Exhibit 10.178

CONFIDENTIAL MATERIAL APPEARING IN THIS DOCUMENT HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION IN ACCORDANCE WITH THE SECURITIES ACT OF 1933, AS AMENDED, AND RULE 24B-2 PROMULGATED THEREUNDER. OMITTED INFORMATION HAS BEEN REPLACED WITH ASTERISKS.

Supplemental Agreement No. 6

to

Purchase Agreement No. 3860

between

The Boeing Company

and

United Airlines, Inc.

Relating to Boeing Model 787 Aircraft

THIS SUPPLEMENTAL AGREEMENT, entered into as of December 31, 2015, by and between THE BOEING COMPANY (Boeing) and UNITED AIRLINES, INC. (a Delaware corporation formerly known as Continental Airlines, Inc. and successor by merger to United Air Lines, Inc.) (Customer);

WHEREAS, the parties hereto entered into Purchase Agreement No. 3860 dated September 27, 2012, as amended and supplemented (Purchase Agreement), relating to the purchase and sale of Boeing model 787 aircraft (Aircraft). This Supplemental Agreement is an amendment to the Purchase Agreement;

WHEREAS, through *** Supplemental Agreement Number 5 to the Purchase Agreement, *** dated May 12, 2015, Boeing and Customer agreed to *** firm 787-*** aircraft for *** 787-*** aircraft and to *** 787-*** firm aircraft with *** 787-***;

WHEREAS, consistent with such prior transactions Boeing and Customer now agree to a *** of *** firm 787-*** aircraft for *** firm 787-*** aircraft and to the *** of these *** 787-*** firm aircraft with *** 787-*** as follows:

(i) Boeing and Customer agree to *** Firm 787-*** Aircraft to the Purchase Agreement as Block C Aircraft as such terms are defined in *** and are further identified in Figure 1 below.

(ii) Boeing and Customer agree to *** the following *** 787-*** Firm Aircraft and *** them with *** 787-*** Aircraft in the delivery months with option exercise expiry dates as specified below:

** - The nominal delivery month above is subject to revision by plus or minus one month. Boeing will advise Customer of the scheduled delivery month at the time of Customer’s exercise of its rights.

WHEREAS, Boeing and Customer agree to apply *** 787-*** previously Firm Aircraft *** under this Supplemental Agreement No. 6 *** for the *** new model 787-*** Aircraft added under this Supplemental Agreement No. 6; and *** Customer’s *** of Supplemental Agreement No. 6 after application of *** (as that term is defined in Letter Agreement No. *** entitled “Option Aircraft” ***) received by Boeing from Customer.

NOW THEREFORE, in consideration of the mutual covenants herein contained, the parties agree to amend the Purchase Agreement as follows:

The “Table of Contents” is deleted in its entirety and replaced with the attached “Table of Contents” (identified by “SA-6”).

2.1. “Table 1”, 787-10 Aircraft with GENX-1B*** Engines Delivery, Description, Price and Advance Payments, is deleted in its entirety and replaced with the attached similarly titled “Table 1” (identified by “SA-6”).

2.2. “Table 1, 787-9 Aircraft Delivery, Description, Price and Advance Payments for *** Firm 787-9 Aircraft” (identified by “SA-6”) is added to the Purchase Agreement.

3.1. Attachment B-1 to Letter Agreement UAL-PA-03860-LA-1209265R1 entitled “787-10 with GENX-1B*** Engines: Option Aircraft Delivery, Description, Price and Advance Payments” is deleted in its entirety and replaced with the attached Attachment B-1 (identified by “SA-6”).

3.2. Letter Agreement UAL-PA-03860-LA-1209413A1R1 entitled “Special Matters — Amendment 1” is deleted in its entirety and replaced with the attached similarly titled Letter Agreement UAL-PA-03860-LA-1209413A1R2 (identified by “SA-6”).

The Purchase Agreement will be deemed to be supplemented to the extent herein provided as of the date hereof and as so supplemented will continue in full force and effect.

The rest of this page is left intentionally blank.

EXECUTED IN DUPLICATE as of the day and year first written above.

TABLE OF CONTENTS

TABLE OF CONTENTS, CONTINUED

TABLE OF CONTENTS, CONTINUED

Table 1 to Purchase Agreement No. 3860

787-10 Aircraft with GENX-1B*** Engines Delivery, Description, Price and Advance Payments

(787-10/GE/***)

Table 1 to Purchase Agreement No. 3860

787-10 Aircraft with GENX-1B*** Engines Delivery, Description, Price and Advance Payments

(787-10/GE/***)

Table 1 to Purchase Agreement No. 3860

787-9 Aircraft Delivery, Description, Price and Advance Payments for *** Firm 787-9 Aircraft

(787-9/GE/***)

Attachment B-1 to Letter Agreement UAL-PA-03860-LA-120965R1

787-10 Option Aircraft with GENX-1B***

Engines Delivery, Description, Price and Advance Payments

(787-10/GE/***)

Attachment B-1 to Letter Agreement UAL-PA-03860-LA-120965R1

787-10 Option Aircraft with GENX-1B***

Engines Delivery, Description, Price and Advance Payments

(787-10/GE/***)

UAL-PA-03860-LA-1209413A1R2

United Airlines, Inc.

233 South Wacker Drive

Chicago, Illinois 60606

This letter agreement UAL-PA-03860-LA-1209413A1R2 (Amending Letter Agreement) amends and supplements the Purchase Agreement and amends and supplements certain terms in Letter Agreement UAL-PA-03860-LA-1209413 (the Letter Agreement) ), and supersedes and replaces in its entirety letter agreement UAL -PA -03860 -LA-1209413A1R1. All terms used and not defined in this Letter Agreement have the same meaning as in the Purchase Agreement.

New Article 1.4 is hereby added as follows:

1.4. The parties agree that the *** 787-*** Aircraft added by SA-6 with delivery months during

(i) ***; and

(ii) ***

shall, if Customer exercises its rights in such 787-*** Aircraft, be deemed to be 787-*** Aircraft for all purposes under the Purchase Agreement and thereby remain entitled to the corresponding 787-*** Aircraft credit memoranda.

4.4    Special *** Provisions for the Launch Aircraft. With respect to

(i) the *** 787-10 Aircraft listed in Table 1 to the Purchase Agreement as of the effective date of Supplemental Agreement No. 6 to the Purchase Agreement and

(ii) the *** 787-*** Aircraft specified in Figure 1 below; (787-10 Aircraft and 787-*** Aircraft specified in (i) and (ii) herein are each a Launch Aircraft), the parties agree that *** of the Article 4.1 *** obligation will be *** delivery of each Launch Aircraft (Launch Aircraft *** Obligation). At time of delivery of each Launch Aircraft, Boeing will *** to *** the Launch Aircraft *** Obligation.

Boeing and Customer understand that certain information contained in this Letter Agreement, including any attachments hereto, is considered by both parties to be confidential. Boeing and Customer agree that each party will treat this Letter Agreement and the information contained herein as confidential and will not, without the other party’s prior written consent, disclose this Letter Agreement or any information contained herein to any other person or entity except as may be required by applicable law or governmental regulations.

Very truly yours,