Document:

<PAGE>

                                                                   EXHIBIT 10(Z)

                               SECOND AMENDMENT
                               ----------------

          SECOND AMENDMENT, dated as of November 19, 1999 (this "Amendment"), to
                                                                 ---------
the Credit Agreement, dated as of August 13, 1997 (as heretofore amended or
otherwise modified, the "Credit Agreement"), among HUMANA INC., a Delaware
                         ----------------
corporation (the "Company"), the several banks and other financial institutions
                  -------
from time to time parties to this Agreement (the "Banks"), BANK OF AMERICA, a
                                                  -----
national banking association, as documentation agent (in such capacity, the
"Documentation Agent") and THE CHASE MANHATTAN BANK, a New York banking
 -------------------
corporation, as administrative agent for the Banks thereunder (in such capacity,
the "Agent") and as CAF Loan agent (in such capacity, the "CAF Loan Agent").
     -----                                                 --------------

                             W I T N E S S E T H:
                             -------------------

     WHEREAS, pursuant to the Credit Agreement, the Banks have agreed to make,
and have made, certain loans and other extensions of credit to the Company; and

     WHEREAS, the Company has requested that certain provisions of the Credit
Agreement be amended in the manner provided for in this Amendment;

     NOW, THEREFORE, the parties hereto hereby agree as follows:

     I.     Defined Terms; Amendments to Section 1.1. (a) Terms defined in the
            ----------------------------------------
Credit Agreement and used herein shall have the meanings given to them in the
Credit Agreement.

     (b)  The following definition of "Second Amendment Effective Date" is
hereby added to Section 1.1 of the Credit Agreement immediately after the
definition of "Riverview Square":

     "Second Amendment Effective Date": as defined in the Second Amendment,
      -------------------------------
     dated as of November 19, 1999."

     (c)  The definition of "Applicable Margin" set forth in Section 1.1 of the
Credit Agreement is hereby deleted in its entirety and replaced with the
following new definition:

     ""Applicable Margin": with respect to each day for each Type of Revolving
       -----------------
     Credit Loan, the higher of (a) the rate per annum determined in accordance
     with Schedule II and (b) the rate per annum determined in accordance with
     Schedule IIA."
<PAGE>

                                                                               2

     (d)  The definition of "Consolidated Net Worth" set forth in Section 1.1 of
the Credit Agreement is hereby deleted in its entirety and replaced with the
following new definition:

     ""Consolidated Net Worth": at any date, the stockholders' equity of the
       ----------------------
     Company and its Subsidiaries at such date, determined in accordance with
     GAAP; provided, that up to $300,000,000 of goodwill written off after the
     Second Amendment Effective Date will be added back to Consolidated Net
     Worth."

     (e)  The definition of "Consolidated EBIT" set forth in Section 1.1 of the
Credit Agreement is hereby amended by replacing the word "and" appearing
immediately before clause (ii) with a comma and adding the following immediately
after the parenthetical at the end:

     "and (iii) for the purpose of calculating Consolidated EBIT for any period
     that includes the first fiscal quarter of 1999, the amount shall be
     adjusted by adding charges recorded in such fiscal quarter, in an amount
     not exceeding $90,000,000, relating to additional medical claims expense
     including $50,000,000 related to premium deficiencies, $35,000,000 to
     strengthen medical claims payable and $5,000,000 for a payment to
     Columbia/HCA, and for the purpose of calculating Consolidated EBIT for any
     period after the Second Amendment Effective Date, the amount shall be
     adjusted by adding back one time charges recorded in such period, in an
     aggregate amount not exceeding $50,000,000, related to the reduction in the
     work force of the Company and its Subsidiaries and an increase in the
     provisions for reserves in Managed Care Indemnity, Inc., the Company's
     wholly-owned professional liability insurance Subsidiary."

     II.  Other Amendments to the Credit Agreement.
          ----------------------------------------

     1    Amendment to Section 2.4.  Section 2.4 of the Credit Agreement is
          ------------------------
hereby amended and restated in its entirety as follows:

          "2.4 Fees. (a) The Company agrees to pay to the Agent, for the account
                    ----
     of each Bank, on the last day of each fiscal quarter, a facility fee in
     respect of the average daily amount of the Commitment of such Bank during
     such fiscal quarter (such amount, the "Average Quarterly Commitment"). Such
                                            ----------------------------
     fee shall be computed at the rate per annum equal to the higher of (i) the
     rate per annum determined in accordance with Schedule II and (ii) the rate
     per annum determined in accordance with Schedule IIA.

          (b)  The Company agrees to pay to the Agent the other fees in the
     amounts, and on the dates, agreed to by the Company and the Agent in the
     fee letter, dated June 4, 1997, between the Agent and the Company.
<PAGE>

                                                                               3

          (c)  For any day on which the Aggregate Outstanding Extensions of
     Credit of all Banks equals or exceeds an amount equal to one third of the
     aggregate Commitments of all the Banks, the Company agrees to pay to the
     Agent for the account of the Banks (ratably in accordance with their
     Commitments) a fee in an amount equal to 0.125% per annum on the Aggregate
     Outstanding Extensions of Credit of all Banks on such day. Any accrued fees
     payable in accordance with the immediately preceding sentence shall be
     payable on the last day of each fiscal quarter and on the earlier of the
     date the Commitments are terminated and the Termination Date."

     2    Amendment to Section 3.3.  Subsection 3.3(a) of the Credit Agreement
          ------------------------
is hereby amended and restated in its entirety as follows:

     "(a) The Company shall pay to the Agent, for the account of the Issuing
     Bank and the L/C Participants, a letter of credit commission with respect
     to each Letter of Credit, computed at the rate per annum equal to the
     higher of (i) the rate per annum determined in accordance with Schedule II
     and (ii) the rate per annum determined in accordance with Schedule IIA, of
     which .100% per annum shall be payable to the Issuing Bank and the balance
     shall be payable to the L/C Participants and the Issuing Bank to be shared
     ratably among them in accordance with their respective Commitment
     Percentages. Such fee shall be payable on each L/C Fee Payment Date and
     shall be nonrefundable."

     3    Amendment to Sections 4.3, 4.4, 4.7 and 4.9. Each of sections 4.3,
          -------------------------------------------
4.4, 4.7 and 4.9 of the Credit Agreement is hereby amended and restated in its
entirety as follows:

     "4.3 Litigation. Except as disclosed in the Company's Annual Report on Form
     10-K for its fiscal year ended December 31, 1996 and the Company's
     Quarterly Report on Form 10-Q for its fiscal quarter ended March 31, 1997
     filed with the Securities and Exchange Commission and previously
     distributed to the Banks or disclosed in writing to the Banks between the
     date thereof and November 19, 1999 (the "Updated Disclosure Letter"), as of
     the Second Amendment Effective Date there is no litigation, at law or in
     equity, or any proceeding before any federal, state, provincial or
     municipal board or other governmental or administrative agency, including
     without limitation, HMO Regulators and Insurance Regulators, pending or to
     the knowledge of the Company threatened which, after giving effect to any
     applicable insurance, may involve any material risk of a Material Adverse
     Effect or which seeks to enjoin the consummation of any of the transactions
     contemplated by this Agreement or any other Loan Document, and no judgment,
     decree, or order of any federal, state, provincial or municipal court,
     board or other governmental or administrative agency, including without
     limitation, HMO Regulators and Insurance Regulators, has been issued
     against the Company or any Subsidiary which has, or may involve, a material
     risk of a Material Adverse Effect. The Company does not believe that the
     final resolution of the matters disclosed in its Annual Report on Form 10-K
     for its fiscal year ended December 31, 1996 and the Company's Quarterly
     Report on Form 10-Q for its fiscal quarter ended March 31, 1997 filed with
     the Securities and Exchange
<PAGE>

                                                                               4

     Commission and previously distributed to the Banks or the Updated
     Disclosure Letter, will have a Material Adverse Effect."

     "4.4 Disclosure. Neither this Agreement nor any agreement, document,
          ----------
     certificate or statement furnished to the Banks by the Company in
     connection herewith (including, without limitation, the information
     relating to the Company and its Subsidiaries included in the Confidential
     Information Memorandum dated July 1997 delivered in connection with the
     syndication of the credit facilities hereunder) contained at the time
     furnished to the Banks any untrue statement of material fact or, taken as a
     whole together with all other information furnished to the Banks by the
     Company, omitted to state a material fact necessary in order to make the
     statements contained herein or therein not misleading. All pro forma
     financial statements made available to the Banks have been prepared in good
     faith based upon reasonable assumptions. There is no fact known to the
     Company which materially adversely affects or could reasonably be expected
     to materially adversely affect the business, operations, affairs or
     condition of the Company and its Subsidiaries on a consolidated basis,
     except to the extent that they may be affected by future general economic
     conditions and except as set forth in paragraph 2 of the Updated Disclosure
     Letter."

     "4.7 Changes in Condition. Since December 31, 1998, there has been no
          --------------------
     development or event nor any prospective development or event, which has
     had, or could reasonably be expected to have, a Material Adverse Effect."

     "4.9 Tax Returns. The Company and each of its Subsidiaries have filed all
     tax returns which are required to be filed and have paid, or made adequate
     provision for the payment of, all taxes which have or may become due
     pursuant to said returns or to assessments received. All federal tax
     returns of the Company and its Subsidiaries through their fiscal years
     ended in 1993 have been audited by the Internal Revenue Service or are not
     subject to such audit by virtue of the expiration of the applicable period
     of limitations, and the results of such audits are fully reflected in the
     balance sheets referred to in subsection 4.6. The Company knows of no
     material additional assessments since said date for which adequate reserves
     have not been established."

     4    Amendment to Section 7.5.  Section 7.5 of the Credit Agreement is
          ------------------------
hereby amended by deleting the word "or" immediately before subsection 7.5(f)
and deleting subsection 7.5(f) in its entirety and substituting the following in
lieu thereof:

     "(f) the sale or other disposition of any other property so long as no
     Default or Event of Default shall have occurred and be continuing; provided
                                                                        --------
     that the aggregate book value of all assets so sold or disposed of in any
     fiscal year of the Company (disregarding the book value of any assets sold
     or disposed of in accordance with subsection 7.5(g) below in the fiscal
     year in which such sale or other disposition is completed) shall not exceed
     in the aggregate 12% of the Consolidated Assets of the Company and its
     Subsidiaries as at the end of the immediately preceding fiscal year of the
     Company; or
<PAGE>

                                                                               5

     (g)  the sale or other disposition of the Company's workers' compensation
     division and the Company's dental division, the aggregate book value of
     which assets shall not exceed $300,000,000"

     III. Amendments to Schedules to the Credit Agreement.
          -----------------------------------------------

     1    Amendment to Schedule II. Schedule II to the Credit Agreement is
          ------------------------
hereby deleted in its entirety and replaced by Schedule II to this Amendment.

     IV.  Addition of Schedule IIA. The Credit Agreement is hereby amended by
          ------------------------
adding Schedule IIA, a copy which is attached hereto.

     V.   Notification of Commitment Reduction. The Agent hereby notifies the
          ------------------------------------
Banks that the Company has, in accordance with Section 2.5(a) of the Credit
Agreement, reduced the amount of the Commitments in an aggregate amount such
that the aggregate amount of the remaining Commitments is $1,000,000,000.

     VI.  Substitution of New Schedule 1. Schedule 1 to the Credit Agreement is
          ------------------------------
hereby revised and restated by reducing the stated amount of each Bank's
Commitment's by an amount equal to one third of the amount presently listed as
such Bank's Commitment.

     VII. Effective Date. This Amendment shall become effective on the date (the
          --------------
"Second Amendment Effective Date") on which the Agent receives (i) counterparts
 -------------------------------
of this Amendment executed by each of the Company, the Agent and the Required
Banks and (ii) an amendment fee from the Company for the account of each Bank
delivering an executed counterpart of this Amendment on or before November 19,
1999 in an amount agreed to between the Company and the Agent.

     VIII. General.
           -------

     1     Representation and Warranties. To induce the Agent and CAF Loan Agent
           -----------------------------
and the Banks parties hereto to enter into this Amendment, the Company hereby
represents and warrants to the Agent and CAF Loan Agent and all of the Banks as
of the Second Amendment Effective Date that (a) this Amendment constitutes the
legal, valid and binding obligation of the Company, enforceable against it in
accordance with its terms, except as such enforcement may be limited by
bankruptcy, insolvency, fraudulent conveyances, reorganization, moratorium or
similar laws affecting creditors' rights generally, by general equitable
principles (whether enforcement is sought by proceedings in equity or at law)
and by an implied covenant of good faith and fair dealing, (b) the
representations and warranties made by the Company in the Credit
<PAGE>

                                                                               6

Agreement (as modified hereby) are true and correct in all material respects on
and as of the date hereof (except to the extent that such representations and
warranties are expressly stated to relate to an earlier date, in which case such
representations and warranties shall have been true and correct in all material
respects on and as of such earlier date), (c) the representation and warranty
set forth in subsection 4.7 of the Credit Agreement (without giving effect to
the modifications effected hereby except that the phrase 'could reasonably be
expected to" shall be used in lieu of the word "may") is true and correct in all
material respects as of the Second Amendment Effective Date and (d) no Default
or Event of Default has occurred and is continuing.

     2    Payment of Expenses. The Company agrees to pay or reimburse the Agent
          -------------------
and CAF Loan Agent for all of its out-of-pocket costs and reasonable expenses
incurred in connection with this Amendment, any other documents prepared in
connection herewith and the transactions contemplated hereby, including, without
limitation, the reasonable fees and disbursements of counsel to the Agent and
CAF Loan Agent.

     3    No Other Amendments; Confirmation. Except as expressly amended,
          ---------------------------------
modified and supplemented hereby, the provisions of the Credit Agreement and the
Notes are and shall remain in full force and effect.

     4    GOVERNING LAW. THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF THE
          -------------
PARTIES HERETO SHALL BE GOVERNED BY, AND CONSTRUED AND INTERPRETED IN ACCORDANCE
WITH, THE LAWS OF THE STATE OF NEW YORK.

     5    Counterparts. This Amendment may be executed by one or more of the
          ------------
parties to this Agreement on any number of separate counterparts, and all of
said counterparts taken together shall be deemed to constitute one and the same
instrument. A set of the copies of this Amendment signed by all the parties
shall be lodged with the Company and the Agent and CAF Loan Agent. This
Amendment may be delivered by facsimile transmission of the relevant signature
pages hereof.
<PAGE>

                                                                               7

                                                                     SCHEDULE II
                                                                     -----------

                              Applicable Margins
                              ------------------

REVOLVING CREDIT LOANS

<TABLE>
<CAPTION>
    Consolidated
   Capitalization          Alternate Base                                Facility Fee            L/C Commission
        Ratio                Rate Loans         Eurodollar Loans       (Rate Per Annum)         (Rate Per Annum)
-------------------      ------------------   --------------------    ------------------       ------------------
<S>                      <C>                  <C>                     <C>                      <C>
less than .20                  .000%                 .1200%                 .0650%                    .2200%

at least .20 but               .000%                 .1300%                 .0700%                    .2300%
less than .30

at least .30 but               .000%                 .1600%                 .0900%                    .2600%
less than .35

at least .35 but               .000%                 .2000%                 .1000%                    .3000%
less than .40

at least .40                   .000%                 .3000%                 .1500%                    .4000%
</TABLE>

     Any change of the Applicable Margin or Facility Fee rate resulting from a
change in the Consolidated Capitalization Ratio shall become effective on the
first Business Day following the date to which the Consolidated Capitalization
Ratio Certificate reflecting such change is applicable.
<PAGE>

                                                                               8

                                                                    SCHEDULE IIA
                                                                    ------------

                              Applicable Margins
                              ------------------

REVOLVING CREDIT LOANS

<TABLE>
<CAPTION>
                Alternate Base     Eurodollar        Facility Fee       L/C Commission
   Rating         Rate Loans         Loans         (Rate Per Annum)    (Rate Per Annum)
------------   ----------------   ------------    ------------------  ------------------
<S>            <C>                <C>             <C>                 <C>
Rating I             .000%            .350%                    .100%               .450%

Rating II            .000%            .375%                    .125%               .475%

Rating III           .000%            .475%                    .150%               .575%

Rating IV            .000%            .575%                    .175%               .675%

Rating V             .000%            .800%                    .200%               .900%
</TABLE>

          For purposes of this Schedule the following terms shall have the
following meanings:

          "Rating": the respective rating of each of the Rating Agencies
           ------
applicable to the long-term senior unsecured non-credit enhanced debt of the
Company, as announced by the Rating Agencies from time to time.

          "Rating Agencies": collectively, S&P and Moody's.
           ---------------

          "Rating Category": each of Rating I, Rating II, Rating III, Rating IV
           ---------------
and Rating V.

          "Rating I, Rating II, Rating III, Rating IV and Rating V": the
           -------------------------------------------------------
respective Ratings set forth below:
<PAGE>

                                                                               9

<TABLE>
<CAPTION>
                   Rating
                  Category               S&P                       Moody's
                  --------               ---                       -------
                  <S>               <C>                       <C>
                  Rating I          greater than or           greater than or
                                    equal to BBB+             equal to Baa1

                  Rating II         lower than BBB+           lower than Baa1
                                    and greater than          and greater than
                                    or equal to BBB           or equal to Baa2

                  Rating III        lower than BBB            lower than Baa2
                                    and greater than          and greater than
                                    or equal to BBB-          or equal to Baa3

                  Rating IV         lower than BBB-           lower than Baa3
                                    and greater than          and greater than
                                    or equal to BB+           or equal to Ba1

                  Rating V          lower than BB+            lower than Ba1
</TABLE>

          ; provided, that (i) if on any day the Ratings of the Rating Agencies
            --------
          do not fall in the same Rating Category, then the Rating Category of
          the lower of such Ratings shall be applicable for such day, (ii) if on
          any day the Rating of only one of the Rating Agencies is available,
          then the Rating Category of such Rating shall be applicable for such
          day and (iii) if on any day a Rating is available from neither of the
          Rating Agencies, then Rating V shall be applicable for such day. Any
          change in the applicable Rating Category resulting from a change in
          the Rating of a Rating Agency shall become effective on the date such
          change is publicly announced by such Rating Agency.
<PAGE>

                                                                              10

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed and delivered by their respective proper and duly authorized
officers as of the day and year first above written.

                                    HUMANA INC.

                                    By:_________________________________________
                                      Name:_____________________________________
                                      Title:____________________________________

                                    THE CHASE MANHATTAN BANK, as Agent, as CAF
                                    Loan Agent and as a Bank

                                    By:_________________________________________
                                      Name:_____________________________________
                                      Title:____________________________________

                                    BANK OF AMERICA NATIONAL ASSOCIATION
                                    (formerly known as BANK OF AMERICA NATIONAL
                                    TRUST & SAVINGS ASSOCIATION), as
                                    Documentation Agent and as a Bank

                                    By:_________________________________________
                                      Name:_____________________________________
                                      Title:____________________________________
<PAGE>

                                            ASAHI BANK, LIMITED, CHICAGO BRANCH

                                            By:_________________________________
                                              Name:_____________________________
                                              Title:____________________________
<PAGE>

                                            BANCA NAZIONALE DEL LAVORO S.P.A.

                                            By:_________________________________
                                              Name:_____________________________
                                              Title:____________________________

                                            By:_________________________________
                                              Name:_____________________________
                                              Title:____________________________
<PAGE>

BANCA MONTE DEI PASCHI DI SIENA, SPA

By:______________________________________
  Name:__________________________________
  Title:_________________________________

By:______________________________________
  Name:__________________________________
  Title:_________________________________
<PAGE>

                                            BANK HAPOALIM B.M.

                                            By:_________________________________
                                              Name:_____________________________
                                              Title:____________________________
<PAGE>

BANK ONE, KENTUCKY, NA

By:______________________________________
  Name:__________________________________
  Title:_________________________________
<PAGE>

                                            BANK OF LOUISVILLE AND TRUST COMPANY

                                            By:_________________________________
                                              Name:_____________________________
                                              Title:____________________________
<PAGE>

BARNETT BANK, N.A.

By:______________________________________
  Name:__________________________________
  Title:_________________________________
<PAGE>

                                            CITIBANK, N.A.

                                            By:_________________________________
                                              Name:_____________________________
                                              Title:____________________________
<PAGE>

FIRST UNION NATIONAL BANK

By:______________________________________
  Name:__________________________________
  Title:_________________________________
<PAGE>

                                            FIFTH THIRD BANK OF KENTUCKY

                                            By:_________________________________
                                              Name:_____________________________
                                              Title:____________________________
<PAGE>

FIRST AMERICAN NATIONAL BANK

By:______________________________________
  Name:__________________________________
  Title:_________________________________
<PAGE>

                                 FIRSTAR BANK, N.A. (formerly known as FIRSTAR
                                 BANK MILWAUKEE, N.A.)

                                 By:____________________________________________
                                   Name:________________________________________
                                   Title:_______________________________________

                                 FIRSTAR BANK, N.A. (as successor by merger to
                                 STAR N.A.)

                                 By:____________________________________________
                                   Name:________________________________________
                                   Title:_______________________________________
<PAGE>

KEYBANK NATIONAL ASSOCIATION

By:______________________________________
  Name:__________________________________
  Title:_________________________________
<PAGE>

                                         MORGAN GUARANTY TRUST COMPANY OF NEW
                                         YORK

                                         By:____________________________________
                                           Name:________________________________
                                           Title:_______________________________
<PAGE>

NATIONAL CITY BANK OF KENTUCKY

By:_________________________________
  Name:_____________________________
  Title:____________________________
<PAGE>

                                          PARIBAS

                                          By:__________________________________
                                            Name:______________________________
                                            Title:_____________________________

                                          By:__________________________________
                                            Name:______________________________
                                            Title:_____________________________
<PAGE>

PNC BANK, KENTUCKY, INC.

By:_________________________________
  Name:_____________________________
  Title:____________________________
<PAGE>

                                          STAR BANK, N.A.

                                          By:__________________________________
                                            Name:______________________________
                                            Title:_____________________________
<PAGE>

SUNTRUST BANK, NASHVILLE, N.A.

By:_________________________________
  Name:_____________________________
  Title:____________________________
<PAGE>

                                          THE BANK OF NEW YORK

                                          By:__________________________________
                                            Name:______________________________
                                            Title:_____________________________
<PAGE>

THE BANK OF NOVA SCOTIA

By:__________________________________
  Name:______________________________
  Title:_____________________________
<PAGE>

                                         BANK ONE, NA (formerly known as THE
                                         FIRST NATIONAL BANK OF CHICAGO)

                                         By:__________________________________
                                           Name:______________________________
                                           Title:_____________________________
<PAGE>

THE BANK OF TOKYO-MITSUBISHI, LTD.,
CHICAGO BRANCH

By:__________________________________
    Name:____________________________
    Title:___________________________
<PAGE>

                                          THE SUMITOMO BANK, LIMITED

                                          By:__________________________________
                                            Name:______________________________
                                            Title:_____________________________
<PAGE>

THE SANWA BANK, LIMITED

By:__________________________________
  Name:______________________________
  Title:_____________________________
<PAGE>

                                          THE NORTHERN TRUST COMPANY

                                          By:__________________________________
                                            Name:______________________________
                                            Title:_____________________________
<PAGE>

WACHOVIA BANK, N.A.

By:__________________________________
    Name:____________________________
    Title:___________________________
<PAGE>

                                          WELLS FARGO BANK, N.A.

                                          By:__________________________________
                                            Name:______________________________
                                            Title:_____________________________

                                          By:__________________________________
                                            Name:______________________________
                                            Title:_____________________________<PAGE>

                                                                    EXHIBIT 4.13

================================================================================

                            AIRTRAN AIRLINES, INC.,
                                  as Issuer,

                          AIRTRAN HOLDINGS, INC. AND
                 THE SUBSIDIARY GUARANTORS SIGNATORIES HERETO,
                                 as Guarantors

                                      and

                             THE BANK OF NEW YORK,
                       as Trustee and Collateral Trustee

                          __________________________

                         SECOND SUPPLEMENTAL INDENTURE

                          Dated as of April 23, 1999

                                      to

                                   INDENTURE

                          Dated as of August 13, 1997

                          __________________________

                     10 1/2% Senior Secured Notes due 2001

================================================================================
<PAGE>

                         SECOND SUPPLEMENTAL INDENTURE

     SECOND SUPPLEMENTAL INDENTURE (this "Supplemental Indenture"), dated as of
                                          ----------------------
April 23, 1999, by and among AirTran Airlines, Inc., a Nevada corporation (the
"Company"), AirTran Holdings, Inc., a Nevada corporation (the "Parent Company"),
--------                                                       --------------
the Subsidiary Guarantors parties hereto (the "Guarantors") and The Bank of New
                                               ----------
York, as trustee and collateral trustee (the "Trustee").
                                              -------

                                   RECITALS

     WHEREAS, the Company, the Parent Company and the Guarantors have heretofore
executed and delivered to the Trustee an Indenture, dated as of August 13, 1997
(the "Indenture"), providing for the issuance of an aggregate principal amount
      ---------
of $80,000,000 of 10 1/2% Senior Secured Notes due 2001 (the "Notes"); and
                                                              -----

     WHEREAS, certain of the Company=s Subsidiaries (i.e., ValuJet Capital
                                                     ----
Corp., ValuJet Management Corp., ValuJet I, Ltd., ValuJet II, Ltd., ValuJet
Reservation Partners, L.P. and ValuJet Corporate Partners, L.P.) have been
liquidated into the Company as their parent entity and the Company has succeeded
to all of the rights and obligations of such Subsidiaries; and

     WHEREAS, pursuant to Section 901 of the Indenture, the Company and the
Trustee may enter into one or more supplemental indentures without the consent
of any Holders to make certain changes to the Indenture; and

     WHEREAS, the Indenture provides for the execution and delivery of a
supplement to the Indenture which shall particularly describe the Collateral
being specifically mortgaged to the Trustee for the benefit and security of the
Holders pursuant to the Indenture; and

     WHEREAS, the Indenture relates to certain Airframes more particularly
described therein, which Indenture was recorded with the FAA on November 4, 1997
and given Conveyance No. GG011890; and

     WHEREAS, it is the desire of the Trustee and the Company to add as
Collateral under the Indenture one (1) McDonnell Douglas DC-9-32 aircraft,
registration number N911VV and serial number 47285 (the "Replacement Aircraft")
                                                         --------------------
in replacement of and substitution for one (1) McDonnell Douglas DC-9-32
aircraft, registration number N925VV and serial number 47319 (the "Released
                                                                   --------
Aircraft");
--------

     NOW THEREFORE, in consideration of the foregoing and for other good and
valuable consideration, the receipt of which is hereby acknowledged, the
Company, the Parent Company, the Guarantors and the Trustee mutually covenant
and agree for the equal and ratable benefit of the Holders of the Notes as
follows:

     1.  Definitions.  Capitalized terms used herein without definition shall
         -----------
have the meanings assigned to them in the Indenture.  For all purposes of this
Supplemental Indenture, except as otherwise herein expressly provided or unless
the context otherwise requires, the words "herein," "hereof" and "hereby" and
other words of similar import used in this Supplemental Indenture refer to this
Supplemental Indenture as a whole and not to any particular section hereof.

                                       2
<PAGE>

     2.  Substitute Security.
         -------------------

          (a) To secure the prompt payment of the principal amount of and
interest on, and all other amounts due with respect to, all Securities from time
to time outstanding under the Indenture and the performance and observance by
the Company of all the agreements, covenants and provisions contained in the
Indenture for the benefit and security of the Holders under the Indenture and
the prompt payment of any and all amounts from time to time owing under the
Indenture by the Company to the Trustee, the Company does hereby grant, sell,
assign, transfer, convey, pledge and confirm unto the Trustee, its successors
and assigns, for the benefit and security of the Holders and the Trustee, a
first priority security interest in all estate, right, title and interest of the
Company in and to the Replacement Aircraft together with all equipment and
accessories, parts and appurtenances pertaining or attached to the Replacement
Aircraft, whether now owned or hereafter acquired, and all warranties of any
manufacturer with respect thereto.

          (b) As evidence of the releasing of all right, title and interest of
the Trustee in, to and under the Released Aircraft and Released Engines, the
Trustee shall execute a separate Partial Release and any other document
reasonably requested by the Company to evidence such release.

          (c) The Company hereby acknowledges that the Replacement Aircraft
referred to in this Supplemental Indenture is owned by and has been delivered to
the Company and is included in the property of the Company subject to the pledge
and mortgage thereof under the Indenture.

     3.  Ratification of Indenture; Supplemental Indentures Part of Indenture.
         --------------------------------------------------------------------
Except as expressly amended hereby, the Indenture is in all respects ratified
and confirmed and all the terms, conditions and provisions thereof shall remain
in full force and effect.  This Supplemental Indenture shall form a part of the
Indenture for all purposes, and every Holder of Notes heretofore or hereafter
authenticated and delivered shall be bound hereby.

     4.  Governing Law.  THIS SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY, AND
         -------------
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK, AS APPLIED TO
CONTRACTS MADE AND PERFORMED WITHIN THE STATE OF NEW YORK, WITHOUT REGARD TO
PRINCIPLES OF CONFLICTS OF LAW.

     5.  Trustee Makes No Representation.  The Trustee makes no representation
         -------------------------------
as to the validity or sufficiency of this Supplemental Indenture.

     6.  Counterparts.  The parties may sign any number of copies of this
         ------------
Supplemental Indenture.  Each signed copy shall be an original, but all of them
together represent the same agreement.

     7.  Effect of Headings.  The Section headings herein are for convenience
         ------------------
only and shall not affect the construction thereof.

                           [SIGNATURES ON NEXT PAGE]

                                       3
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed as of the date first above written.

                                    AIRTRAN AIRLINES, INC., the Company

                                    By: /s/ Joseph B. Leonard
                                        --------------------------------
                                        Name:  Joseph B. Leonard
                                        Title: CEO

                                    AIRTRAN HOLDINGS, INC., the Parent
                                    Company

                                    By: /s/ Joseph B. Leonard
                                        --------------------------------
                                        Name:  Joseph B. Leonard
                                        Title: CEO

                                    AIRTRAN AIRWAYS, INC., as Guarantor

                                    By: /s/ Joseph B. Leonard
                                        --------------------------------
                                        Name:  Joseph B. Leonard
                                        Title: CEO

                                    VALUJET INVESTMENT CORP., as
                                    Guarantor

                                    By: /s/ Joseph B. Leonard
                                        --------------------------------
                                        Name:  Joseph B. Leonard
                                        Title: CEO

                                    THE BANK OF NEW YORK, as Trustee and
                                    Collateral Trustee

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

                                       4

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