Farecast expands airfare predictions; just know when to use them.

UpdatedFarecast, the start-up that predicts whether airfares are about to go up or down (earlier coverage here), has expanded predictions for 20 more airport destinations, bringing the total to 75 airports.

Until now, Farecast has offered airfare search results for only those destinations where it offers fare predictions. On Monday, it launches airfare search results for all airports in the U.S.

This area of forecasting is a tough one, because the airline industry makes so many changes to its prices all the time.

We asked a few questions of Farecast’s VP of Marketing, Mike Fridgen, and here are a few things we learned about how Farecast deals with the complexity. Airlines issue raw data some eight times a day, and tens of thousands of fares can change that many times a day, making it difficult to keep up with fares. Farecast has invested a lot into its database, but it is still limited in what it can do. It manages, Fridgen says, by looking out a maximum of 90 days out into the future. Most leisure trips are two to eight days in length, and so Farecast predicts fare prices for two-to-eight day trip. If you are booking travel more than 90 days out, or if your trip is longer than eight days, Farecast isn’t relevant.

Still, only 20 percent of all trips fall into this category, so Farecast is a useful tool for 80 percent of all trips.