Below is an edited note about how Delta Airlines recognizes revenue. Suppose you are a Delta Sky Miles Member, and you make sure that you always use your miles for travel when you have accumulated enough miles. If you purchased a ticket for $600 cash for travel next week, how would Delta account for the revenue on the date you purchased your ticket and on the date of your trip?

If another person is not a member of Sky Miles and purchases the same ticket, how would Delta account for the revenue?

Passenger Tickets. We defer sales of passenger tickets to be flown by us or that we sell on behalf of other airlines in our air traffic liability. Passenger revenue is recognized when we provide transportation or when ticket breakage occurs. For tickets that we sell on behalf of other airlines, we reduce the air traffic liability when consideration is remitted to those airlines. The air traffic liability primarily includes sales of passenger tickets to be flown in the future and credits which can be applied as payment toward the cost of a ticket ("travel credits"). Travel credits are typically issued as a result of ticket cancellations prior to their expiration dates. We periodically evaluate the estimated air traffic liability and record any adjustments in our income statement. These adjustments relate primarily to refunds, exchanges, ticket breakage, transactions with other airlines and other items for which final settlement occurs in periods subsequent to the sale of the related tickets at amounts other than the original sales price.

Approximately $3.1 billion, $3.8 billion and $3.5 billion of the prior year air traffic liability related to passenger ticket sales (which excludes those tickets sold on behalf of other airlines) was recognized in passenger revenue during the years ended December 31, 2020, 2019 and 2018, respectively.

Ticket Breakage. We estimate the value of tickets that will expire unused and recognize revenue at the scheduled flight date. We periodically evaluate our breakage estimates, which are based on historical experience, ticket contract terms and customers travel behavior, and may adjust our estimates in the future.

Loyalty Travel Awards

Loyalty travel awards revenue is related to the redemption of miles for travel. We recognize loyalty travel awards revenue in passenger revenue as miles are redeemed and transportation is provided. See below for discussion of our loyalty program accounting policies.

Loyalty Program

Our SkyMiles loyalty program generates customer loyalty by rewarding customers with incentives to travel on Delta. This program allows customers to earn mileage credits ("miles") by flying on Delta, Delta Connection and other airlines that participate in the loyalty program. When traveling, customers earn miles based on the passenger's loyalty program status and ticket price. Customers can also earn miles through participating companies such as credit card companies, hotels, car rental agencies and ridesharing companies. Miles are redeemable by customers in future periods for air travel on Delta and other participating airlines, membership in our Sky Club and other program awards. To facilitate transactions with participating companies, we sell miles to non-airline businesses, customers and other airlines.

The loyalty program includes two types of transactions that are considered revenue arrangements with multiple performance obligations (1) passenger ticket sales earning miles and (2) sale of miles to participating companies.

Passenger Ticket Sales Earning Miles. Passenger ticket sales earning miles provide customers with (1) miles earned and (2) air transportation, which are each considered performance obligations. We value each performance obligation on a standalone basis. To value the miles earned, we consider the quantitative value a passenger receives by redeeming miles for a ticket rather than paying cash, which is referred to as equivalent ticket value ("ETV"). Our estimate of ETV is adjusted for miles that are not likely to be redeemed ("breakage"). We use statistical models to estimate breakage based on historical redemption patterns. A change in assumptions to the redemption activity for miles or the estimated fair value of miles expected to be redeemed could have a material impact on our revenue in the year in which the change occurs and in future years. We recognize breakage proportionally during the period in which the remaining miles are actually redeemed.

We defer revenue for the miles when earned and recognize loyalty travel awards in passenger revenue as the miles are redeemed and transportation is provided. We record the air transportation portion of the passenger ticket sales in air traffic liability and recognize passenger revenue when we provide transportation or if the ticket goes unused.