The Aerodata planning and balance system, which is used to evenly distribute weight on planes, was down, affecting Delta Air Lines, American Airlines, United Airlines, Southwest Airlines and more.

The Federal Aviation Administration says the issue has been resolved, but warned passengers to contact their carriers to assess delays in a tweet. Flight tracking service FlightAware said over 2,900 flights had already been delayed today as of 10:39am ET.

Southwest was the carrier with the most delays (18%), but that was likely due to the carrier’s continuing issues revolving around the grounding of the Boeing 737 Max jetliner.

AeroData is critical software for pilots. A report from Dell’s VMWare cloud computing platform put it succinctly in a consumer case study focusing on the company.

“AeroData’s flight deck client-server application is the last application used by pilots before the aircraft entry door is closed prior to takeoff,” it wrote. “As a result, just five minutes of system downtime can result in over 100 delayed flights and loss of revenue.”

The company has data centers in three physical locations in Arizona and Colorado, which makes today’s outage curious, especially as experts warn that hackers are showing increased interest in the airline industry.

Monday’s outage follows problems last week with Sabre, which many airlines use for ticketing and reservations. That outage led to massive lines and delays at airports across the U.S. last Tuesday.