Exhibit 10.6(b) CONFIDENTIAL PORTIONS HAVE BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT TO A CONFIDENTIAL TREATMENT REQUEST. FOURTH AMENDMENT TO THE UNITED EXPRESS AGREEMENT [United Airlines Letterhead] December 11, 1998 Mr. Kerry B. Skeen President & CEO Atlantic Coast Airlines 515A Shaw Road Dulles, VA 20166 Re: Fourth Amendment to United Express Agreement, dated October 1, 1991, and as amended from time to time thereafter (the "United Express Agreement") Dear Kerry: This letter, when countersigned by you, will constitute a further amendment to the United Express Agreement as follows: 1. Parties. Atlantic Coast Airlines Holdings, Inc. will be a party to the United Express Agreement, and, together with Atlantic Coast Airlines, will be referred to in the United Express Agreement as "Contractor". 2. Extension of the United Express Agreement a. Article 2.B of the United Express Agreement is hereby amended to read as follows: "B. This Agreement becomes effective on October 1, 1991 ("Effective Date"), and, subject to termination at an earlier date pursuant to one or more provisions of this Agreement, will continue in effect until March 31, 2009; provided, however, United may terminate this Agreement without cause effective no earlier than March 31 2004 upon one (1) years' prior written notice; provided further, however, if United so terminates this Agreement upon one (1) years prior written notice, the liquidated damages under Article 16.G will not apply. In the event, however, that by January 2, 2001 United fails to grant Contractor the ability, separate and apart from the allocations of fifty seat regional jet aircraft, to operate regional jet aircraft of 44 seats or less capacity under this Agreement, then Contractor will have the right to notify United that the Agreement will terminate as of March 31, 2004, which notice must be given by Contractor by no later than January 2, 2002; provided, however, if Contractor timely gives such notice but United then makes such allocation on or before January 2, 2002 then Contractor's aforesaid notice will be null and avoid." b. The fifth and sixth lines of Article 16.G (2) are amended to read as follows: ". . . during the period commencing with the date of termination through the end of the term of this Agreement in accordance with Article 2.B; provided, however, if United secures . . ." c. To the extent the agreements entered into between the parties related to the United Express Agreement (including agreements relating to Mileage Plus participation, prorate/revenue sharing, interline travel, groundhandling, and emergency response) contain a termination date, all such agreements are hereby amended such that each of such agreements will continue in effect for a term equal to that stated in Article 2.B of the United Express Agreement; provided, however, that all such agreements automatically will terminate contemporaneously with the termination of the United Express Agreement. The parties confirm that previous extensions of the United Express Agreement were intended to effectuate extensions of these other agreements as well. 3. Program Fee. Appendix I of the United Express Agreement is amended by the addition of the following paragraph: "Notwithstanding anything else contained in this United Express Agreement to the contrary, the current Program Fee will be fixed through March 31, 2000 and will be subsequently adjusted only in accordance with the terms of this paragraph. [ * ] 4. Regional Jets. A new Article 4.F, entitled "Regional Jets" is hereby added to the United Express Agreement as follows: "F. Regional Jets (1) Contractor is authorized to operate up to 43 regional jet aircraft, 50-seat capacity, as United Express under the terms of this Agreement. The schedule for the introduction of the 34th through 43 such aircraft will be as set forth in a schedule to this Fourth Amendment to this Agreement to be agreed upon by December 21, 1998. The deployment of any regional jet aircraft operated by Contractor, as United Express or operated with United Express livery, must be approved by United on a city pair by city pair basis. [ * ] The allocation of any such aircraft to Contractor thereafter will be governed by subparagraph F.(2), below. (2) If the operation of greater than a total of [* ] fifty seat regional jet aircraft within the United Express system is approved and allocated by United, Contractor will be authorized to operate a minimum of [ * ] of any such aircraft above [ *]. Said percentage will be based upon the total number of such aircraft approved and allocated for operation in the total United Express fleets of all United Express carriers, in excess of the initial [ *] fifty seat regional jet aircraft, measured as of the end of each calendar year; provided, however, in applying the foregoing percentage to determine the number of aircraft allocable to Contractor fractional aircraft shall be disregarded." 5. Additional Terms a. The parties agree to revise the Operating Performance Standards contained in the United Express Agreement to mutually agreed levels and agree upon mutually agreed revised incentives and consequences based upon achievement or lack thereof of those Operating Performance Standards. b. The terms of the letter of understanding, dated November 2, 1998, regarding Contractor's use of Orion and its IRS database are incorporated into the United Express Agreement. c. The following Article 4.G is hereby added to the United Express Agreement: "G. [ * ] d. Article 16.D of the United Express Agreement is amended to read as follows: "D. United may immediately terminate this Agreement if Contractor enters into a similar arrangement (including, without limitation, a code share arrangement) with any other carrier, unless United has given Contractor its express prior written approval of such arrangement." e. A new Article 12.G is hereby added to the United Express Agreement as follows: G. United hereby assumes liability for and agrees to indemnify, release, defend, protect, save and hold Contractor, its officers, directors, agent, and employees harmless from and against any and all liabilities, damages, expenses, losses, claims, demands, suits, fines, or judgments, including, but not limited to, attorneys' and witnesses' fees, costs, and expenses incident thereto, which may be suffered by, accrue against, be charges to or be recovered from Contractor, its officers, directors, employees, or agents, by reason of any injuries to or deaths of persons or the loss of, damage to, or destruction of property, including the loss of use thereof, arising out of, in connection with, or in any way related to any act, error, omission, operation, performance or failure of performance of United or its officers, directors, employees or agents, which is in any way related to the services of United contemplated by or provided pursuant to this Agreement." f. Pursuant to Article 3.1 of the United Express Agreement, United will provide a minimum co-op advertising budget for calendar year 1999 of [ *]. 6. Other Terms and Conditions. Except as specifically stated above, all other terms and conditions of the United Express Agreement remain in full force and effect. Please confirm the concurrence of Atlantic Coast Airlines and Atlantic Coast Airlines Holdings, Inc. to the foregoing by signing below. Very truly yours United Air Lines, Inc. By: /s/ Thomas Hanley_________ Thomas Hanley By: /s/ Rono Dutta____________ Rono Dutta Accepted and Agreed to this 17th day of December, 1998 Atlantic Coast Airlines Atlantic Coast Airlines Holdings, Inc. By: /s/Kerry B. Skeen__________ Kerry B. Skeen President and CEO