The Federal Aviation Administration (FAA) has halted flights nationwide with a rare ground stop, citing "extraordinary circumstances."

Details remain thin at this point.

Reports of an unusual RF outage in a number of the most heavily used aviation corridors in the U.S. (the Northeast and Chicago among them) have triggered a series of targeted ground stops which have rapidly cascaded out well beyond the areas initially affected.

The FAA's own status page currently has ground stops in place at both O’Hare and Midway and attributes the problem to an “equipment outage”. They haven’t specified what equipment failed, why it failed in multiple locations at the same time, or for how long passengers should expect to remain at the gate.

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Radio comms are the lifeblood of ATC. If controllers can’t talk to pilots, aircraft don’t move. A failure at a major hub like O’Hare doesn’t just stay in Chicago for a day or two. It spreads across the country in hours to a point where you’ve got a complete gridlock in terms of delays and cancellations.

For perspective, the last time the FAA ordered a nationwide grounding of all departing aircraft was Sept. 11, 2001. There is an important distinction, however: today's order does not direct planes already in the air to land immediately, as the FAA did on 9/11.

There is currently no indication that the outage is related to a terrorist threat or other sinister activity.

We are monitoring this story and will provide updates.