<PAGE>
                                                                   Exhibit 10.15

                                             * TEXT OMITTED AND FILED SEPARATELY
                                                CONFIDENTIAL TREATMENT REQUESTED
                                           UNDER 17 C.F.R. SECTION 200.80(B)(4),
                                                            200.83 AND 240.24b-2

                        SIXTH AMENDMENT TO CODE SHARE AND
                          REVENUE SHARING AGREEMENT AND
                              SETTLEMENT AGREEMENT

     THIS SIXTH AMENDMENT TO CODE SHARE AND REVENUE SHARING AGREEMENT AND
SETTLEMENT AGREEMENT ("Sixth Amendment") is made and entered into as of July 27,
2005 (the "Effective Date"), between AMERICA WEST AIRLINES, INC., a Delaware
corporation ("AWA"), MESA AIRLINES, INC., a Nevada corporation ("Mesa"), AIR
MIDWEST, INC., a Kansas corporation ("AM"), and FREEDOM AIRLINES, INC., a Nevada
corporation ("Freedom"). Mesa, AM and Freedom are referred to collectively as
the "Mesa Group").

     RECITALS:

     A. AWA and the Mesa Group are parties to that certain Code Share and
Revenue Sharing Agreement, dated to be effective February 1, 2001, as amended by
that certain First Amendment to Code Share and Revenue Sharing Agreement, dated
to be effective April 27, 2001, that certain Second Amendment to Code Share and
Revenue Sharing Agreement, dated to be effective October 24, 2002, that certain
Third Amendment to Code Share and Revenue Sharing Agreement, dated to be
effective January 29, 2003, that certain Fourth Amendment to Code Share and
Revenue Sharing Agreement and Release, dated to be effective September 5, 2003
(the "Fourth Amendment"), and that certain Fifth Amendment to Code Share and
Revenue Agreement, dated to be effective January 28, 2005 (collectively, the
"Code Share Agreement"). All capitalized terms used herein, but not otherwise
defined herein, shall have the meanings given to such terms in the Code Share
Agreement.

     B. The Code Share Agreement requires the Mesa Group to provide certain
Flight Services and Other Services for AWA, pursuant to the terms and conditions
of the Code Share Agreement.

     C. The Mesa Group and AWA desire to amend the Code Share Agreement pursuant
to the terms and conditions of this Sixth Amendment.

     D. The Mesa Group and AWA desire to settle certain amounts payable to each
other pursuant to the Code Share Agreement.

     NOW, THEREFORE, in consideration of the promises, covenants,
representations and warranties hereinafter set forth, and for other valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, AWA
and the Mesa Group agree as set forth below:

     AMENDMENTS:

<PAGE>

     1. The parties agree that Section 2.2.2(f) of the Code Share Agreement is
amended in its entirety as follows:

          CRJ 900 Put Option. AWA agrees that Mesa may add to the CRJ Subfleet
          and place into Flight Services [*] CRJ Model 900s that, pursuant to
          Exhibit A, were scheduled to be placed into Flight Services in [*]
          (the "Put CRJ 900s") on [*]. Each Put CRJ 900 shall be marked with
          AWA's livery and shall be configured with all coach seating in a
          manner required by AWA, at Mesa's sole cost and expense, upon being
          placed into Flight Services. Notwithstanding anything contained in
          Section 7.2 to the contrary, AWA shall not be required to pay
          "Ownership" Guaranteed Non-Maintenance Costs for each of the Put CRJ
          900s for a period of 30 days after each Put CRJ 900 commences Flight
          Services.

     2. The parties agree that Section 2.2.6 of the Code Share Agreement is
amended by deleting the parenthetical "(other than overhead)" in the last
sentence and replacing it with the following: "(other than Overhead and Crew
RON)".

     3. The parties agree that Section 2.6.3 of the Code Share Agreement is
amended in its entirety as follows:

          2.6.3 Location. Mesa shall maintain maintenance bases as follows: (i)
          one in Grand Junction, Colorado for Dash 8s; (ii) one in Phoenix,
          Arizona for CRJ Model 200s; (iii) one in Phoenix, Arizona for CRJ
          Model 900s; and (iv) commencing on or after December 1, 2005, one in a
          mutually acceptable location on the East Coast for CRJ Model 900s (the
          "Maintenance Bases"). In addition, Mesa intends to provide CRJ Model
          200 and 900 maintenance at certain outstation or Mesa provided
          maintenance bases (the "Outstation Bases"). Each Schedule prepared by
          AWA shall provide for not less than 20% of the Dash 8s, 33% of the CRJ
          Model 200s, and 33% of the CRJ Model 900s to remain overnight at the
          applicable Maintenance Bases and Outstation Bases each night. AWA and
          Mesa intend that 20% to 25% of the CRJ will be maintained at the
          Maintenance Bases and the balance will be maintained at the Outstation
          Bases in order to reach the 33% overnight threshold. One aircraft
          shall remain overnight for 10 hours and the remainder for at least 8
          hours. Mesa shall not relocate any Maintenance Base without prior
          written consent of AWA, which consent may be withheld if the new
          location fails to meet AWA's schedule requirements. Each CRJ
          Maintenance Base shall be staffed and equipped to maintain a fleet of
          up to 25 aircraft. Mesa shall add maintenance bases as necessary to
          provide the Flight Services and Other Services at locations which meet


                                       2

<PAGE>

          AWA's maintenance base requirements and are approved by AWA. AWA, by
          providing Mesa with at least 180 days' prior written notice, may
          require Mesa to close any Maintenance Base. Upon Mesa assigning to AWA
          all of its rights, title and interest in the lease of the Maintenance
          Base that is closed (together with any required landlord consent), AWA
          shall reimburse Mesa for all actual out-of-pocket costs and expenses
          incurred by Mesa in closing such Maintenance Bases.

     4. The parties agree that Section 5 of the Code Share Agreement is amended
by adding the following new paragraph 5.7:

          5.7 Performance Penalties Waiver. Mesa and AWA agree that neither Mesa
          nor AWA shall be required to pay any performance penalties or bonuses,
          pursuant to Sections 5.1 through 5.4, based on Mesa's provision of
          Flight Services for the period of [*]. If AWA exercises its right to
          slot substitutions pursuant to that certain Slot Substitution
          Agreement, dated July 27, 2005, between AWA and Mesa (the
          "Substitution Agreement"), then, for the purposes of calculating
          performance penalties and bonuses for the applicable year, month, Six
          Month Period or Calendar Quarter, as applicable, pursuant to Sections
          5.1 through 5.4, the Flight Services and AWA flight operations
          occurring on any date on which AWA exercises its rights under the
          Substitution Agreement shall not be included. For example only, if AWA
          exercised its rights under the Substitution Agreement on August 15,
          2005, then the Flight Services and AWA flight operations on such day
          shall not be used in calculating the DOT Complaint Rate and AWA's DOT
          Complaint Rate for calendar year 2005, the MBR for the month of
          August, 2005, the OTP Rate Threshold and OTP Rate for the Six Month
          Period between July 1, 2005 and December 31, 2005, and the FCF
          Threshold and FCF for the Calendar Quarter commencing July 1, 2005 and
          expiring on September 30, 2005.

     5. The parties agree that the Code Share Agreement is amended by adding the
following as a new Section 7.1.9:

          7.1.9 Actual costs invoiced to and paid by Mesa for the maintenance of
          CRJs at the Outstation Bases, including depreciation of parts,
          equipment and tooling and Mesa on-site personnel costs (net of any
          discounts or promotions and provided a good faith estimate of the
          general scope and costs for the Outstation Bases was initially
          pre-approved, in writing, by AWA's scheduling and planning group).


                                       3

<PAGE>

     6. The parties agree that the Code Share Agreement is amended by adding the
following as a new last paragraph to Section 7.2:

          Notwithstanding anything above to the contrary, subsequent to December
          31, 2004, AWA shall be entitled to a credit equal to the sum of: (i)
          the greater of $[*] per day that an applicable CRJ is out of Flight
          Services for the performance of the Reliability Improvement Program
          ("RIMP") or the actual ownership reimbursement received by Mesa from
          Bombardier for the applicable CRJ undergoing such RIMP work; plus (ii)
          an amount equal to a per diem allocation of Crew RON and Engine and
          APU Depreciation chargeable by Mesa as Guaranteed Maintenance Costs
          for the CRJ undergoing such RIMP work, based on the number of days the
          CRJ is out of service for RIMP work.

     7. The parties agree that Exhibit C of the Code Share Agreement is amended
by: (i) deleting the reference to $[*] in the "Ownership" line under the CRJ 200
column and replacing such number with a reference to Note 5; (ii) replacing the
CRJ 200 A/C Ownership Schedule attached as page 2 to Exhibits C and D with page
2, attached hereto; and (iii) adding the following Note 5:

          [*]

     AGREEMENTS:

     8. Settlement Amounts. AWA and Mesa agree to the following settlement of
certain outstanding liabilities, duties and obligations contained in the Code
Share Agreement and disputes between Mesa and AWA:

          8.1 AWA and Mesa agree that, pursuant to Section 19 of the Fourth
Amendment, the Reimbursement Payment is $[*];

          8.2 AWA and Mesa agree that Mesa shall reimburse AWA for Mesa's
overcharge of "overhead costs" for spare aircraft, pursuant to Section 2.2.6, in
an amount equal to $[*];

          8.3 AWA and Mesa agree that AWA shall pay to Mesa the sum of $[*] in
settlement of outstanding CRJ Model 200 Ownership costs payable through [*],
pursuant to Section 7.2(a);

          8.4 AWA and Mesa agree that AWA shall pay to Mesa the sum of $[*] for
actual outstation maintenance costs incurred by Mesa for the periods of [*]
through [*] (the "Outstation Costs");

          8.5 AWA and Mesa agree that Mesa shall pay to AWA the sum of $[*] in
settlement of all performance penalties incurred by Mesa, pursuant to Sections
5.1 through 5.4, through [*];


                                       4

<PAGE>

          8.6 AWA and Mesa agree that AWA shall pay to Mesa the sum of $[*] in
settlement of all transition costs incurred by Mesa in connection with the
elimination of CRJ Model 700s, pursuant to Section 2.3.3;

          8.7 AWA and Mesa agree that AWA shall pay to Mesa the sum of $[*] in
settlement of all turn costs payable by AWA for CRJ Model 900s at the Phoenix
Hub, pursuant to Exhibit B, for the period of [*];

          8.8 AWA and Mesa agree that AWA shall pay to Mesa the sum of $[*] in
settlement of disputed sums due for the provision of Flight Services to
Guadalajara, Mexico (the "GDC Services"); and

          8.9 AWA and Mesa agree that AWA shall pay to Mesa the sum of $[*] for
RIMP work performed by Mesa between March 1, 2004 and December 31, 2004 (the
"RIMP Work").

          The net amount due pursuant to Sections 7.1 through 7.9, above, from
AWA to Mesa is $[*], which sum shall be paid by AWA to Mesa within three (3)
business days after the Effective Date.

     9. Releases.

          9.1 The Mesa Group fully and finally release, acquit and forever
discharge AWA and its parent companies and subsidiaries from any and all claims
or demands for: (i) the Reimbursement Payment; (ii) CRJ Model 200 Ownership
costs through and including June 30, 2005; (iii) Outstation Costs through
December 31, 2004; (iv) costs and expenses incurred in connection with the RIMP
Work; (v) reimbursement of actual out-of-pocket costs and expenses incurred in
connection with the elimination of CRJ Model 700s from the Fleet; (vi) turn
costs for CRJ Model 900s pursuant to Exhibit B for the periods of October 1,
2004 through June 30, 2005; and (vii) payment for the GDC Services.

          9.2 AWA fully and finally releases, acquits and forever discharges
Mesa and its parent company and subsidiaries from any and all claims and demands
for: (i) reimbursement of improperly charged overhead costs prior to the
Effective Date pursuant to Section 2.2.6; (ii) performance penalties pursuant to
Sections 5.1 through 5.4 for all periods prior to December 31, 2004; and (iii)
Crew RON costs charged for spare aircraft prior to July 1, 2005.

     10. Tail N937/TSA Expenses. AWA and Mesa agree that: (i) no amount is
payable by AWA to Mesa for CRJ Model 900 tail number N937 for the month of June,
2005; and (ii) no payment is due to AWA for the double billed TSA expenses.

     11. Effect. Except as set forth in this Sixth Amendment, all of the terms
and conditions of the Code Share Agreement shall remain in full force and effect
and be applicable to this Sixth Amendment.


                                       5

<PAGE>

     12. Counterparts. This Sixth Amendment may be executed in counterparts, all
of which when taken together shall be one and the same document.

     13. Entire Agreement. This Sixth Amendment constitutes the entire agreement
between the parties with respect to the subject matter hereof and supersedes all
prior understandings with respect thereto.

                        [SIGNATURES APPEAR ON NEXT PAGE]


                                       6

<PAGE>

                                        AMERICA WEST AIRLINES, INC.


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


                                        MESA AIRLINES, INC.


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


                                        FREEDOM AIRLINES, INC.


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


                                        AIR MIDWEST, INC.


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


                                       7

<PAGE>

                               Exhibit C - Page 2


CRJ-200 A/C Ownership Schedule

<TABLE>
<CAPTION>
                 Monthly
    Tail #        Amount
--------------   -------
<S>              <C>
    1   7178       $[*]
    2   7218       $[*]
    3   7228       $[*]
    4   7231       $[*]
    5   7260       $[*]
    6   7264       $[*]
    7   7275       $[*]
    8   7278       $[*]
    9   7286       $[*]
   10   7291       $[*]
   11   7302       $[*]
   12   7305       $[*]
   13   7314       $[*]
   14   7318       $[*]
   15   7325       $[*]
   16   7337       $[*]
   17   7353       $[*]
   18   7358       $[*]

Agreed Rate(1)     $[*]
</TABLE>

(1)  [*]