Who this is for: anyone who found a fare, came back an hour later, and found it gone, and wants to know what actually happened.
How we know: Sources are listed at the end. The mechanics come from published descriptions of airline fare filing and booking classes; the cookies question is answered with the two most-cited independent tests, quoted directly. No figure in this guide is a Wildly measurement.
Why the flight price you saw disappeared is almost never the answer people suspect. It was not your cookies. It was, in roughly descending order of likelihood, a fare bucket that sold its last seat, a search result that was already stale when you saw it, an airline that refiled its fares, or a different product you did not notice you were comparing. Each of those has a specific mechanism, and knowing them changes how you shop. This guide explains the four, then deals with the cookies myth using the actual tests people cite.
Mechanism 1: the bucket sold out
Airlines do not sell "economy seats" at one price. They file many fares for the same cabin, each attached to a booking class. A fare basis code is "an alphabetic or alpha-numeric code used by airlines to identify a fare type", its first letter is the booking class, and booking codes "are the identifiers used by the airline's revenue management department to control how many seats can be sold at a particular fare level" (Wikipedia: Fare basis code). The same article gives the shape of it: a plane with 25 economy seats left might show availability as "Y7 K5 M4 T6 E3", meaning seven seats sellable in Y, five in K, and so on.
The cheap fare you saw was the lowest class with seats still allocated. When the last seat in that class is sold, or the airline closes the class, the next price up becomes the lowest available. Nothing about the flight changed; the cheapest bucket emptied. This is the most common reason a price vanishes between two searches, and it is also why a fare can reappear: a cancellation or a revenue-management decision can reopen the class.
Mechanism 2: the airline refiled its fares
The prices themselves, as distinct from seat allocations, are filed centrally. ATPCO "engages in the collection and distribution of fare and fare-related data for the airline and travel industry", works with more than 440 airlines, and handles "over 446 million active fares in its database" with "an average of 18 million fare changes each day" (Wikipedia: ATPCO). Eighteen million changes a day means the fare you saw may simply have been replaced by a new filing. Refiling tends to happen in waves rather than continuously, which is one reason prices can look stable for days and then jump.
Mechanism 3: the result you saw was already stale
Search engines and travel agencies do not query every airline live for every search. They cache. Kayak's help page says plainly that "the price displayed may occasionally no longer be available, resulting in an outdated price", and, for its Explore product, that "airline seats are priced dynamically and can change frequently" and "the prices shown reflect limited seats that may not be available on all flights or days" (KAYAK Help). The fare you saw in a results list may have been minutes or hours old; the real check happens when you click through and the booking system prices the itinerary again.
This is also why a price on an aggregator can differ from the airline's own site at the same moment, and why a fare that appears once and never again may never have been bookable in the first place.
Mechanism 4: you compared two different products
Two results at different prices are often two different fares, not one fare that moved. The booking class determines refundability, change rules, bag allowance and seat selection. A basic economy fare and a standard economy fare on the same flight are different filings in different classes. If you searched with different passenger counts, a different cabin, or with a stop filter on one search and off the other, you asked a different question and got a different answer. Our guide on Is this a good flight price? walks through checking what a fare includes before comparing it.
The cookies myth, tested
The belief is that airlines watch your searches and raise the price when you come back. Here is what the two tests people actually cite found.
Consumer Reports, 2016. CR ran 372 simultaneous searches on nine sites, one browser with a full search history and one "scrubbed" of cookies. It found 42 pairs where the two browsers showed different prices at the same moment; of those, 25 were higher on the scrubbed browser and 17 were lower. CR's conclusion was that it could not say whether the fare differences were due to pricing based on browser histories. A Kayak spokesman told CR: "Cookies or no cookies, it is impossible for us to show different prices to different users," suggesting a gap of even a few seconds between searches could explain the differences (Consumer Reports).
Note the direction: the browser with no history was more often the one showing the higher fare. That is the opposite of the myth.
TIME, 2017. TIME's piece quotes FareCompare's Rick Seaney that airlines say "prices change not because of a consumer's search history... but because of inventory updates or glitches on the website", and that if airlines were doing it "there would be whistleblowers and senators running to microphones for legislation". The article's own explanation for what shoppers see is "inventory changes, data caching techniques and the fact that prices generally get more expensive closer to departure date" (TIME).
Neither test proves a negative, and the airline industry is opaque. But the evidence people point to does not show cookie-based price increases, and the mechanisms above explain the observations without them. Clearing cookies costs nothing; expecting it to lower a fare is not supported.
What is protected once you buy
None of the above applies after purchase. US rules make it "an unfair and deceptive practice" for a seller of air transportation to "increase the price of that air transportation" after the ticket is bought, with the sole exception of a government-imposed tax or fee increase that the buyer agreed to in writing before purchase (14 CFR 399.88). A disappeared price hurts only if you had not yet bought it. (Fares that were errors are a separate case; see mistake fares explained.)
How to catch the fare next time
- Search the airline directly before deciding. Aggregator caches lag; the airline's booking engine prices live.
- Read Google's insight labels. Google shows "Prices are less than usual" when a fare is significantly below its expectation from past similar trips, and "Prices are likely to increase" when its model expects a rise (Google Travel Help). If a fare is flagged low and the bucket is thin, buy it.
- Set a threshold alert. A Wildly alert checks the route you name on Google Flights once a day and emails you when the lowest fare is at or below your target, with plus or minus 3 days of flexibility if you want it. A daily check will not catch a fare that lives for an hour, but it will catch one that reopens for a day or two, which is the common case. The free plan covers this; see /pricing.
- Know the route's range. Route pages such as New York to London, Seattle to Tokyo, Los Angeles to Paris and Miami to Punta Cana show the lowest fare from the most recent daily check with its date and, once enough days accumulate, typical, good and great bands, so you can tell whether the fare that vanished was unusual or ordinary. How the observations are made is on /methodology. Wildly records daily prices on about 240 routes; route pages show observed ranges as they accumulate.
FAQ
Why did the price go up when I went back to book? Most likely the lowest booking class sold out or was closed between your searches, or the first result was cached and stale. Both are described above.
Do airlines really change prices 18 million times a day? ATPCO reports handling an average of 18 million fare changes a day across more than 440 airlines. That is the industry total, not one airline or one route.
Does incognito mode get cheaper flights? The Consumer Reports test found the cookie-free browser showed a higher price more often than a lower one when the two differed, and could not attribute the differences to browsing history. Incognito is harmless but not a discount.
Can the airline raise the price after I have paid? Not for anything other than a government tax or fee increase you agreed to in advance, under 14 CFR 399.88.
Sources
- Wikipedia: Fare basis code: https://en.wikipedia.org/wiki/Fare_basis_code
- Wikipedia: ATPCO (Airline Tariff Publishing Company): https://en.wikipedia.org/wiki/ATPCO
- KAYAK Help: Pricing and Price Alerts: https://www.kayak.com/c/help/pricing/
- TIME: Do Airlines Raise Prices on Repeat Flight Searches?: https://time.com/4899508/flight-search-history-price/
- Consumer Reports: How to Get the Lowest Airfares: https://www.consumerreports.org/airline-travel/how-to-get-the-lowest-airfares
- Google Travel Help: How to find the best fares with Google Flights: https://support.google.com/travel/answer/7664728
- 14 CFR 399.88, Prohibition on post-purchase price increase (Cornell LII): https://www.law.cornell.edu/cfr/text/14/399.88