For travelers, a canceled flight often seems to be the result of bad weather, mechanical problems or random bad luck. But according to data that have only recently been made public, some flights are much more prone to being axed.

United Airlines flight 650 from Chicago to New York's LaGuardia Airport, for example, was canceled 25 percent of the time last month. Delta Air Lines flight 778 from Atlanta to Huntsville, Ala., was canceled 38.7 percent of the time.

The most troubled flights seem to be those that are lightly booked and travel into or out of big hubs. These flights are most vulnerable because when airlines are faced with bad weather, mechanical issues or crew shortages that leave them with fewer resources or planes, they opt to cancel flights with the fewest passengers.

Now, a new database is making it possible for consumers to identify -- and avoid -- the most frequently canceled flights. FlightStats, a Portland, Ore., unit of Conducive Technology Corp. that once developed Web-based products for American Airlines, has built a database of information about most every airline flight in the U.S. FlightStats can now tell you how often a particular flight is canceled, diverted or tardy, along with a maximum, minimum and average delay. The company retrieves its data from sources such as the Federal Aviation Administration and airline reservation systems. The database has been used by freight firms and the U.S. Postal Service to route cargo to dependable flights and track the progress of flights.

Beginning in August, FlightStats opened the data to the public at www.flightstats.com. (You need to download "Crystal Viewer" software from the site to drill into the data and get the most out of FlightStats reports.) Soon, FlightStats data are likely to be incorporated into other Internet and corporate booking engines so consumers will be able to search for flights not only by price, but also by reliability. Today, FlightStats is expected to announce a deal with PriceGrabber.com to include FlightStats ratings on dependability on PriceGrabber's new airline booking site.

Other flight information databases also exist. The Department of Transportation's Bureau of Transportation Statistics does have a searchable database (www.bts.gov) where you can look at delays and cancellations for particular flights. But the information is 30-60 days older and not as detailed as FlightStats.

Bankruptcy, a mechanics strike, high oil prices and three big hurricanes all caused flight cancellations this fall. But cancellations were already becoming a bigger issue for travelers because the European Union began forcing airlines to compensate customers in cash for cancellations earlier this year.

In the U.S., airline schedules are not guaranteed, and carriers pledge to put you on the next available flight, if there's an open seat. It's up to the discretion of the airline whether to book you on another carrier. And if you get stranded overnight, airlines generally promise meals and hotel accommodations only if the problem was within the airline's control -- weather cancellations aren't covered. As always with airlines, elite frequent fliers, high-dollar ticket holders and persistent customers can get better service.

Flight cancellations are rare -- major airlines generally operate 98 percent or more of their schedule each month. But the FlightStats data show that certain flights turn up over and over again as canceled.

Delta's Flight 778 is supposed to leave Atlanta at 12:42 p.m. for a one-hour hop to Huntsville, then turns around and comes back to Atlanta. In October, it was canceled 12 times. Seven of those were because of mechanical problems, according to FlightStats. Three times Delta canceled because of weather issues, and twice, Delta listed the reason for cancellation in a computer reservation system as "fuel conservation measures."

A Delta spokesman said the high number of cancellations was because of "various operational reasons." "Nothing was wrong with the trip. This was an anomaly within the month," spokesman Anthony Black said. Indeed, the Delta flight had a better record in August and September. It was canceled a total of five times in those months.

Alaska Airlines Flight 228 leaves Seattle at 6 a.m. -- or at least it's supposed to. It was canceled more than 35 percent of the time in October. Alaska says the flight fell victim to Hurricane Wilma damage in Cancun, Mexico. Flight 228 continues on from Los Angeles to Cancun, and since most of its customers are headed all the way to Cancun, the Los Angeles leg was canceled when the Cancun portion was suspended because of hurricane damage in Mexico.

Passengers were moved to later morning flights to Los Angeles, a spokeswoman says.

US Airways Flight 436 from Philadelphia to Boston was canceled 28.6 percent of the time in October, and United Airlines Flight 650 from Chicago-O'Hare to New York-La Guardia canceled one-quarter of its flights. Both flights ran into weather problems on the East Coast in October. But they were both more likely than others to be canceled because they are smaller planes -- both fly with Boeing 737s -- at midday times. Smaller planes are more vulnerable to being canceled because that means airlines have fewer passengers to rebook.

"We look at what's going to be the least amount of impact on our customers and our operation," said United spokesman Jeff Green.

Sometimes the data can be misleading. According to FlightStat data, Mesaba Airways Flight 2755, operated on behalf of Northwest Airlines, was canceled 80 percent of the time last month, as was its return to Minneapolis as Flight 3035.

But Mesaba says the dismal record is actually the result of a quirk of airline systems. The carrier, a unit of Mair Holdings Inc., frequently substituted a Canadair regional jet on that trip in order to give crews training with new airplanes. To make the switch, Mesaba canceled the flights that were to be flown with turboprop Saab 340 turboprops, and created new flight numbers for the jets. While it looked like the flight was canceled, a plane actually flew at the scheduled times 30 of 31 days last month on that route, a Mesaba spokeswoman said.