Until Wednesday, few travelers had ever heard of a Notice to Air Missions, or NOTAM, nor did they know that the system used to generate those notices could cause widespread travel misery.

As they arrived at airports in the morning, they quickly found out.

The Federal Aviation Administration computer system that compiles and distributes essential safety information for pilots went kaput. That temporarily grounded all flights nationwide and touched off a cascading air traffic jam that will take at least a day to unclog. More than 1,300 flights were canceled and 9,000 delayed by early evening on the East Coast because of the outage, according to flight-tracking website FlightAware.

The system has been around for more than a half century and it has evolved from paper to computers. It's in the process of being updated.

WHAT ARE NOTAMs?

They are compilations of essential preflight information for pilots, airline dispatchers and others that include details about things such as potential bad weather on the route, runway and taxiway changes at airports and closed airspace that must be avoided. The notices began in 1947 and were modeled after a system used to warn ship captains of hazards on the seas.

A pilot can't legally take off until he or she has reviewed the information. John Hansman, an aeronautics professor at MIT, said most airlines subscribe to services that gather NOTAM information from the FAA and package it for each flight. Airline dispatch centers relay it to pilots. In this case, the services couldn't get the information because the FAA system malfunctioned, he said.

WHAT WENT WRONG WITH THE SYSTEM?

The FAA said preliminary indications “traced the outage to a damaged database file.” The agency said it would take steps to avoid another similar disruption.

The system stopped working at 8:28 p.m. Eastern time on Tuesday, but because there weren't many departures at that hour, pilots were able to get the information verbally. At daybreak in the East, the system was still out, and there were too many flights leaving to brief pilots individually.

It's likely that the main system had a problem, and the backup didn't work correctly. The FAA rebooted the main system around 5 a.m., but it took a while to verify that all the information was validated and available, Transportation Secretary Pete Buttigieg said. So the FAA ordered all flights grounded Wednesday morning and planes were stuck for hours "to make absolutely sure the messages were moving correctly and the information for safety purposes is working the way it should,” Buttigieg said.

Longtime aviation insiders could not recall a systemwide FAA outage of this magnitude caused by a technology breakdown.

HOW IS THE GOVERNMENT RESPONDING?

Buttigieg said that the NOTAM system is constantly being updated, and a key question is whether it’s outdated.

“We will not allow anything to take place that is not safe,” Buttigieg told reporters. “This is precisely why our focus right now is on understanding, identifying and correcting anything related to the root cause of how this happened in the first place.”

Rep. Sam Graves, R-Mo., said the NOTAM malfunction is inexcusable, and a result of a Transportation Department and FAA “failure to properly maintain and operate the air traffic control system.”

Graves, chairman of the House Transportation Committee, said the FAA has been without a permanent leader for nearly a year. He said he expects a full briefing on the outage.

HAS THERE BEEN A PROBLEM WITH THE SYSTEM BEFORE THIS?

Not of this magnitude. "Periodically there have been local issues here or there, but this is pretty significant historically,” said Tim Campbell, a former senior vice president of air operations at American Airlines and now a consultant in Minneapolis. While the cause of Tuesday’s breakdown was not immediately clear, Campbell said there was concern about FAA’s technology, and not just the NOTAM system.

Other FAA technology also is aging, Campbell said. “So much of their systems are old mainframe systems that are generally reliable but they are out of date,” he said.

The bipartisan infrastructure law passed last year allocated $25 billion to airports, with roughly $5 billion for air traffic control facilities. Congressional staffers said some of that money could be spent on equipment. But Buttigieg said any NOTAM upgrade may have to wait for a new FAA funding bill.

“I think this gives us a really important data point and a really important moment to understand what we're going to need moving forward,” he said.

The National Transportation Safety Board has criticized the way NOTAMs are presented after an Air Canada jet nearly landed on four other planes waiting to take off from San Francisco International Airport in 2017. Pilots had missed information about a closed runway that was buried among numerous notices.

WILL THE GOVERNMENT HAVE TO COMPENSATE PASSENGERS?

Airlines will be on the hook for refunds and other compensation, even though the FAA was at fault for the outage. Kurt Ebenhoch, a consumer travel advocate and former airline executive, said passengers are entitled to a full refund if an airline cancels a flight for any reason.

Major airlines including Delta, American, Southwest and United were waiving change fees for Wednesday flights — and, in some cases, Thursday flights — to make it easier for passengers to change their travel plans.

The government has no legal obligation to reimburse travelers, which is maddening, said Brett Snyder, a travel agent and author of the “Cranky Flier” travel blog.

“Secretary Buttigieg should set the right example here and reimburse people directly from the government coffers,” he said.

COULD THIS HAPPEN AGAIN?

Not clear. Congress already was going to look into aviation technology after Southwest Airlines' crew-scheduling system went haywire during the holidays, leading to nearly 17,000 canceled flights in the last 10 days of December. Now the system that sends NOTAMs, and backup systems, will be part of the inquiry.

“We will be looking into what caused this outage and how redundancy plays a role in preventing future outages,” Sen. Maria Cantwell, chair of the Senate Transportation Committee, said in a statement. "The public needs a resilient air transportation system.”