So here's a story that makes you think about how much empty space there is in the sky, and how we still manage to almost fill it up sometimes. The Federal Aviation Administration said Thursday it's investigating what it calls a "close call" between an Army Black Hawk helicopter and a Boeing Co. (BA) 737-800 operated by United Airlines Holdings Inc. (UAL) in California.

Think about it: Tuesday night, 8:40 PM, a United jet is coming in to land at John Wayne Airport in Santa Ana. And a Sikorsky Black Hawk—you know, the kind the Army flies—passes in front of it. Flight tracking data from Flightradar24 suggests they were separated by more than 500 feet vertically, which in aviation terms is... not nothing, but also not exactly a country mile when you're talking about large metal objects moving at speed.

Now, the FAA isn't just looking at what happened. It's also checking whether everyone was following the new rules. See, after some previous incidents, the agency told controllers and pilots to basically stop using "visual separation" between airplanes and helicopters during flights. Visual separation is exactly what it sounds like—relying on people looking out the window to keep things apart, rather than just radar and instruments. The FAA wants to know if that procedure was being used Tuesday night.

MarketDash reached out to United and the FAA for comment, but didn't immediately hear back.

This isn't the first time a Black Hawk and a commercial jet have had a bad day. You might remember last January, when an American Airlines Group Inc. (AAL) regional jet collided with a Black Hawk helicopter near Ronald Reagan Washington National Airport. That one didn't end with just an investigation—67 people died. Since then, the FAA has been tightening the screws, issuing restrictions for helicopter traffic around Reagan Airport and other places.

It's a reminder that the sky, for all its vastness, is a carefully managed workspace. And when the rules of that workspace get tweaked—like telling people not to rely on their eyes as much—it takes time for everyone to adjust.

Meanwhile, in a separate but not entirely unrelated corner of the aviation world, United CEO Scott Kirby has been sounding the alarm about costs. He says airfares could be heading up, thanks to the partial government shutdown and surging jet fuel prices linked to the situation in the Middle East. It's one of those moments where the business of flying people around intersects with the mechanics of keeping them safe in the air—both requiring constant attention and, apparently, more money.