Southwest Airlines, BWI Marshall's largest carrier, saw a larger-than-usual percentage of flights delayed in recent months — which it blamed on weather conditions along the East Coast.
Those federal aviation statistics also showed Southwest had some of the worst on-time rates in America, and BWI ranked below many other major airports for on-time performance.
But the data may not be telling the whole story.
In a new report, the office of the inspector general at the Department of Transportation found that the agency's flight data "present the public with an incomplete picture of the number of delays that actually occur."
Under current rules, for instance, airlines such as United Airlines and American Airlines that contract regional flights out to smaller carriers don't have to report on those contracted flights. Airlines such as Southwest, which doesn't contract with smaller carriers, have to report all flights.
If those rules and others change, as the inspector general report recommends, the performance of airports like Baltimore-Washington International Thurgood Marshall Airport could rise in national rankings. Data showing improvements in national airline performance also could be revisited as the Department of Transportation pushes new ways of calculating airline delays.
Airline delays fell by 33 percent from 2000 to 2012, while flight cancellations dropped by 56 percent at the nation's largest airports, according to the inspector general's report. However, that only looks at flight data from the 16 largest airlines, which account for just 76 percent of domestic flights.
The Department of Transportation fails to collect data from "code sharing partners" of major airlines, or the smaller carriers that operate regional flights under contract, said Michael Boyd, president of the aviation consulting firm Boyd Group International.
Comparing all airlines as equal in a system that doesn't treat them equally isn't fair, Boyd said.
"The numbers are bogus," he said. "We don't know whether Southwest has better or worse on-time performance. All we know is that they report all their flights. …There are whole chunks of what's flown by other airlines that we don't know about."
The inspector general also found that the airlines padded flight schedules with extra time between departure and destination on almost all flights.
In 2000, the time that airlines scheduled for a flight exceeded the actual flight time on 73 percent of routes analyzed by the inspector general. By 2012, this rate had grown to 98 percent of all routes. For example, the scheduled flight time from LaGuardia to Indianapolis increased by 21 minutes from 2000 to 2012, the report found.
Airlines say that they don't inflate the scheduled flight time to avoid delays but try to be realistic about the time each flight needs.
"Airline scheduling is based on the realities of the air travel system, taking into account conditions such as airspace and ground congestion or weather that can impact gate-to-gate time," said Vaughn Jennings, a spokesman for Airlines for America, the trade group for the nation's airlines.
Southwest flights were on time about 78.5 percent of the time in October, according to the most recent Department of Transportation statistics. That compared to an 84 percent on-time rate for all airlines.
Southwest also was among the worst-performing carriers at BWI.
Brian Parrish, a Southwest spokesman, said in a statement that a "combination of unexpected summer weather and changes made to our schedule put us behind the pack in the recent on-time performance rankings."
He did not address the questions raised by the office of the inspector general. "We are aware of the pain points in our network, and we continue to work on schedule adjustments that will improve our future performance," Parrish said.
If all of the inspector general's recommendations — many of which the Department of Transportation concurred with — were implemented, it's unclear how the statistics would change.
"Before we jump and say Southwest is becoming a troubled operator or a troubled carrier because of that, we have to get better data," Boyd said. "It wasn't like Southwest had airplanes that were routinely three hours late."
Based on reported statistics, BWI ranked 16th out of 29 major airports for on-time arrival performance in October. For on-time departure performance, BWI fared worse, ranking 26th.
But BWI had fewer unreported delays than other airports, according to the inspector general's report.
BWI reported a total of 17,557 delays in 2012, while 1,839 delays — or about 9 percent — went unreported, the report found.
At Ronald Reagan Washington National Airport, 12,287 delays were reported and 9,600 delays — or 44 percent — went unreported. And at Washington Dulles International Airport, 14,117 delays were reported and 5,506 delays — or 28 percent — went unreported.
Tribune newspapers contributed to this article.