SECOND
SUPPLEMENTAL INDENTURE SECOND
  SUPPLEMENTAL INDENTURE (this Second Supplemental Indenture) dated effective
  as of April 3, 2006, among Frontier Airlines, Inc., a Colorado corporation
  (Frontier), a wholly-owned subsidiary of Frontier Airlines Holdings, Inc.,
  a Delaware corporation (Parent), Parent and U.S. Bank National Association,
  a nationally chartered banking association, as trustee under the indenture
  referred to below (the Trustee). WITNESSETH: WHEREAS
Frontier has heretofore executed and delivered to the Trustee a Base Indenture
dated as of December 7, 2005, and a First Supplemental Indenture dated as of
December 7, 2005 (collectively, the Indenture; capitalized terms used but not
defined herein having the meanings assigned thereto in the Indenture),
providing for the issuance by Frontier of its 5% Convertible Debentures Due
2025; WHEREAS
the Indenture requires that, as a condition to any Parent Holding Company
Transaction, (1) Parent enter into a supplemental indenture to assume all the
obligations of Frontier under the Indenture and (2) Frontier fully and
unconditionally guarantee all the obligations of Parent under the Indenture; WHEREAS
the Indenture permits Frontier to execute and deliver to the Trustee a
supplemental indenture pursuant to which Frontier shall unconditionally
guarantee all the Companys obligations under the Debt Securities pursuant to a
Guarantee on the terms and conditions set forth herein; WHEREAS
the Guarantee contained in this Second Supplemental Indenture shall constitute
a Guarantee and Frontier shall constitute a Guarantor for all purposes of
the Indenture; and WHEREAS
pursuant to Section 901(1) and (11) of the Indenture, the Trustee, Parent and
Frontier are authorized to execute and deliver this Second Supplemental
Indenture; NOW
THEREFORE, in consideration of the foregoing and for other good and valuable
consideration, the receipt of which is hereby acknowledged, Parent, Frontier
and the Trustee mutually covenant and agree for the equal and ratable benefit
of the holders of the Debt Securities as follows: 1.          Parent
Substituted. Parent hereby agrees
to succeed to and be substituted for the Company under the Indenture with the
same effect as if Parent had been named as the Company in the Indenture, and on
and from the date of this Second Supplemental Indenture, Parent shall succeed
to, and be substituted for, and may exercise every right and power under the
Indenture with the same effect as if Parent had been named as the Company
therein.   2.          Agreement
to Guaranty. Frontier hereby
agrees to unconditionally guarantee the Companys obligations under the Debt
Securities on the terms and subject to the conditions set forth in Article 16
of the Indenture and to be bound by all other applicable provisions of the
Indenture and the Debt Securities.

3.          Successors
and Assigns. This Second
Supplemental Indenture shall be binding upon Parent and Frontier and their
respective successors and assigns and shall enure to the benefit of the
successors and assigns of the Trustee and the Holders and, in the event of any
transfer or assignment of rights by any Holder or the Trustee, the rights and
privileges conferred upon that party in the Indenture and in the Debt
Securities shall automatically extend to and be vested in such transferee or
assignee, all subject to the terms and conditions of this Second Supplemental
Indenture and the Indenture.

4.          No
Waiver. Neither a failure nor a delay
on the part of either the Trustee or the Holders in exercising any right, power
or privilege under this Second Supplemental Indenture, the Indenture or the
Debt Securities shall operate as a waiver thereof, nor shall a single or
partial exercise thereof preclude any other or further exercise of any right,
power or privilege. The rights, remedies and benefits of the Trustee and the
Holders herein and therein expressly specified are cumulative and not exclusive
of any other rights, remedies or benefits which either may have under this
Second Supplemental Indenture, the Indenture or the Debt Securities at law, in
equity, by statute or otherwise.

5.          Opinion
of Counsel. Concurrently with the
execution and delivery of this Second Supplemental Indenture, the Company shall
deliver to the Trustee an Opinion of Counsel to the effect that (1) all
conditions to a Parent Holding Company Transaction set forth in Section 8.03 of
the Indenture have been satisfied; (2) this Second Supplemental Indenture has
been duly authorized, executed and delivered by each of Parent and Frontier;
and (3) (A) each of the Indenture and any outstanding Debentures is a legal,
valid and binding obligation of the Parent, enforceable against the Parent in
accordance with its terms, in substantially the form of, and subject to
substantially the same limitations and qualifications as set forth in, the
Opinion of Counsel dated as of December 7, 2005 delivered to the Trustee in connection
with the execution and delivery of the First Supplemental Indenture and (B) the
Guarantee of Frontier is a legal, valid and binding obligation of Frontier,
enforceable against Frontier in accordance with its terms, subject to the
application of bankruptcy, insolvency, moratorium, fraudulent conveyance or
transfer and other similar laws relating to creditors rights generally and to
principles of equity, whether considered in a proceeding at law or in equity,
and such other reasonable and customary limitations and qualifications.

6.          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 Second
Supplemental Indenture shall form a part of the Indenture for all purposes, and
every holder of Debt Securities heretofore or hereafter authenticated and
delivered shall be bound hereby.

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7.          Governing
Law. THIS SECOND SUPPLEMENTAL INDENTURE SHALL BE GOVERNED
BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

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

9.          Effect
of Headings. The Section headings
herein are for convenience only and shall not effect the construction thereof.

[Signature pages follow]

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

FRONTIER AIRLINES, INC.By:Paul H.
TateSenior Vice
  President andChief Financial
  OfficerFRONTIER AIRLINES
  HOLDINGS, INC.By:Paul H.
TateSenior Vice
  President andChief Financial
  OfficerU.S. BANK NATIONAL
  ASSOCIATION,
  as TrusteeBy:Name:Title: