EXHIBIT 10.4
                                                                  CONFORMED COPY
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                         INDENTURE SUPPLEMENT [N583ML]
          INDENTURE SUPPLEMENT NO. 1 [N583ML] dated August 16, 1999 (this
"Indenture Supplement") of FIRST UNION TRUST COMPANY, NATIONAL ASSOCIATION, not
in its individual capacity except as expressly provided in the Operative
Agreements, but solely as Owner Trustee under the Trust Agreement (in such
capacity, herein called the "Owner Trustee").
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                              W I T N E S S E T H
          WHEREAS, the Indenture provides for the execution and delivery of this
Indenture Supplement, which shall particularly describe the Aircraft and any
Replacement Airframe or Replacement Engine included in the property covered by
the Indenture.
          WHEREAS, the Trust Indenture and Security Agreement [N583ML] dated as
of August 10, 1999 (herein called the "Indenture") between the Owner Trustee and
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Allfirst Bank, as Indenture Trustee (herein called the "Indenture Trustee"),
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provides for the execution and delivery of a supplement thereto substantially in
the form hereof which shall particularly describe the Aircraft (such term and
other defined terms in the Indenture being herein used with the same meanings)
and any Replacement Airframe or Replacement Engine included in the Indenture
Estate, and shall specifically mortgage such Aircraft, Replacement Airframe or
Replacement Engine, as the case may be, to the Indenture Trustee.
          WHEREAS, the Indenture relates to the Airframe and Engines described
below and a counterpart of the Indenture is attached hereto and made a part
hereof and this Indenture Supplement, together with such counterpart of the
Indenture, is being filed for recordation on the date hereof with the Federal
Aviation Administration as one document.
          NOW, THEREFORE, this Indenture Supplement witnesseth, that, to secure
the prompt payment of the principal of and Make-Whole Amount (if any) and
interest on, and all other amounts due with respect to, all Equipment Notes from
time to time outstanding under the Indenture and the performance and observance
by the Owner Trustee of all the agreements, covenants and provisions for the
benefit of the Noteholders in the Indenture and in the Participation Agreement
and the Equipment Notes, and the prompt payment of any and all amounts from time
to time owing under the Indenture or the Participation Agreement or the other
Operative Agreements by the Owner Trustee, the Owner Participant or the Lessee
to the Noteholders, and for the uses and purposes and subject to the terms and
provisions of the Indenture, and in consideration of the premises and of the
covenants contained in the Indenture, and of the acceptance of the Equipment
Notes by the Noteholders, and of the sum of $1 paid to the Owner Trustee by the
Indenture Trustee at or before the delivery hereof, the receipt whereof is
hereby acknowledged, the Owner Trustee has granted,
bargained, sold, assigned, transferred, conveyed, mortgaged, pledged and
confirmed, and does hereby grant, bargain, sell, assign, transfer, convey,
mortgage, pledge and confirm, unto the Indenture Trustee and its successors and
assigns, for the security and benefit of the Noteholders, in the trust created
by the Indenture, a first priority security interest in and first mortgage lien
upon, all right, title and interest of the Owner Trustee in, to and under the
following described property:
                                    AIRFRAME
                      one airframe identified as follows:
                                                      FAA Registration         Manufacturer' s Serial
Manufacturer                Model                          Number                       Number
------------                -----                          ------                       -----
                                                                      
Bombardier Inc.        Canadair Regional Jet               N583ML                       7327
                          CL-600-2B19
together with all Parts, appliances, equipment, instruments and accessories
(including, without limitation, radio and radar) from time to time thereto
belonging, owned by the Owner Trustee and installed in or appurtenant to said
aircraft and all records, logs, manuals and data, inspection, modification and
overhaul records and all other documents at any time maintained with respect to
the foregoing property.
                               AIRCRAFT ENGINES
                    (2) aircraft engines, each such engine
                  having 750 or more rated takeoff horsepower
               or the equivalent thereof, identified as follows:
Manufacturer                Model          Manufacturer's  Serial
------------                -----          ----------------------
                                           Numbers
                                           -------
General Electric Company    CF34-3B1       GE-E-872529
                                           GE-E-872530
together with all Parts, equipment and accessories thereto belonging, by
whomsoever manufactured, owned by the Owner Trustee and installed in or
appurtenant to said aircraft engines and all records, logs, manuals and data,
inspection, modification and overhaul records and all other documents at any
time maintained with respect to the foregoing property.
          Together with all substitutions, replacements and renewals of the
property above described, and all property which shall hereafter become
physically attached to or incorporated in the property above described, whether
the same are now owned by the Owner Trustee or shall hereafter be acquired by
it.
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          As further security for the obligations referred to above and secured
by the Indenture and hereby, the Owner Trustee has granted, bargained, sold,
assigned, transferred, conveyed, mortgaged, pledged and confirmed, and does
hereby grant, bargain, sell, assign, transfer, convey, mortgage, pledge and
confirm, unto the Indenture Trustee, its successors and assigns, for the
security and benefit of the Noteholders, in the trust created by the Indenture,
all of the right, title and interest of the Owner Trustee in, to and under Lease
Supplement No. 1 of even date (other than Excluded Payments, if any) covering
the property described above.
          TO HAVE AND TO HOLD all and singular the aforesaid property unto the
Indenture Trustee, its successors and assigns, for the benefit and security of
the Noteholders for the uses and purposes and subject to the terms and
provisions set forth in the Indenture.
          This Indenture Supplement shall be construed as supplemental to the
Indenture and shall form a part thereof, and the Indenture is hereby
incorporated by reference herein and is hereby ratified, approved and confirmed.
          AND, FURTHER, the Owner Trustee hereby acknowledges that the
Airframe/Engines referred to in this Indenture Supplement and the aforesaid
Lease Supplement have been delivered to the Owner Trustee and are included in
the property of the Owner Trustee and are covered by all the terms and
conditions of the Indenture and are subject to the Lien of the Indenture.
          This Indenture Supplement is being delivered in the State of New York.
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          IN WITNESS WHEREOF, the Owner Trustee has caused this Indenture
Supplement to be duly executed by one of its officers thereunto authorized, this
16/th/ day of August, 1999.
                                                  FIRST UNION TRUST COMPANY,
                                                  NATIONAL ASSOCIATION,
                                                  not in its individual capacity
                                                  except as otherwise expressly
                                                  provided in the Operative
                                                  Agreements, but solely as
                                                  Owner Trustee
                                                  By:  /s/ ▇▇▇▇▇▇▇ ▇. ▇▇▇▇
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                                                       Name: ▇▇▇▇▇▇▇ ▇. ▇▇▇▇
                                                       Title: Vice President
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                             Note to Exhibit 10.4
The following Indenture Supplement is substantially identical in all material
respects to the Indenture Supplement filed herewith except as follows:
                     FAA Registration No.     Aircraft          Engine
                           or                 Manufacturer's    Manufacturer's
Date                 Aircraft Tail No.        Serial No.        Serial Nos.
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August 10, 1999            N584ML                 7330          GE-E-872547 and
                                                                GE-E-872546
September 10, 1999          585ML                 7334          GE-E-872527 and
                                                                GE-E-872528