Exhibit 10.20

6-1162-ELP-0792

United Air Lines, Inc.

P. O. Box 66100

Chicago, IL 60601

This letter agreement (Letter Agreement) amends and supplements the Purchase Agreement. All terms used but not defined in this Letter Agreement have the same meaning as in the Purchase Agreement.

Customer has requested and Boeing has agreed that Customer may delay the selection of engines for the Aircraft beyond the signing date of the Purchase Agreement. The engine model and prices shown in Table 1 to the Purchase Agreement, Supplemental Exhibit EE1 to the Purchase Agreement will be revised [CONFIDENTIAL MATERIAL OMITTED AND FILED SEPARATELY WITH THE SEC PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT]. The engine price and escalation in effect at the time of engine selection will apply to the engine model selected.

The Model 787-8 Aircraft is offered to Customer powered by [CONFIDENTIAL MATERIAL OMITTED AND FILED SEPARATELY WITH THE SEC PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT]. Customer shall notify Boeing of Customer’s final engine selection (Default Engine Type) on or before [CONFIDENTIAL MATERIAL OMITTED AND FILED SEPARATELY WITH THE SEC PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT], and Boeing and Customer shall execute a supplemental agreement to the Purchase Agreement conforming Table 1 to Customer’s engine selection and incorporate an Attachment A to this Letter Agreement which includes Aircraft delivery, configuration , pricing and advance payments for the Aircraft equipped with [CONFIDENTIAL MATERIAL OMITTED AND FILED SEPARATELY WITH THE SEC PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT].

Assignment.

The rights and obligations described in this Letter Agreement are provided to Customer in consideration of Customer becoming the operator of the Aircraft and can only be assigned, in whole or in part, pursuant to Article 9 of the AGTA as amended by Letter Agreement No. 6-1162-IRS-0184.

Confidential Treatment.

Customer and Boeing understand that certain commercial and financial information contained in this Letter Agreement are considered by Boeing and Customer as confidential and are subject to the terms and conditions set forth in Letter Agreement No. 6-1162-IRS-0182.