The latest major air travel meltdown has brought to light a system critical to the safety of flights and airports across the country.

Early Wednesday morning, a failure of the Notice to All Air Missions (NOTAM) system forced the Federal Aviation Administration to temporarily ground departing flights nationwide for the first time since the Sept. 11, 2001, terrorist attacks, a move described by the agency as an “abundance of caution.”

Although the grounding only lasted approximately 90 minutes, the impact on flights—particularly during the peak morning rush period—was immense. More than 1,300 flights were canceled that day and another 10,000 were delayed, according to flight tracking site FlightAware.

This latest in a series of air travel meltdowns since the start of the COVID-19 pandemic has left travelers asking questions about the cause of delays and cancellations—and about a new acronym in the headlines: NOTAM.

What is a NOTAM?

According to the FAA, a Notice to All Air Missions (formerly Notice to All Airmen), or NOTAM, is a notice issued for any irregularity within the National Airspace System. It contains information essential to personnel concerned with flight operations but not known far enough in advance to be publicized by other means.

In essence, a NOTAM alerts flight crews of any potential hazards and conditions that could impact flights, much like a navigation app alerts you to a lane closure on a highway. Prior to every departure, flight crews are required to check notices, which are updated in real time with safety information regarding current conditions for airport and flight operations.

For example, Daytona Beach International Airport (DAB) currently has a NOTAM issued for increased bird activity near the airfield in effect from 9:34 p.m. on Dec. 31, 2022, to 4 a.m. on Feb. 2, 2023. It looks like this:

!DAB 12/074 DAB AD AP BIRD ACT INCREASED SEAGULLS, EGRETS AND VULTURES 2212312134-2302010400

At Phoenix-Sky Harbor International Airport, this NOTAM warns of a closure for Runway 7L/25R in effect between 6:59 a.m.-3 p.m. on Jan. 14:

!PHX 01/057 PHX RWY 07L/25R CLSD 2301140659-2301141500

And at Tampa International Airport, this NOTAM advises of a taxiway restriction for aircraft with a wingspan greater than 171 feet in effect from December 2022 to June 2023:

!TPA 12/198 TPA TWY J BTN TWY E AND TXL K CLSD TO ACFT WINGSPAN MORE THAN 171FT 2212221017-2306302359

At airports, it is common to have multiple NOTAMs issued throughout a day. Boston Logan International Airport, for example, had a total of 37 NOTAMs issued by the FAA on Jan. 13. That same day, Dallas-Fort Worth International Airport had 99 listed.

To view a list of active NOTAMs for any U.S. airport or airfield, a search can be done through the FAA’s website.

All eyes on IT infrastructure

Following a preliminary investigation, the FAA announced that the outage was caused by a damaged database file affecting the primary and backup systems of the NOTAM network. The agency and most airlines said flight operations were back to normal by Thursday morning.

Both the FAA and Department of Transportation stated there was no evidence of a cyberattack.

Transportation Secretary Pete Buttigieg announced the DOT launched an “after-action process” into the outage’s cause and directed the FAA to identify steps to prevent such an occurrence from happening again in the future.

“These kinds of disruptions should not happen,” Buttigieg told CNN. “My primary interest now that we’ve gotten through the immediate disruptions of the morning is understanding exactly how this was possible and exactly what steps are needed to make sure it doesn’t happen again.”

The FAA’s NOTAM outage comes weeks after Southwest Airlines canceled nearly 15,000 flights over the holiday period after weather and staffing shortages overwhelmed some of the airline’s computer systems that are decades old.

These recent events have placed information technology infrastructure under a microscope in the airline industry as leaders and experts have discussed a need to upgrade aging systems vulnerable to widespread failures.

Government officials and industry experts have called on carriers to invest in modernizing equipment. Airline leaders, meanwhile, have requested Congress to increase funding to allow the FAA access to more resources, such as equipment and staffing.

“This [outage] is not the FAA’s fault,” Delta Air Lines CEO Ed Bastian told CNBC. “I lay this on the fact that we’re not giving them the resources, funding, staffing, tools, technology that they need to modernize.”