On the morning of Jan. 26, as two Alaska Airlines flights from Seattle to Hawaii departing six minutes apart experienced a tail strike.

The pilots of each flight felt a slight bump and the flight attendants at the back of the cabin heard a scraping noise. As the noses of both Boeing 737s lifted skyward on takeoff, their tails had scraped the runway.

Both planes circled back immediately and landed again at Seattle-Tacoma International Airport. The incident grounded both flights and forced a temporary shutdown of Alaska’s flight activity nationwide.

Horrifyingly, investigators have now discovered that a software glitch was responsible for the incident. According to the Seattle Times, the tailstrikes occurred largely as the result of a bug in a program sold by a Swedish firm called DynamicSource.

The program is supposed to deliver “crucial weight and balance data” that pilots enter into their flight computers to help determine stuff like “how much thrust the engines will provide and at what speed the jet will be ready to lift off.”

The data [delivered] was on the order of 20,000 to 30,000 pounds light. With the total weight of those jets at 150,000 to 170,000 pounds, the error was enough to skew the engine thrust and speed settings.

Both planes headed down the runway with less power and at lower speed than they should have. And with the jets judged lighter than they actually were, the pilots rotated too early.

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