If only it were an April Fools’ joke: On Monday, hundreds of flights were delayed after a brief software glitch affected U.S. airlines Delta, Southwest, United, American, Alaska, and JetBlue. The issue was due to a third-party, non-FAA system called AeroData, which some airlines use to determine the plane’s weight and balance data, which is necessary for takeoff.

“Much like any software that any company would use, you would hope that your provider has developed adequate backup processes so that if the primary system fails, a backup system kicks in almost instantaneously,” says Henry Harteveldt, travel industry analyst and the president of Atmosphere Research Group. “Clearly, that didn’t happen.”

After implementing an internal ground stop for 40 minutes on Monday morning, Southwest said it was anticipating “scattered flight delays” and that customers should check with the airline for the latest updates. Delta, meanwhile, cited a “brief third-party technology issue” that affected some Delta Connection flights and said it was working to “resolve some resulting delays.” United, which had “about 150” delays, advised travelers check the website for the most up-to-date information. American, Alaska, and JetBlue have all said they are working with customers.

If “technology problems cause headaches for fliers” sounds familiar, it is: Last week, travelers on JetBlue, American, and Alaska flights were stuck in long lines after Sabre, an airline reservations company, went down for 30 minutes.

Despite the length of this outage (relatively short) and the number of delays (relatively small), the AeroData outage has brought to light some of the unknowns of air travel, including a pilot’s pre-launch plan and keeping track of where—and how much—luggage is placed. (Entering incorrect baggage weights, as seen in the case of Southwest, is dangerous.) It’s also shown, once again, just how dependent airlines are these entrusted systems to do some of their most important work.

So where does AeroData fit into takeoff—and what power does the FAA have? Harteveldt says in this case, it helps to think of the FAA as a traffic cop. “They don’t directly employ AeroData,” he says. “AeroData works directly with the airlines. But the airlines have to file their flight plans with the FAA, and weight and balance data is a component of that flight plan.” In other words: If you’re an airline that uses AeroData to crunch your weight and balance information and that system is suddenly no longer available to you, you won’t get clearance to fly, because you can’t provide that required information. (This also explains why airlines that don’t use AeroData had no issues; other weight-and-balance technology systems include Evinta, LodeStar, and Jeppesen.)

AeroData, for its part, has no social media presence, a bare-bones website, and an address in Scottsdale, Arizona. (AeroData has not returned phone calls seeking comment.) Yet the available intel shows just how critical it is to flight operations: A 2017 consumer case study of the company says AeroData’s “flight deck client-server application is the last application used by pilots before the aircraft entry door is closed prior to takeoff,” presciently noting that “just five minutes of system downtime can result in over 100 delayed flights and loss of revenue.” More than 50 percent of all North American flights depend on AeroData, and the company says it’s “soon to be 85 percent.”

A plane’s correct weight and balance are essential to safe flight, and have been required for decades. “This is not new,” says Harteveldt. “One of the first things you learn when you’re starting to fly is the importance of weight and balance and how you calculate it on the little planes that you learn on as a student pilot. So it’s fundamental.”

Chalk it up to physics: Both weight and balance can dramatically affect takeoff speed, cruising speeds, and maneuverability while flying. Based on a plane’s design, the FAA assigns a “maximum allowable weight” for commercial aircraft. Still, a plane’s operational weight may be lower than its maximum, due to factors like high-density altitudes or shorter runways, both of which are considerations figuring into how much weight a plane can carry. (Think of it sort of like a much higher-stakes trip to the grocery store: Just because you can carry up to 100 pounds of fruit without hurting yourself, you may not do it every time, because the weather could be nasty or you might have to walk farther than normal.) There are also important pre-flight considerations: when every seat is occupied and the fuel tanks and baggage compartments are full, the airline is “grossly overloaded,” according to the FAA’s comprehensive “Weight Balance Handbook”. This means something has to give: either the pilot leaves baggage or passengers behind in order to reach the plane’s maximum flying range, or they’ll have to sacrifice range and find a way to take a shorter route. If this all sounds complicated, it is—which is why figuring out if a plane’s weight and balance are safe is largely left up to technology.

For now, the FAA has said it’s looking into the cause of the outage. But in situations like these, where flights are delayed because of technology glitches, do travelers have any recourse? The short answer: not really.

“A passenger on a U.S.-based airline has fewer rights than one flying on a European-based airline,” says air travel expert and airfarewatchdog.com founder George Hobica. “In the U.S., passengers only have the right to get a fare refund if a flight is canceled or severely delayed; to received compensation if bumped from a flight; and to be compensated for lost or delayed baggage. That’s about it.” As we’ve previously reported, passengers flying on a European-based airline have more protections; if the outage had affected one of these airlines, passengers would probably have a case, says Hobica.