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                                                                   EXHIBIT 10.21

                              FLEET NATIONAL BANK
                               100 Federal Street
                           Boston, Massachusetts 02110


                            As of September 21, 2001

Mercury Air Group, Inc.
5456 McConnell
Los Angeles, CA 90066

Attn: Randolf E. Ajer, Chief Financial Officer

       Re: Limited Waiver

Ladies and Gentlemen:

       Reference is made to the Revolving Credit and Term Loan Agreement dated
as of March 2, 1999 (as amended, modified, supplemented or restated and in
effect from time to time, the "Credit Agreement") by and among (a) MERCURY AIR
GROUP, INC., a Delaware corporation having its principal place of business at
5456 McConnell Avenue, Los Angeles, California 90066 (the "Borrower"), (b) FLEET
NATIONAL BANK (formerly known as BankBoston, N.A.), a national banking
association, as agent (in such capacity the "Agent") for itself and the other
Banks referred to below; and (c) FLEET NATIONAL BANK and the other financial
institutions from time to time parties thereto (collectively, the "Banks").
Capitalized terms defined in the Credit Agreement that are not otherwise defined
in this letter agreement shall have the respective meanings assigned to such
terms in the Credit Agreement.

       Pursuant to Section 14.7 of the Credit Agreement, the Borrower has agreed
that it will not permit Consolidated Net Income of the Borrower and its
Subsidiaries for any fiscal quarter ending after the Closing Date to be less
than $1.00. The Borrower has advised the Banks and the Agent that, as a result
of a special charge in the approximate amount of $1,250,000.00 taken by the
Borrower and its Subsidiaries in connection with the disposition of
substantially all of the assets of its wholly-owned Subsidiary Jupiter Airline
Automation Services, Inc. (then known as RPA Airline Automation Services, Inc.
and previously known as Rene Perez and Associates, Inc.), the Borrower expects
Consolidated Net Income of the Borrower and its Subsidiaries for the fiscal
quarter ended June 30, 2001 to be approximately negative Eight Hundred
Eighty-Six Thousand Dollars ($-886,000.00).

       Subject to the satisfaction of the conditions set forth below, the Agent
and the Majority Banks hereby waive compliance by the Borrower and its
Subsidiaries with the covenant set forth in Section 14.7 solely for the fiscal
quarter ended June 30, 2001; provided that the Borrower shall not permit
Consolidated Net Income of the Borrower and its Subsidiaries for the fiscal
quarter ended June 30, 2001 to be less than negative Nine Hundred Thousand
Dollars ($-900,000.00).

       The limited waiver set forth in this letter agreement shall be effective
upon the satisfaction of each of the following conditions, each to be in form
and substance satisfactory to the Agent:


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                                      -2-

              (i) delivery to the Agent of original counterpart signature pages
       to this letter agreement, duly executed and delivered by the Majority
       Banks, the Borrower and each of the Guarantors;

              (ii) payment by the Borrower to the Agent, for the account of the
       Banks, of a waiver fee in the amount of $6,500.00 for each Bank
       ($26,000.00 in the aggregate); and

              (iii) payment by the Borrower of all fees and expenses of the
       Agent's special counsel in connection with this letter agreement, the
       Credit Agreement, any amendments thereto, and the transactions
       contemplated hereby and thereby.

       The waiver contained in this letter agreement is limited strictly to its
terms, shall apply only to the specific matters and events described herein,
shall not extend to or affect any of the Borrower's or the Guarantors' other
obligations contained in the Credit Agreement or any other Loan Document. Except
as expressly set forth herein, nothing contained herein shall be deemed to be a
waiver of, or shall in any way impair or prejudice, any rights of the Agent or
the Banks under the Credit Agreement or any other Loan Document. Nothing
contained in this letter agreement shall be construed to imply a willingness on
the part of the Agent and the Banks to grant any similar or other future waivers
of any of the terms and conditions of the Credit Agreement or the other Loan
Documents.

       This letter agreement may be executed in any number of counterparts, each
of which when so executed shall be deemed to be an original and all of which
taken together shall constitute one and the same instrument. THIS LETTER
AGREEMENT SHALL BE AN INSTRUMENT UNDER SEAL AND SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE COMMONWEALTH OF MASSACHUSETTS
(WITHOUT REFERENCE TO PRINCIPLES OF CONFLICTS OR CHOICE OF LAW).

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                                      -3-

       Kindly acknowledge your agreement with the terms of this letter agreement
by signing in the space indicated below and returning a copy of this letter to
the Agent.

                                        FLEET NATIONAL BANK
                                        (f/k/a BankBoston, N.A.),
                                        individually and as Agent


                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:



                                        UNITED CALIFORNIA BANK
                                        FORMERLY KNOWN AS SANWA BANK CALIFORNIA


                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:



                                        MELLON BANK, N.A.


                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:



                                        UNION BANK OF CALIFORNIA, N.A.


                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:

ACCEPTED AND AGREED:

MERCURY AIR GROUP, INC.



By:
   --------------------------
  Name:
  Title:


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                                      -4-

Each of the Guarantors hereby consents to the terms of the foregoing letter
agreement, and each confirms its obligation to the Agent and the Banks under its
Guaranty and agrees that its guaranty of the Obligations thereunder shall extend
to and include the Obligations under the Credit Agreement and the other Loan
Documents as modified by this letter agreement, the Assumption Agreement and the
other transaction documents contemplated hereby.


                                        MERCFUEL, INC.


                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:



                                        MAYTAG AIRCRAFT CORPORATION


                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:



                                        MERCURY AIR CARGO, INC.


                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:



                                        MERCURY ACCEPTANCE CORPORATION


                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:



                                        EXCEL CARGO, INC.


                                        By:
                                           -------------------------------------
                                           Name:
                                           Title:


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                                      -5-

                                       MERCURY AIR CENTERS, INC.
                                       (f/k/a Wofford Flying Service, Inc.)


                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:



                                       AEG FINANCE CORPORATION


                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:



                                       JUPITER AIRLINE AUTOMATION
                                       SERVICES, INC.
                                       (f/k/a RPA Airline Automation Services,
                                       Inc. and Rene Perez and Associates, Inc.)


                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:



                                       HERMES AVIATION, INC.


                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:



                                       VULCAN AVIATION, INC.


                                       By:
                                          --------------------------------------
                                          Name:
                                          Title: