless otherwise specified herein, capitalized
                  -----------
terms used herein without definition shall have the respective meanings
specified heretofore in the Basic Agreement or in the Intercreditor Agreement.

                                  ARTICLE IV
                                  THE TRUSTEE

     Section 4.01 The Trustee. The Trustee shall not be responsible in any
                  -----------
manner whatsoever for or in respect of the validity or sufficiency of this Trust
Supplement or the due execution hereof by the Company or for or in respect of
the recitals and statements contained herein, all of which recitals and
statements are made solely by the Company.

     Except as herein otherwise provided, no duties, responsibilities or
liabilities are assumed, or shall be construed to be assumed by the Trustee by
reason of this Trust Supplement other than as set forth in the Basic Agreement,
and this Trust Supplement is executed and accepted on behalf of the Trustee,
subject to all the terms and conditions set forth in the Basic Agreement, upon
the effectiveness thereof, as fully to all intents as if the same were herein
set forth at length.

                                   ARTICLE V
                           MISCELLANEOUS PROVISIONS

     Section 5.01  Basic Agreement Ratified. Except and so far as herein
                   ------------------------
expressly provided, all of the provisions, terms and conditions of the Basic
Agreement are in all respects ratified and confirmed; and the Basic Agreement
and this Trust Supplement shall be taken, read and construed as one and the same
instrument.

     Section 5.02  GOVERNING LAW.  THIS TRUST SUPPLEMENT AND THE CLASS A-2
                   -------------
CERTIFICATES SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF
THE STATE OF NEW YORK (WITHOUT REGARD TO THE PRINCIPLES OF CONFLICTS OF LAWS
THEREOF (OTHER THAN SECTION 5-1401 OF THE NEW YORK GENERAL OBLIGATIONS LAW)) AND
THE OBLIGATIONS, RIGHTS AND REMEDIES OF THE PARTIES UNDER THIS AGREEMENT SHALL
BE DETERMINED IN ACCORDANCE WITH SUCH LAWS.

                                       3
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

     Section 5.03 Execution in Counterparts. This Trust Supplement 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.

                                       4
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

     IN WITNESS WHEREOF, the Company and the Trustee have caused this Trust
Supplement to be duly executed by their respective officers thereto duly
authorized, as of the day and year first written above.

                                   UNITED AIR LINES, INC.

                                   By: /s/ Jeffrey T. Kawalsky
                                      ------------------------------------------
                                   Name: Jeffrey T. Kawalsky
                                   Title: Assistant Treasurer

                                   STATE STREET BANK AND TRUST
                                   COMPANY OF CONNECTICUT,
                                   NATIONAL ASSOCIATION, as Trustee

                                   By: /s/ John G. Correia
                                      ------------------------------------------
                                   Name: John G. Correia
                                   Title: Assistant Vice President

                                       5
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

                                   EXHIBIT A

                              FORM OF CERTIFICATE

                             CLASS A-2 CERTIFICATE

                           REGISTERED $______________
                         Fractional Undivided Interest

No. R - __                                           CUSIP NO.__________________

     UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE
DEPOSITORY TRUST COMPANY, A NEW YORK CORPORATION ("DTC"), TO THE TRUSTEE OR ITS
                                                   ---
AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY CERTIFICATE
ISSUED IN EXCHANGE FOR THIS CERTIFICATE IS REGISTERED IN THE NAME OF CEDE & CO.
OR IN SUCH OTHER NAME AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC
(AND ANY PAYMENT HEREON IS MADE TO CEDE & CO.  OR TO SUCH OTHER ENTITY AS IS
REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE OR OTHER
USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS
THE REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

     TRANSFERS OF THIS CERTIFICATE SHALL BE LIMITED TO TRANSFERS IN WHOLE, BUT
NOT IN PART, TO NOMINEES OF DTC OR TO A SUCCESSOR THEREOF OR SUCH SUCCESSOR'S
NOMINEE AND TRANSFERS OF PORTIONS OF THIS CERTIFICATE SHALL BE LIMITED TO
TRANSFERS MADE IN ACCORDANCE WITH THE RESTRICTIONS SET FORTH IN SECTION 3.05 OF
THE PASS THROUGH TRUST AGREEMENT REFERRED TO HEREIN.

     EITHER:  (A) THE HOLDER IS NOT ACQUIRING THIS CERTIFICATE OR AN INTEREST
HEREIN WITH PLAN ASSETS OF ANY PLAN OR AN INDIVIDUAL RETIREMENT PLAN SUBJECT TO
TITLE I OF THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974, AS AMENDED
("ERISA"), OR SECTION 4975 OF THE INTERNAL REVENUE CODE OF 1986, AS AMENDED (THE
"CODE"); OR (B) THE HOLDER'S PURCHASE AND HOLDING OF THIS CERTIFICATE OR AN
INTEREST HEREIN ARE EXEMPT FROM THE PROHIBITED TRANSACTION RESTRICTIONS OF
SECTION 406(A) OF ERISA AND SECTION 4975 OF THE CODE BY AN ADMINISTRATIVE CLASS
PROHIBITED TRANSACTION EXEMPTION GRANTED BY THE DEPARTMENT OF LABOR.

                                  Exhibit A
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

             UNITED AIRLINES 2001-1A-2 ENHANCED PASS THROUGH TRUST

                     UNITED AIRLINES ENHANCED PASS THROUGH
                         CERTIFICATE, SERIES 2001-1A-2

           Final Expected Regular Distribution Date:  ______________

evidencing a fractional undivided interest in a trust, the property of which
includes certain equipment notes each secured by an Aircraft owned by United Air
Lines, Inc.

     THIS CERTIFIES THAT CEDE & CO., for value received, is the registered owner
of a _________________________________ ($______________) Fractional Undivided
Interest in the United Airlines 2001-1A-2 Pass Through Trust (the "Trust")
created by State Street Bank and Trust Company of Connecticut, National
Association, as trustee (the "Trustee") pursuant to a Pass Through Trust
Agreement, dated as of August 22, 2001 (the "Basic Agreement"), as supplemented
by Trust Supplement No. 2001-1A-2 (collectively, the "Agreement") between the
Trustee and United Air Lines, Inc., a Delaware corporation (the "Company"), a
summary of certain of the pertinent provisions of which is set forth below.  To
the extent not otherwise defined herein, the capitalized terms used herein have
the meanings assigned to them in the Agreement.  This Certificate is one of the
duly authorized Certificates designated as "United Airlines Enhanced Pass
Through Certificates, Series 2001-1A-2" (the "Certificates").  This Certificate
is issued under and is subject to the terms, provisions and conditions of the
Agreement.

     By virtue of its acceptance hereof, the Certificateholder of this
Certificate assents to and agrees to be bound by the provisions of the
Agreement, the Intercreditor Agreement and the Note Purchase Agreement.  The
property of the Trust includes (i) certain Notes and all monies at any time paid
thereon and all monies due and to become due thereunder, (ii) funds from time to
time deposited in the related Escrow Account, the related Certificate Account
and the related Special Payments Account, (iii) all rights of such Trust and the
Trustee, on behalf of such Trust, under the Intercreditor Agreement and the Note
Purchase Agreement, including all rights to receive certain payments thereunder
and all monies paid to such Trustee on behalf of such Trust pursuant to the
Intercreditor Agreement and the Note Purchase Agreement and (iv) all monies
receivable by the Subordination Agent under the Liquidity Facility for the Trust
(the "Trust Property").  Each issue of the Notes is secured directly or
indirectly by, among other things, a security interest in the Aircraft owned by
the Company.

     The Certificates represent Fractional Undivided Interests in the Trust and
the Trust Property, and have no rights, benefits or interest in respect of any
assets or property of any trust of another class.

     Interest applicable to this Certificate will be payable at a rate equal to
_____%.

     Subject to and in accordance with the terms of the Agreement, the
Intercreditor Agreement and the Note Purchase Agreement, from and to the extent
of funds then available to the Trustee, there shall be distributed on each
[    ] and [    ] (a "Regular Distribution Date"), commencing on [    ], to the
Person in whose name this Certificate is registered at

                                  Exhibit A
                                    Page 2
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

the close of business on the 15th day preceding such Regular Distribution Date,
an amount in respect of the Scheduled Payments on the Notes due on such Regular
Distribution Date, the receipt of which has been confirmed by the Trustee, equal
to the product of the percentage interest in the Trust evidenced by this
Certificate and an amount equal to the sum of such Scheduled Payments. Subject
to and in accordance with the terms of the Agreement, the Intercreditor
Agreement and the Note Purchase Agreement, if Special Payments on the Notes are
received by the Trustee, from funds then available to the Trustee, there shall
be distributed on the applicable Special Distribution Date, to the Person in
whose name this Certificate is registered at the close of business on the 15th
day preceding the Special Distribution Date, an amount in respect of such
Special Payments on the Notes, the receipt of which has been confirmed by the
Trustee, equal to the product of the percentage interest in the Trust evidenced
by this Certificate and an amount equal to the sum of such Special Payments so
received. If a Regular Distribution Date or Special Distribution Date is not a
Business Day, distribution shall be made on the next Business Day with the same
force and effect as if made on such Regular Distribution Date or Special
Distribution Date and interest shall accrue during the intervening period. The
Trustee shall mail notice of each Special Payment and the Special Distribution
Date therefor to the Certificateholder of this Certificate.

     Except as otherwise provided in the Agreement and notwithstanding the
above, the final distribution on this Certificate shall be made after notice
mailed by the Trustee of the pendency of such distribution and only upon
presentation and surrender of this Certificate at the office or agency of the
Trustee specified in such notice.

     THE AGREEMENT AND THIS CERTIFICATE SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK (WITHOUT REGARD TO THE
PRINCIPLES OF CONFLICTS OF LAWS  THEREOF (OTHER THAN SECTION 5-1401 OF THE NEW
YORK GENERAL OBLIGATIONS LAW)) AND THE OBLIGATIONS, RIGHTS AND REMEDIES OF THE
HOLDER OF THIS CERTIFICATE UNDER THE AGREEMENT SHALL BE DETERMINED IN ACCORDANCE
WITH SUCH LAWS.

     Reference is hereby made to the further provisions of this Certificate set
forth in the reverse hereof, which further provisions shall for all purposes
have the same effect as if set forth at this place.

     Unless the certificate of authentication hereon has been executed by the
Trustee, by manual signature, this Certificate shall not be entitled to any
benefit under the Agreement or be valid or obligatory for any purpose.

                                  Exhibit A
                                    Page 3
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

     IN WITNESS WHEREOF, the Trustee, on behalf of the Issuer and not in its
individual capacity, has caused this Certificate to be duly executed.

                                        UNITED AIRLINES 2001-1A-2 PASS THROUGH
                                        TRUST

                                        By: STATE STREET BANK AND TRUST COMPANY
                                            CONNECTICUT, NATIONAL ASSOCIATION,
                                            not in its individual capacity but
                                            solely as Trustee

Dated:____________________________      By:_________________________________
                                        Name:_______________________________
                                        Title:______________________________

                                   Exhibit A
                                    Page 4
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

                    TRUSTEE'S CERTIFICATE OF AUTHENTICATION

     This is one of the Certificates referred to in the within-mentioned Trust
Agreement.

STATE STREET BANK AND TRUST COMPANY        STATE STREET BANK AND TRUST COMPANY
OF CONNECTICUT, NATIONAL ASSOCIATION,      OF CONNECTICUT, NATIONAL ASSOCIATION,
not in its individual capacity but         not in its individual capacity but
solely as Trustee                          solely as Trustee

By:__________________________________      By:__________________________________
                                                     Authenticating Agent

                                           By:__________________________________

                                   Exhibit A
                                    Page 5
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

                           [REVERSE OF CERTIFICATE]

     The Certificates do not represent a direct obligation of, or an obligation
guaranteed by, or an interest in, the Company or the Trustee or any of their
Affiliates.  The Certificates are limited in right or payment, all as more
specifically set forth on the face hereof and in the Agreement.  All payments or
distributions made to Certificateholders under the Agreement shall be made only
from the Trust Property and only to the extent that the Trustee shall have
sufficient income or proceeds from the Trust Property to make such payments in
accordance with the terms of the Agreement.  Each Certificateholder of this
Certificate, by its acceptance hereof, agrees that it shall look solely to the
income and proceeds from the Trust Property to the extent available for
distribution to such Certificateholder as provided in the Agreement.  This
Certificate does not purport to summarize the Agreement and reference is made to
the Agreement for information with respect to the interests, rights, benefits,
obligations, proceeds and duties evidenced hereby.  A copy of the Agreement may
be examined during normal business hours at the principal office of the Trustee,
and at such other places, if any, designated by the Trustee, by any
Certificateholder upon request.

     The Agreement permits, with certain exceptions therein provided, the
amendment thereof and the modification of the rights and obligations of the
Company and the rights of the Certificateholders under the Agreement at any time
by the Company and the Trustee with the consent of the Certificateholders
holding Certificates evidencing Fractional Cumulative Interests aggregating not
less than a majority in interest in the Trust.  Any such consent by the
Certificateholder of this Certificate shall be conclusive and binding on such
Certificateholder and upon all future Certificateholders of this Certificate and
of any Certificate issued upon the transfer hereof or in exchange hereof or in
lieu hereof whether or not notation of such consent is made upon this
Certificate.  The Agreement also permits the amendment thereof, in certain
limited circumstances, without the consent of the Certificateholders of any of
the Certificates.

     As provided in the Agreement and subject to certain limitations therein set
forth, the transfer of this Certificate is registrable in the Register upon
surrender of this Certificate for registration of transfer at the offices or
agencies maintained by the Trustee in its capacity as Registrar, or by any
successor Registrar, at its Corporate Trust Office, duly endorsed or accompanied
by a written instrument of transfer in form satisfactory to the Trustee and the
Registrar duly executed by the Certificateholder hereof or such
Certificateholder's attorney duly authorized in writing, and thereupon one or
more new Certificates of authorized denominations evidencing the same aggregate
Fractional Undivided Interest in the Trust shall be issued to the designated
transferee or transferees.

     The Certificates are issuable only as registered Certificates without
coupons in minimum denominations of $1,000 Fractional Undivided Interest and
integral multiples of $1,000 in excess thereof.  As provided in the Agreement
and subject to certain limitations therein set forth, the Certificates are
exchangeable for new Certificates of authorized denominations evidencing the
same aggregate Fractional Undivided Interest in the Trust, as requested by the
Certificateholder surrendering the same.

                                   Exhibit A
                                    Page 6
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

     No service charge shall be made for any such registration of transfer or
exchange, but the Trustee shall require payment by the Certificateholder of a
sum sufficient to cover any tax or governmental charge payable in connection
therewith.

     The Trustee, the Registrar, and any agent of the Trustee or the Registrar
may treat the person in whose name this Certificate is registered as the owner
hereof for all purposes, and neither the Trustee, the Registrar, nor any such
agent shall be affected by any notice to the contrary.

     The obligations and responsibilities created by the Agreement and the Trust
created thereby shall terminate upon the distribution to Certificateholders of
all amounts required to be distributed to them pursuant to the Agreement and the
disposition of all property held as part of the Trust Property.

                                   Exhibit A
                                    Page 7
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

                            FORM OF TRANSFER NOTICE

     FOR VALUE RECEIVED the undersigned registered holder hereby sell(s),
assign(s) and transfer(s) unto

Insert              Taxpayer                 Identification                No.

_______________________________________________________________________________
please print or typewrite name and address including zip code of assignee

_______________________________________________________________________________
the within Certificate and all rights thereunder, hereby irrevocably
constituting and appointing

_______________________________________________________________________________
attorney to transfer said Certificate on the books of the Trustee with full
power of substitution in the premises.

Date:                                   [Name of Transferor]

                                        _______________________________________

                                        NOTE: The signature must correspond with
                                        the name as written upon the face of the
                                        within-mentioned instrument in every
                                        particular, without alteration or any
                                        change whatsoever.

Signature Guarantee:________________

                                   Exhibit A
                                    Page 8
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

                                   EXHIBIT B

                         DTC LETTER OF REPRESENTATIONS

                                   Exhibit B
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

                                   EXHIBIT C

                        SCHEDULED PAYMENTS OF PRINCIPAL
                           ON CLASS A-2 CERTIFICATES

           Payment Date                             Principal Payment
-------------------------------------   ----------------------------------------

                                   Exhibit C
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

                                   EXHIBIT D

                       PRINCIPAL AMOUNTS AND MATURITIES

          Notes                 Principal Amount               Maturity
-------------------------  --------------------------  -------------------------

                                   Exhibit D
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

                                   EXHIBIT E

                 AIRCRAFT, REGISTRATION NUMBERS AND MATURITIES

                                    Aircraft
                                  Registration
        Aircraft                     Number                    Maturity
-------------------------  --------------------------  -------------------------

                                   Exhibit E
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

                                   EXHIBIT F

                                NOTE DOCUMENTS

<TABLE>
<CAPTION>
                                       Aircraft
                                     Registration
      Aircraft                          Number                            Note Document
 ----------------            ----------------------------        ---------------------------------
<S>                          <C>                                 <C>
      A319-131                          N831UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A319-131                          N833UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A319-131                          N834UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A319-131                          N835UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D
</TABLE>

                                   Exhibit F
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

<TABLE>
<S>                                     <C>                        <C>
      A319-131                          N836UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A319-131                          N837UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A319-131                          N838UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A319-131                          N839UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D
</TABLE>

                                   Exhibit F
                                    Page 2
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

<TABLE>
<S>                                     <C>                        <C>
      A319-131                          N840UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A319-131                          N841UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A320-232                          N467UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A320-232                          N468UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A320-232                          N469UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D
</TABLE>

                                   Exhibit F
                                    Page 3
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

<TABLE>
<S>                                     <C>                        <C>
      A320-232                          N470UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A320-232                          N471UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      A320-232                          N472UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B747-422                          N117UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B747-422                          N118UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D
</TABLE>

                                   Exhibit F
                                    Page 4
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

<TABLE>
<S>                                     <C>                        <C>
      B747-422                          N122UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B747-422                          N127UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B747-422                          N128UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B767-322                          N664UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B767-322                          N667UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D
</TABLE>

                                   Exhibit F
                                    Page 5
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

<TABLE>
<S>                                     <C>                        <C>
      B767-322                          N669UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B767-322                          N670UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B767-322                          N671UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B777-222                          N794UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B777-222                          N795UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-2
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D
</TABLE>

                                   Exhibit F
                                    Page 6
<PAGE>

                                                [Trust Supplement No. 2001-1A-2]

<TABLE>
<S>                                     <C>                        <C>
      B777-222                          N204UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D

      B777-222                          N205UA                     Participation Agreement
                                                                   Trust Indenture and Mortgage
                                                                   Equipment Note, Series A-1
                                                                   Equipment Note, Series A-3
                                                                   Equipment Note, Series B
                                                                   Equipment Note, Series C
                                                                   Equipment Note, Series D
</TABLE>

                                   Exhibit F
                                    Page 7<PAGE>

                                                                     Exhibit 4.9

                        TRUST SUPPLEMENT NO. 2001-1A-3
                                      TO
                         PASS THROUGH TRUST AGREEMENT

                          dated as of August 22, 2001

                                    between

                            UNITED AIR LINES, INC.

                                      and

                      STATE STREET BANK AND TRUST COMPANY
                     OF CONNECTICUT, NATIONAL ASSOCIATION,
                                  as Trustee

                                 $372,987,000

     United Airlines Enhanced Pass Through Certificates, Series 2001-1A-3

<PAGE>

                               TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                        Page
<S>                                                                     <C>
Article I     Declaration of Trust....................................     1
              Section 1.01 Declaration of Trust.......................     1
Article II    The Certificates........................................     2
              Section 2.01 The Certificates...........................     2
Article III   Definitions.............................................     3
              Section 3.01 Definitions................................     3
Article IV    The Trustee.............................................     3
              Section 4.01 The Trustee................................     3
Article V     Miscellaneous Provisions................................     3
              Section 5.01 Basic Agreement Ratified...................     3
              Section 5.02 Governing Law..............................     3
              Section 5.03 Execution in Counterparts..................     4
</TABLE>

Exhibit A      Form of Certificate
Exhibit B      DTC Letter of Representations
Exhibit C      Scheduled Payments of Principal on Class A-3 Certificates
Exhibit D      Notes, Principal Amounts and Maturities
Exhibit E      Aircraft, Registration Numbers and Maturities
Exhibit F      Note Documents

                        TRUST SUPPLEMENT NO. 2001-1A-3

                          Dated as of August 22, 2001

              Enhanced Pass Through Certificates, Series 2001-1A-3

                                       i
<PAGE>

                        TRUST SUPPLEMENT NO. 2001-1A-3

     THIS TRUST SUPPLEMENT NO. 2001-1A-3, dated as of August 22, 2001 (herein
called the "Trust Supplement"), between UNITED AIR LINES, INC., a Delaware
corporation (the "Company"), and STATE STREET BANK AND TRUST COMPANY OF
CONNECTICUT, NATIONAL ASSOCIATION, a national banking association (the
"Trustee"), to the Pass Through Trust Agreement, dated as of August 22, 2001
(the "Basic Agreement"), between the Company and the Trustee.

     WHEREAS, the Basic Agreement, which is unlimited as to the aggregate
principal amount of Certificates that may be issued thereunder, has heretofore
or concurrently herewith been executed and delivered;

     WHEREAS, pursuant to each Indenture with respect to the Aircraft, United
proposes to issue on a recourse basis the Notes to refinance its current
indebtedness originally incurred to finance the Aircraft;

     WHEREAS, the Trustee intends to declare the creation of this 2001-1A-3
Trust (the "Class A-3 Trust") for the benefit of the Certificateholders of the
Class A-3 Certificates to be issued in respect of such Class A-3 Trust, and the
initial Certificateholders of the Class A-3 Certificates, as grantors of such
Class A-3 Trust, by their respective acceptances of the Class A-3 Certificates,
shall join in the creation of the Class A-3 Trust with the Trustee;

     WHEREAS, all Certificates to be issued by the Class A-3 Trust will evidence
fractional undivided interests in the Class A-3 Trust created hereby and will
convey no rights, benefits or interests in respect of any property other than
the Trust Property of the Class A-3 Trust;

     WHEREAS, pursuant to the terms and conditions of the Basic Agreement, as
supplemented by this Trust Supplement, and the Note Purchase Agreement, the
Trustee on behalf of the Class A-3 Trust shall purchase the Notes issued by the
Company, in each case having the identical interest rate as, and final maturity
dates not later than the final Regular Distribution Date of, the Class A-3
Certificates issued hereunder and shall hold such Notes in trust for the benefit
of the Class A-3 Certificateholders;

     WHEREAS, 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;

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

                                   ARTICLE I
                             DECLARATION OF TRUST

     Section 1.01  Declaration of Trust. The Trustee hereby declares the
                   --------------------
creation of this Class A-3 Trust for the benefit of the Certificateholders of
the Certificates issued hereunder, and
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

the initial Certificateholders, as the grantors of the Class A-3 Trust, by their
respective acceptances of the Certificates issued hereunder, join in the
creation of this Class A-3 Trust with the Trustee.

                                  ARTICLE II
                               THE CERTIFICATES

     Section 2.01  The Certificates. There is hereby created a class of
                   ----------------
Certificates to be issued under the Agreement, designated and to be
distinguished and known as "United Airlines Enhanced Pass Through Certificates,
Series 2001-1A-3" (the "Class A-3 Certificates"). Each Certificate represents a
Fractional Undivided Interest in the Class A-3 Trust created hereby. The terms
and conditions applicable to the Class A-3 Certificates are as follows:

             1.    The maximum aggregate principal amount of the Class A-3
Certificates that shall be authenticated under the Agreement (except for Class
A-3 Certificates authenticated and delivered pursuant to Sections 3.04, 3.07 and
3.10 of the Basic Agreement) upon their initial issuance is $372,987,000.

             2.    The Cut-off Date is August 22, 2001.

             3.    The Regular Distribution Dates with respect to any payment of
Scheduled Payments means each March 1 and September 1, commencing on March 1,
2002, until payment of all of the Scheduled Payments to be made under the Notes
described in Exhibit D.
             ---------

             4.    The Special Distribution Dates with respect to the Class A-3
Certificates are any Business Day on which a Special Payment is to be
distributed pursuant to the Agreement.

             5.    The Class A-3 Certificates shall be in the form attached
hereto as Exhibit A, shall be Global Certificates and shall be subject to the
          ---------
conditions set forth in the Letter of Representations between the Company and
The Depository Trust Company, as initial Clearing Agency, attached hereto as
Exhibit B.
---------

             6.    The Scheduled Payments of principal shall be as set forth in
Exhibit C.
---------

             7.    The proceeds of the Class A-3 Certificates issued under the
Class A-3 Trust shall be used to purchase the Notes described in Exhibit D.
                                                                 ---------

             8.    The Notes described in paragraph 7 relate to the Aircraft
listed in Exhibit E.
          ---------

             9.    The related Note Documents are listed in Exhibit F.
                                                            ---------

             10.   The Class A-3 Certificates shall bear the following legend:

     EITHER: (A) THE HOLDER IS NOT ACQUIRING THIS CERTIFICATE OR AN INTEREST
HEREIN WITH PLAN ASSETS OF ANY PLAN OR AN INDIVIDUAL RETIREMENT PLAN SUBJECT TO
TITLE I OF THE EMPLOYEE RETIREMENT INCOME

                                       2
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

SECURITY ACT OF 1974, AS AMENDED ("ERISA"), OR SECTION 4975 OF THE INTERNAL
REVENUE CODE OF 1986, AS AMENDED (THE "CODE"); OR (B) THE HOLDER'S PURCHASE AND
HOLDING OF THIS CERTIFICATE OR AN INTEREST HEREIN ARE EXEMPT FROM THE PROHIBITED
TRANSACTION RESTRICTIONS OF SECTION 406(A) OF ERISA AND SECTION 4975 OF THE CODE
BY AN ADMINISTRATIVE CLASS PROHIBITED TRANSACTION EXEMPTION GRANTED BY THE
DEPARTMENT OF LABOR.

     11.  The Class A-3 Certificates shall have the benefit of the Class A-3
Liquidity Facility.

                                  ARTICLE III
                                  DEFINITIONS

     Section 3.01  Definitions. Unless otherwise specified herein, capitalized
                   -----------
terms used herein without definition shall have the respective meanings
specified heretofore in the Basic Agreement or in the Intercreditor Agreement.

                                  ARTICLE IV
                                  THE TRUSTEE

     Section 4.01  The Trustee. The Trustee shall not be responsible in any
                   -----------
manner whatsoever for or in respect of the validity or sufficiency of this Trust
Supplement or the due execution hereof by the Company or for or in respect of
the recitals and statements contained herein, all of which recitals and
statements are made solely by the Company.

     Except as herein otherwise provided, no duties, responsibilities or
liabilities are assumed, or shall be construed to be assumed by the Trustee by
reason of this Trust Supplement other than as set forth in the Basic Agreement,
and this Trust Supplement is executed and accepted on behalf of the Trustee,
subject to all the terms and conditions set forth in the Basic Agreement, upon
the effectiveness thereof, as fully to all intents as if the same were herein
set forth at length.

                                   ARTICLE V
                           MISCELLANEOUS PROVISIONS

     Section 5.01  Basic Agreement Ratified. Except and so far as herein
                   ------------------------
expressly provided, all of the provisions, terms and conditions of the Basic
Agreement are in all respects ratified and confirmed; and the Basic Agreement
and this Trust Supplement shall be taken, read and construed as one and the same
instrument.

     Section 5.02  GOVERNING LAW. THIS TRUST SUPPLEMENT AND THE CLASS A-3
                   -------------
CERTIFICATES SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF
THE STATE OF NEW YORK (WITHOUT REGARD TO THE PRINCIPLES OF CONFLICTS OF LAWS
THEREOF (OTHER THAN SECTION 5-1401 OF THE NEW YORK GENERAL OBLIGATIONS LAW)) AND
THE OBLIGATIONS, RIGHTS AND REMEDIES OF THE PARTIES UNDER THIS AGREEMENT SHALL
BE DETERMINED IN ACCORDANCE WITH SUCH LAWS.

                                       3
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

     Section 5.03  Execution in Counterparts. This Trust Supplement 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.

                                       4
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

     IN WITNESS WHEREOF, the Company and the Trustee have caused this Trust
Supplement to be duly executed by their respective officers thereto duly
authorized, as of the day and year first written above.

                                        UNITED AIR LINES, INC.

                                        By:/s/ Jeffrey T. Kawalsky
                                           ----------------------------
                                        Name: Jeffrey T. Kawalsky
                                        Title: Assistant Treasurer

                                        STATE STREET BANK AND TRUST
                                        COMPANY OF CONNECTICUT,
                                        NATIONAL ASSOCIATION, as Trustee

                                        By:/s/ John G. Correia
                                           ----------------------------
                                        Name: John G. Correia
                                        Title: Assistant Vice President

                                       5
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

                                   EXHIBIT A

                              FORM OF CERTIFICATE

                             CLASS A-3 CERTIFICATE

                          REGISTERED $______________
                         Fractional Undivided Interest

No. R - __                                           CUSIP NO. ________________

     UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE
DEPOSITORY TRUST COMPANY, A NEW YORK CORPORATION ("DTC"), TO THE TRUSTEE OR ITS
                                                   ---
AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY CERTIFICATE
ISSUED IN EXCHANGE FOR THIS CERTIFICATE IS REGISTERED IN THE NAME OF CEDE & CO.
OR IN SUCH OTHER NAME AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC
(AND ANY PAYMENT HEREON IS MADE TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS
REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE OR OTHER
USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS
THE REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

     TRANSFERS OF THIS CERTIFICATE SHALL BE LIMITED TO TRANSFERS IN WHOLE, BUT
NOT IN PART, TO NOMINEES OF DTC OR TO A SUCCESSOR THEREOF OR SUCH SUCCESSOR'S
NOMINEE AND TRANSFERS OF PORTIONS OF THIS CERTIFICATE SHALL BE LIMITED TO
TRANSFERS MADE IN ACCORDANCE WITH THE RESTRICTIONS SET FORTH IN SECTION 3.05 OF
THE PASS THROUGH TRUST AGREEMENT REFERRED TO HEREIN.

     EITHER: (A) THE HOLDER IS NOT ACQUIRING THIS CERTIFICATE OR AN INTEREST
HEREIN WITH PLAN ASSETS OF ANY PLAN OR AN INDIVIDUAL RETIREMENT PLAN SUBJECT TO
TITLE I OF THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974, AS AMENDED
("ERISA"), OR SECTION 4975 OF THE INTERNAL REVENUE CODE OF 1986, AS AMENDED (THE
"CODE"); OR (B) THE HOLDER'S PURCHASE AND HOLDING OF THIS CERTIFICATE OR AN
INTEREST HEREIN ARE EXEMPT FROM THE PROHIBITED TRANSACTION RESTRICTIONS OF
SECTION 406(A) OF ERISA AND SECTION 4975 OF THE CODE BY AN ADMINISTRATIVE CLASS
PROHIBITED TRANSACTION EXEMPTION GRANTED BY THE DEPARTMENT OF LABOR.

                                   Exhibit A
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

             UNITED AIRLINES 2001-1A-3 ENHANCED PASS THROUGH TRUST

                     UNITED AIRLINES ENHANCED PASS THROUGH
                         CERTIFICATE, SERIES 2001-1A-3

           Final Expected Regular Distribution Date: ______________

evidencing a fractional undivided interest in a trust, the property of which
includes certain equipment notes each secured by an Aircraft owned by United Air
Lines, Inc.

     THIS CERTIFIES THAT CEDE & CO., for value received, is the registered owner
of a _________________________________ ($______________) Fractional Undivided
Interest in the United Airlines 2001-1A-3 Pass Through Trust (the "Trust")
created by State Street Bank and Trust Company of Connecticut, National
Association, as trustee (the "Trustee") pursuant to a Pass Through Trust
Agreement, dated as of August 22, 2001 (the "Basic Agreement"), as supplemented
by Trust Supplement No. 2001-1A-3 (collectively, the "Agreement") between the
Trustee and United Air Lines, Inc., a Delaware corporation (the "Company"), a
summary of certain of the pertinent provisions of which is set forth below. To
the extent not otherwise defined herein, the capitalized terms used herein have
the meanings assigned to them in the Agreement. This Certificate is one of the
duly authorized Certificates designated as "United Airlines Enhanced Pass
Through Certificates, Series 2001-1A-3" (the "Certificates"). This Certificate
is issued under and is subject to the terms, provisions and conditions of the
Agreement.

     By virtue of its acceptance hereof, the Certificateholder of this
Certificate assents to and agrees to be bound by the provisions of the
Agreement, the Intercreditor Agreement and the Note Purchase Agreement. The
property of the Trust includes (i) certain Notes and all monies at any time paid
thereon and all monies due and to become due thereunder, (ii) funds from time to
time deposited in the related Escrow Account, the related Certificate Account
and the related Special Payments Account, (iii) all rights of such Trust and the
Trustee, on behalf of such Trust, under the Intercreditor Agreement and the Note
Purchase Agreement, including all rights to receive certain payments thereunder
and all monies paid to such Trustee on behalf of such Trust pursuant to the
Intercreditor Agreement and the Note Purchase Agreement and (iv) all monies
receivable by the Subordination Agent under the Liquidity Facility for the Trust
(the "Trust Property"). Each issue of the Notes is secured directly or
indirectly by, among other things, a security interest in the Aircraft owned by
the Company.

     The Certificates represent Fractional Undivided Interests in the Trust and
the Trust Property, and have no rights, benefits or interest in respect of any
assets or property of any trust of another class.

     Interest applicable to this Certificate will be payable at a rate equal to
_____%.

     Subject to and in accordance with the terms of the Agreement, the
Intercreditor Agreement and the Note Purchase Agreement, from and to the extent
of funds then available to the Trustee, there shall be distributed on each [   ]
and [   ] (a "Regular Distribution Date"), commencing on [   ], to the Person in
whose name this Certificate is registered at

                                   Exhibit A
                                    Page 2
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

the close of business on the 15th day preceding such Regular Distribution Date,
an amount in respect of the Scheduled Payments on the Notes due on such Regular
Distribution Date, the receipt of which has been confirmed by the Trustee, equal
to the product of the percentage interest in the Trust evidenced by this
Certificate and an amount equal to the sum of such Scheduled Payments. Subject
to and in accordance with the terms of the Agreement, the Intercreditor
Agreement and the Note Purchase Agreement, if Special Payments on the Notes are
received by the Trustee, from funds then available to the Trustee, there shall
be distributed on the applicable Special Distribution Date, to the Person in
whose name this Certificate is registered at the close of business on the 15th
day preceding the Special Distribution Date, an amount in respect of such
Special Payments on the Notes, the receipt of which has been confirmed by the
Trustee, equal to the product of the percentage interest in the Trust evidenced
by this Certificate and an amount equal to the sum of such Special Payments so
received. If a Regular Distribution Date or Special Distribution Date is not a
Business Day, distribution shall be made on the next Business Day with the same
force and effect as if made on such Regular Distribution Date or Special
Distribution Date and interest shall accrue during the intervening period. The
Trustee shall mail notice of each Special Payment and the Special Distribution
Date therefor to the Certificateholder of this Certificate.

     Except as otherwise provided in the Agreement and notwithstanding the
above, the final distribution on this Certificate shall be made after notice
mailed by the Trustee of the pendency of such distribution and only upon
presentation and surrender of this Certificate at the office or agency of the
Trustee specified in such notice.

     THE AGREEMENT AND THIS CERTIFICATE SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK (WITHOUT REGARD TO THE
PRINCIPLES OF CONFLICTS OF LAWS THEREOF (OTHER THAN SECTION 5-1401 OF THE NEW
YORK GENERAL OBLIGATIONS LAW)) AND THE OBLIGATIONS, RIGHTS AND REMEDIES OF THE
HOLDER OF THIS CERTIFICATE UNDER THE AGREEMENT SHALL BE DETERMINED IN ACCORDANCE
WITH SUCH LAWS.

     Reference is hereby made to the further provisions of this Certificate set
forth in the reverse hereof, which further provisions shall for all purposes
have the same effect as if set forth at this place.

     Unless the certificate of authentication hereon has been executed by the
Trustee, by manual signature, this Certificate shall not be entitled to any
benefit under the Agreement or be valid or obligatory for any purpose.

                                   Exhibit A
                                    Page 3
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

      IN WITNESS WHEREOF, the Trustee, on behalf of the Issuer and not in its
individual capacity, has caused this Certificate to be duly executed.

                                        UNITED AIRLINES 2001-1A-3 PASS
                                        THROUGH TRUST

                                        By:  STATE STREET BANK AND TRUST
                                             COMPANY OF CONNECTICUT,
                                             NATIONAL ASSOCIATION, not in its
                                             individual capacity
                                             but solely as Trustee

Dated:____________________________      By:___________________________________
                                        Name:_________________________________
                                        Title:________________________________

                                   Exhibit A
                                    Page 4
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

                    TRUSTEE'S CERTIFICATE OF AUTHENTICATION

     This is one of the Certificates referred to in the within-mentioned Trust
Agreement.

STATE STREET BANK AND TRUST             STATE STREET BANK AND TRUST
COMPANY OF CONNECTICUT,                 COMPANY OF CONNECTICUT,
NATIONAL ASSOCIATION, not in its        NATIONAL ASSOCIATION, not in its
individual capacity but                 individual capacity but
solely as Trustee                       solely as Trustee

By:______________________________       By:__________________________________
                                                  Authenticating Agent

                                        By:__________________________________

                                   Exhibit A
                                    Page 5
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

                           [REVERSE OF CERTIFICATE]

     The Certificates do not represent a direct obligation of, or an obligation
guaranteed by, or an interest in, the Company or the Trustee or any of their
Affiliates. The Certificates are limited in right or payment, all as more
specifically set forth on the face hereof and in the Agreement. All payments or
distributions made to Certificateholders under the Agreement shall be made only
from the Trust Property and only to the extent that the Trustee shall have
sufficient income or proceeds from the Trust Property to make such payments in
accordance with the terms of the Agreement. Each Certificateholder of this
Certificate, by its acceptance hereof, agrees that it shall look solely to the
income and proceeds from the Trust Property to the extent available for
distribution to such Certificateholder as provided in the Agreement. This
Certificate does not purport to summarize the Agreement and reference is made to
the Agreement for information with respect to the interests, rights, benefits,
obligations, proceeds and duties evidenced hereby. A copy of the Agreement may
be examined during normal business hours at the principal office of the Trustee,
and at such other places, if any, designated by the Trustee, by any
Certificateholder upon request.

     The Agreement permits, with certain exceptions therein provided, the
amendment thereof and the modification of the rights and obligations of the
Company and the rights of the Certificateholders under the Agreement at any time
by the Company and the Trustee with the consent of the Certificateholders
holding Certificates evidencing Fractional Cumulative Interests aggregating not
less than a majority in interest in the Trust. Any such consent by the
Certificateholder of this Certificate shall be conclusive and binding on such
Certificateholder and upon all future Certificateholders of this Certificate and
of any Certificate issued upon the transfer hereof or in exchange hereof or in
lieu hereof whether or not notation of such consent is made upon this
Certificate. The Agreement also permits the amendment thereof, in certain
limited circumstances, without the consent of the Certificateholders of any of
the Certificates.

     As provided in the Agreement and subject to certain limitations therein set
forth, the transfer of this Certificate is registrable in the Register upon
surrender of this Certificate for registration of transfer at the offices or
agencies maintained by the Trustee in its capacity as Registrar, or by any
successor Registrar, at its Corporate Trust Office, duly endorsed or accompanied
by a written instrument of transfer in form satisfactory to the Trustee and the
Registrar duly executed by the Certificateholder hereof or such
Certificateholder's attorney duly authorized in writing, and thereupon one or
more new Certificates of authorized denominations evidencing the same aggregate
Fractional Undivided Interest in the Trust shall be issued to the designated
transferee or transferees.

     The Certificates are issuable only as registered Certificates without
coupons in minimum denominations of $1,000 Fractional Undivided Interest and
integral multiples of $1,000 in excess thereof. As provided in the Agreement and
subject to certain limitations therein set forth, the Certificates are
exchangeable for new Certificates of authorized denominations evidencing the
same aggregate Fractional Undivided Interest in the Trust, as requested by the
Certificateholder surrendering the same.

                                   Exhibit A
                                    Page 6
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

     No service charge shall be made for any such registration of transfer or
exchange, but the Trustee shall require payment by the Certificateholder of a
sum sufficient to cover any tax or governmental charge payable in connection
therewith.

     The Trustee, the Registrar, and any agent of the Trustee or the Registrar
may treat the person in whose name this Certificate is registered as the owner
hereof for all purposes, and neither the Trustee, the Registrar, nor any such
agent shall be affected by any notice to the contrary.

     The obligations and responsibilities created by the Agreement and the Trust
created thereby shall terminate upon the distribution to Certificateholders of
all amounts required to be distributed to them pursuant to the Agreement and the
disposition of all property held as part of the Trust Property.

                                   Exhibit A
                                    Page 7
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

                            FORM OF TRANSFER NOTICE

     FOR VALUE RECEIVED the undersigned registered holder hereby sell(s),
assign(s) and transfer(s) unto

Insert Taxpayer Identification No.

________________________________________________________________________________
please print or typewrite name and address including zip code of assignee

________________________________________________________________________________
the within Certificate and all rights thereunder, hereby irrevocably
constituting and appointing

________________________________________________________________________________
attorney to transfer said Certificate on the books of the Trustee with full
power of substitution in the premises.

Date:                                   [Name of Transferor]

                                        ________________________________________
                                        NOTE: The signature must correspond with
                                        the name as written upon the face of the
                                        within-mentioned instrument in every
                                        particular, without alteration or any
                                        change whatsoever.

Signature Guarantee:_________________

                                   Exhibit A
                                    Page 8
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

                                    EXHIBIT B

                          DTC LETTER OF REPRESENTATIONS

                                   Exhibit B
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

                                   EXHIBIT C

                        SCHEDULED PAYMENTS OF PRINCIPAL
                           ON CLASS A-3 CERTIFICATES

          Payment Date                             Principal Payment
----------------------------------      ---------------------------------------

                                   Exhibit C
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

                                   EXHIBIT D

                       PRINCIPAL AMOUNTS AND MATURITIES

        Notes                   Principal Amount               Maturity
---------------------    -----------------------------    -------------------

                                   Exhibit D
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

                                   EXHIBIT E

                 AIRCRAFT, REGISTRATION NUMBERS AND MATURITIES

                                     Aircraft
                                   Registration
        Aircraft                      Number                     Maturity
------------------------    --------------------------    ----------------------

                                   Exhibit E
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

                                   EXHIBIT F

                                NOTE DOCUMENTS

                                 Aircraft
                               Registration
       Aircraft                   Number                   Note Document
----------------------  --------------------------  ----------------------------
      A319-131                    N831UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A319-131                    N833UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A319-131                    N834UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A319-131                    N835UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

                                   Exhibit F
                                    Page 1
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

      A319-131                    N836UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A319-131                    N837UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A319-131                    N838UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A319-131                    N839UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

                                   Exhibit F
                                    Page 2
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

      A319-131                    N840UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A319-131                    N841UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A320-232                    N467UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A320-232                    N468UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A320-232                    N469UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

                                   Exhibit F
                                    Page 3
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

      A320-232                    N470UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A320-232                    N471UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      A320-232                    N472UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B747-422                    N117UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B747-422                    N118UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

                                   Exhibit F
                                    Page 4
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

      B747-422                    N122UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B747-422                    N127UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B747-422                    N128UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B767-322                    N664UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B767-322                    N667UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

                                   Exhibit F
                                    Page 5
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

      B767-322                    N669UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B767-322                    N670UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B767-322                    N671UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B777-222                    N794UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B777-222                    N795UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-2
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

                                   Exhibit F
                                    Page 6
<PAGE>

                                                [Trust Supplement No. 2001-1A-3]

      B777-222                    N204UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

      B777-222                    N205UA            Participation Agreement
                                                    Trust Indenture and Mortgage
                                                    Equipment Note, Series A-1
                                                    Equipment Note, Series A-3
                                                    Equipment Note, Series B
                                                    Equipment Note, Series C
                                                    Equipment Note, Series D

                                   Exhibit F
                                    Page 7

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