Turning your phone on “airplane mode” once your plane is preparing for takeoff is a standard part of any frequent flier’s onboard routine. In airplane mode, your phone is disconnected from cellular networks, but you can still use the device — and even connect it to in-flight Wi-Fi if it’s available.

But what happens if you accidentally leave your phone on?

Don’t worry — it’s not likely that your plane will fall out of the sky. In fact, a 2017 survey by Allianz Travel Insurance found that 17.2 percent of passengers never put their phones in airplane mode during flights. The problem is, we don’t really know what might happen because the interactions between cell phones and planes aren’t particularly well studied. Though there have been anecdotal reports of personal electronic devices (PEDs) potentially interfering with avionics, “it has proven almost impossible to duplicate these events,” according to Boeing.
But that doesn’t mean there is zero risk.

“If you do not turn off your cell phone, it has the potential to interfere with navigation instruments,” Dan Bubb, a professor at the University of Nevada, Las Vegas and a former airline pilot, tells Travel + Leisure. “Potential” is the operative word here — if there’s any potential for danger, whatever the cause, the Federal Aviation Administration (FAA) and the Federal Communications Commission (FCC) are going to avoid it.

That’s why the Code of Federal Regulations in the U.S. currently states the following: “Cellular telephones installed in or carried aboard airplanes, balloons, or any other type of aircraft must not be operated while such aircraft are airborne (not touching the ground). When any aircraft leaves the ground, all cellular telephones on board that aircraft must be turned off.” And if you don’t comply, you may be fined.

Another issue with in-flight cell phone use is the possibility of overloading the cell towers on the ground. When you’re flying at low altitudes — that is, during takeoff and landing, which are considered crucial phases of flight — your phone is connecting to multiple towers at a time as you move. Given all the passengers in the sky at any moment, that could certainly be a burden on the network near particularly busy airports. (Once you’re at cruising altitude, however, you’re probably going to be out of range of the towers, so your phone’s battery will drain faster as it attempts to connect to nothing.)

As technology changes, new threats are introduced in the phone-plane relationship. And right now, the biggest threat is 5G networks.

“With many phone companies now offering 5G bandwidth, that has the potential to interfere with the radio altimeter, which is the instrument upon which pilots rely to indicate when they need to flare, or lift the airplane’s nose wheel, to land the plane,” says Bubb. The specific bandwidth that U.S. cell carriers use for 5G services is very close to the same bandwidth used by the radio altimeter, which increases the potential for interference.

And the radio altimeter isn’t something to mess with. “Because pilots sit so high in the cockpit, it is difficult for them to see the runway when they land, which is why they rely upon the radio altimeter for guidance,” says Bubb. (Generally speaking, radio altimeters are used during low-visibility landings.) For what it’s worth, nearly 60 percent of all aviation accidents in the last 20 years occurred during the landing phase of flight, per a report by Airbus. “So, when flight attendants ask passengers to put their phones on ‘airplane mode,’ or turn them off, there is a good reason why passengers should comply with that request,” says Bubb.

Interestingly, the European Union is requiring the implementation of 5G networks on aircraft, allowing passengers to communicate freely with their mobile devices, including making calls. But there’s a key difference between European 5G networks and American ones: the European networks operate at a different frequency, which adds more separation between the cell usage and the radio altimeter. Therefore, there’s less chance of interference. For that reason, you shouldn’t expect the U.S. to follow suit any time soon.

And even if we did live in a world where phone calls are possible on flights, remember that planes are shared — and typically very cramped — spaces. It would be far from polite to be chatting on the phone loudly on a plane. At the very least, let’s just stick to texts.