<PAGE>

                                                                    EXHIBIT 10.6

                       ECA FACILITY AGREEMENT SIDE LETTER

To: (1)   Credit Lyonnais
          as Security Trustee, French National Agent, Agent and Joint Arranger

    (2)   Halifex plc
          as British National Agent and Joint Arranger

    (3)   Bayerische Landesbank Girozentrale
          as German National Agent

    (4)   Kreditanstalt fur Wiederaufbau
          as German Parallel Lender

    (the "National Agents")

                                                              20th December 2001

ECA SUPPORTED FINANCING OF UP TO NINETEEN (19) AIRBUS AIRCRAFT (THE "AIRCRAFT")

1.    We refer to the Facility Agreement of even date herewith between, inter
      alias, the banks and financial institutions named therein, as Lenders,
      Credit Lyonnais, as Security Trustee, Credit Lyonnais, as Agent, O'Farrell
      Leasing Limited, as Cayman Export Lessee, O'Fanell Leasing (Ireland)
      Limited, as Irish Export Lessee, EFG Aircraft Limited, as Cayman Borrower,
      EFG Aircraft (Ireland) Limited, as Irish Borrower, and ourselves (the
      "Facility Agreement").

2.    Capitalised terms used herein shall have the meanings given to them in the
      Facility Agreement (whether defined expressly therein or by reference to
      another document).

3.    In consideration of your agreeing to enter into the Facility Agreement and
      the other Transaction Documents, we hereby agree as follows:

3.1   If an event or circumstance occurs which in the reasonable opinion of any
      National Agent (acting on the instructions of the relevant Export Credit
      Agency) has or would have a material adverse effect on our ability to
      perform or comply with our material obligations under the Transaction
      Documents (a "Material Adverse Change") then any such National Agent
      (acting solely on the instructions of the relevant Export Credit Agency)
      may issue a notice (a "Review Notice") to us (with copy to each of the
      other National Agents) accordingly.

3.2   Thereafter, without prejudice to any of your rights under the Transaction
      Documents, the relevant Export Credit Agencies, the National Agents and we
      will consult with each other in good faith for a period of up to 30 days
      (or such later date as may be agreed by

<PAGE>

      the National Agents and us) after issue of a Review Notice to determine
      what additional security (if any) with respect to the financing of any or
      all of the Aircraft would be acceptable to the National Agents (acting
      solely on the instructions of the relevant Export Credit Agency) having
      regard to the relevant circumstances. We understand that the National
      Agents (or any of them) may, acting solely on the instructions of the
      relevant Export Credit Agency, request us to provide adequate additional
      security in these circumstances

3.3   In the event that after completion of the procedures described in
      paragraph 3.2 hereof:

      3.3.1 in the opinion of any National Agent (acting solely on the
            instructions of the relevant Export Credit Agency) either:

            (i)   no additional security is available to alter its opinion as
                  mentioned in paragraph 3.1 above; or

            (ii)  we do not provide any additional security requested by any
                  National Agent within 10 Business Days; and

      3.3.2 the Ratings Event has occurred and is continuing and such Material
            Adverse Change is continuing,

      any National Agent may (acting solely on the instructions of its Export
      Credit Agency), by notice given not earlier than 30 days and not later
      than 45 days (or such other period as may be agreed by the National Agents
      and us) after the date of the Review Notice, issue a notice to us (with a
      copy to the other National Agents and the Agent) stating that a Mandatory
      Prepayment Event shall have occurred and is continuing for the purposes of
      Clause 4.10.1(d) of the Facility Agreement and instructing the Agent to
      demand repayment of the relevant Loan and/or Loans thereunder.

3.4   It is expressly acknowledged that any failure by the relevant Borrower to
      pay or procure the payment of the sums referred to in paragraph 3.3 within
      the time periods specified in the Facility Agreement shall constitute a
      Termination Event.

4.    For the purposes of:

      (i)   this letter, a "Rating Event" shall have occurred and be continuing;
            and

      (ii)  the Transaction Documents, a "Utilisation Block Event" shall have
            occurred and be continuing,

      if, at any such time, our long term debt obligations are rated (a) less
      than "single B plus" by Standard & Poor's Corporation, (b) less than "Bl"
      by Moody's Investor Service, Inc. and (c) less than "B+" by Fitch IBCA,
      Duff & Phelps.

5.    This letter is a Transaction Document.

6.    The provisions of Clause 22 and 23 of the Facility Agreement shall apply
      to this letter as if the same were set out in full herein, mutatis
      mutandis,

<PAGE>

Yours faithfully

GATX FINANCIAL CORPORATION

By: /s/[ILLEGIBLE]
    --------------------
Title:_________________

Accepted and agreed:

CREDIT LYONNAIS

By: /s/[ILLEGIBLE]
    --------------------

Title:_________________

HALIFAX PIC

By: /s/[ILLEGIBLE]
    --------------------

Title:_________________

BAYERISCHE LANDESBANK GIROZENTRALE

By: /s/[ILLEGIBLE]
    --------------------

Title:__________________

KREDITANSTALT FIIR WIEDERAUFBAU

By: /s/ A. Ufer
    -------------------
Title: Vice President

<PAGE>

Acknowledged and agreed:

EFG AIRCRAFT LIMITED

By: /s/[ILLEGIBLE]
    --------------------

Title:_________________

EFG AIRCRAFT (IRELAND) LIMITED

By: /s/[ILLEGIBLE]
    --------------------

Title:_________________