This article originally appeared on Motherboard.
I was flying in the United States, from Nashville to Charlotte, a few days ago when an announcement bled through my headphones. I ignored it, assuming it was the normal we're-making-a-descent speech, and went back to my episode of the Longform podcast.
Then, a couple of minutes later, another announcement, and this time, with a tap on the arm from my neighbor. "You have to turn off your phone," she said. Apparently we were making a "special approach" into Charlotte that required us to power down completely, no airplane mode allowed. A few minutes later, an air hostess came by to check that we all complied.
Cellular signals can mess up the flight's navigation system since the device is communicating with several towers at once. But I wondered why airplane mode—which is designed to isolate your phone from these signals, as well as GPS or Bluetooth activity—wasn't enough. What exactly was this "special approach" and why was it happening on a regular day?
First, I reached out to to the US Federal Aviation Agency (FAA) to find out what could be going on. They said there are circumstances, usually during reduced visibility, where mobile signals can interfere with the aircraft's autopilot function, which is linked to the landing systems at the airport.
"In such cases, it's important to make absolutely sure there is no possibility of interference with the airplane's navigation systems by telling passengers to turn off their electronic devices," the spokesperson told me.
She also pointed me to a 2013 statement that expanded the ability of US air passengers to use devices while in flight. Part of the statement recognizes that "most commercial airplanes can tolerate radio interference signals" as well, so airplane mode means extra precaution.
Unsatisfied with this response, I turned to an expert, Captain Ross Aimer, a former US air force and commercial pilot who is now an aviation consultant with Aero Consultants. His first guess what that there was bad weather, or low visibility, which would potentially force a pilot to make a Category III (CAT III) landing.
A CAT III landing means the pilot is no longer relying on autopilot, and has to use precision instruments to land without the usual visibility of the runway. The instrument landing system lets pilots uses radio signals to help the pilot determine a reference point for the landing if they can't see the runway.
In this case, even the slightest cell phone signal could interfere with the little information pilots have to work with, which can happen pretty easily.
"It actually happened to me one time I was flying the aircraft, it was a private jet. I had four passengers, all ladies, and the wifi was on. Maybe they were on Facebook," Aimer said. "And it did affect our navigation system."
When I told Aimer that the weather was actually perfect—we saw a great sunset settling along clear blue skies as we coasted in—he had two other theories. Maybe, he said, one of the flight instruments was acting up with interference, and the pilot didn't want to take chances. There's also the possibility that the pilot was doing a practice CAT III landing.
"We do that often when the weather is good," the captain said.
So the good news is, there's a reason we have airplane mode, and it allows us to use our devices most of the time. The bad news is I had to wait 30 minutes to finish my riveting podcast episode so that a pilot could make sure he knew how to save lives in the future.
Meanwhile, my friend revealed to me later that he left his phone on the whole time.