LONDON (AP) — A fire that closed London's Heathrow Airport has sparked one of the most serious disruptions to air travel in years.

More than 1,300 flights were cancelled and hundreds of thousands of journeys disrupted following the fire at an electrical substation, whose cause is under investigation.

Here is a look at some past incidents:

A glitch at Britain’s National Air Traffic Services in August 2023 meant flight plans had to be processed manually, rather than automatically. Hundreds of flights were delayed or canceled at the height of the summer holidays. The NATS system had already suffered several software-related failures in the years after it opened in 2002.

As a new coronavirus spread around the globe in early 2020, the world’s airports shut down, as many governments closed national borders and imposed travel restrictions. By April, the number of flights around the world had fallen by 80%. When air travel resumed, it was with masks, mandatory coronavirus tests and other measures that made flying more onerous and expensive. It wasn’t until 2024 that global passenger numbers reached 2019 levels again.

More than 140,000 travelers were stranded or delayed after dozens of drone sightings shut down Gatwick south of London, Britain’s second-busiest airport, for parts of three consecutive days before Christmas. A months-long police investigation failed to identify the culprits or determine how many of the sightings were real.

A computer failure at a British Airways data center forced the airline to cancel all flights from Heathrow and Gatwick on a holiday weekend. The airline blamed a power-supply issue for the incident which affected some 75,000 travelers.

Delta planes around the world were grounded when an electrical component failed and led to a shutdown of the transformer that provides power to the airline’s data center. Delta said it canceled more than 2,000 flights and lost $100 million in revenue as a result of the outage.

People around the world learned how to pronounce the name of Iceland’s tongue-twisting Eyjafjallajökull volcano (ay-yah-FYAH-lah-yer-kuhl) after it roared to life, sending plumes of ash and dust into the atmosphere. Airspace over northern Europe was shut for several days and airlines canceled flights between Europe and North America because of concerns the ash could damage jet engines. More than 100,000 flights were canceled, stranding millions of passengers, at an estimated cost of $3 billion.

U.S. airspace was closed to commercial flights on Sept. 11, 2011 after hijackers crashed planes into the World Trade Center towers, the Pentagon and a field in Pennsylvania. Thousands of planes were grounded and flights in the air heading for the U.S. were diverted to Canada and Mexico. Flights began to resume two days later, but air travel was forever altered, with passengers facing more rigorous security, more intrusive scrutiny and longer lines.