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After troubles with its 32-year-old crew-scheduling software spawned huge holiday flight delays, Delta Airlines’ subsidiary Comair resumed its usual flight schedule in the middle of this week. But Comair is now under the glare of an investigation by the U.S. Department of Transportation (DOT).

Comair’s problems began on Christmas Day, when the failure of a legacy software program from SBS International—now a subsidiary of Boeing—was apparently to blame for bringing a Comair computer server to a total halt.

Comair returned to 75 percent of its normal flight schedule on Tuesday of this week, and to 100 percent on Wednesday and Thursday.

But U.S. Transportation Secretary Norman Y. Mineta on Monday asked the DOT’s inspector general to join an investigation into holiday flight delays impacting customers of both Comair and U.S. Airways over the holidays.

“As you know, problems experienced this past holiday weekend by two carriers—U.S. Airways and Comair—severely disrupted travel for tens of thousands of passengers,” Mineta wrote in a memo.

“U.S. Airways has attributed its troubles to a combination of weather and an unusually high number of employees calling in sick,” according to Mineta.

“Comair’s problems reportedly stem from a major computer malfunction that prevented the carrier from scheduling sufficient flight crews. The airline canceled its flights for several days, affecting passengers in more than a hundred cities.

While the weather was also cited as a factor by Comair, no other major carrier reported weather-related difficulties over the weekend.”

A few days before Christmas, Comair’s Cincinnati hub got socked by an early Midwestern blizzard.

In early accounts in other publications, and in Internet newsgroups, speculation ran rampant that Comair—now on the verge of upgrading to a newer SBS software program called Maestro—found that its current software was unable to handle the unusual number of crew scheduling changes spurred by the storm.

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Comair’s 32-year-old crew scheduling software—which is no longer on the market—was variously referred to in other media reports as “Track” and “Crew Check.” Boeing acquired SBS in 2001.

In newsgroups, users discussed a Cincinnati Post story that pinned the blame on an overflowed 16-bit counter. They also debated whether Comair should foot the bill for the hotel rooms required by stranded travelers.

But by Thursday, officials from the Department of Transportation, Comair and the SBS were mainly mum about both the travel delays and the DOT investigation.

David Barnes, a spokesperson for the DOT’s inspector general, was willing to confirm only that the investigation started this Monday and that it is ongoing.

SBS referred questions to Mike Pound, a spokesperson for Jeppesen, another Boeing subsidiary. Earlier this year, the two Boeing subsidiaries integrated their software suites.

Pound told that SBS’ legacy software program was actually called “Track.” Beyond that, he referred all questions to Comair.

“We’re not saying anything else to anybody. We’ve already said enough,” he said.

A Comair spokesperson failed to return phone calls in time to meet’s deadline..

Boeing’s Web site refers to several other airlines that have previously moved to Maestro, including DHL Airways (now Astar Air Cargo) and Alaska Airlines.

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