Risk of not turning Airplane Mode on, on a mobile device while in the air?

There are two significant areas – not what you think.

Interference with cell-phone towers

Cell phone technology puts a staggering number of radio transceivers sharing surprisingly little bandwidth. How does this work? By reducing transmit power, and having more and smaller cell towers. That’s a big part of 3G/4G/5G. In the old days, an AMPS cell phone might have had 20 miles of signal radius. Now if your 4G phone is 1 mile from a tower, it down-tunes to 1 mile of radius. Your phone speaks softly, because it doesn’t need to shout.

That way someone else 2 miles away can use that same bandwidth, which they couldn’t under the older systems. However, if you’re in a bad reception zone, your phone may punch it up high power with 10 miles of throw. That means no one else in a 10 mile radius can use that frequency: your phone is “stepping on their signal”.

So there you are, at 7 miles of altitude. Your signal has to punch through seats, carpets, several layers of aluminum, a bunch of mixed cargo, and the fact that the cell antennas are aimed at the ground not the sky (so attenuated)… then make it 7-10 miles (albeit line-of-sight) to a cell tower. So your phone will punch it up to high power: it will be shouting.

That means you’re reaching several towers at once, in nearly equal strength since it’s line-of-sight and mostly “up”, and sweeping maybe a 7 mile radius on the ground with that loud signal, so about 150 square miles. That frequency is unusable in that entire area, because you are using it, and stepping on any other signal that tries to be transmitted.

The tower system is designed to “hand-off” your cell phone signal to the next tower, as you travel… by land … at sane speeds such as 70 MPH. On the ground, it figures out which tower to hand you off to based on your directionality and which other towers you get good signal from. But in the air, it’s very unclear: you have good signals to towers all over the place – should your next tower be east, north or west? Further, this handoff is occurring much more often than the system is designed for, because of your 500 mph speed.

So a cell phone in the sky confounds and overloads the cellular networks.

Lithium battery fires

The closest real life gets to a horror movie is fire on an airplane. It’s nightmare fuel for any pilot. There have been several incidents with massive loss of life on the runway, after a successful landing — take Saudia 163, the plane screeches to a halt, firemen line up to help people down the slides, and the door … never … opens. There have been more incidents where crews detected smoke, but dawdled or favored a convenient airport, and that delay allowed fire to destroy the flight controls. UPS 6 (which could’ve made it to an Iranian island), Swissair 111, Air Canada 797, etc.

There is an aviation safety reporting system for crews to report genuine safety issues. It’s not anonymous but reports are protected by law. NASA, who runs the system, has done aggregate analysis of risks relating to cell phones. From their report, the #1 risk is battery fire.

In fact, that’s why there’s a watt-hour limit on the size of accessory battery boosters you can bring onto an airplane.

They do have containment methods for burning consumer devices, but that is not a cure-all: Fire is a wildcard. You never know for sure what fire is going to do, when combined with all the random things consumers bring onto airplanes, and that consumers do. Imagine if a customer tries to self-manage the burning cell phone by throwing it into a galley trash can. Fire + panic = anything can happen.

Now, this seems hypocritical, since lots of consumer devices contain lithium batteries. But cell phones are the most prevalent, and the most likely to be operating unusually (cranking up the transmitter to max, as discussed).

And keep in mind, aviation has a long track record of underestimating the threat of fire. So over-reaction it may be, but the alternative has not worked out well for aviation.

I’ve answered similarly on another question about on taxiing and landing.

From the physical, every single transmitting device on a plane has to be certified to legally fly on the plane in transmitting mode. Most devices ‘probably’ won’t interfere, but even adding a wifi router onboard takes hours of testing, compliance, and certification (source: I was on the Qantas wifi programme team). Since they can’t possibly certify every single variant of mobile phone, it’s easier to just say ‘turn it to airplane mode’. There’s a theoretical possibility the signals could interfere, but it’s unlikely – but thus the testing required. We’ve seen interference in our testing before.

What could have actually happened? Physically, not much, probably. From a legal point, If you forget, or don’t bother (and our scanners could see this happening on almost every flight), it almost certainly won’t make a difference, but you are technically transmitting onboard an aircraft with a non-certified/compliant device, which is illegal in many countries.

You’re also meant to legally always follow instructions of the flight staff. They might seem like just friendly hosties, but there are serious consequences available in most countries for disobeying instructions of the flight crew.

Short answer: If you landed safely, don’t worry about it. Try to double check next time though.

Would this cause any real risk/worry or even bring the plane down?

If it was this easy to bring a plane down, we would be in deep trouble. There’s plenty of things that generate more interference than a cellphone. Heck, there’s a RADAR right on the plane’s nose.

Note that the rule disallowing cellphone usage on a flight is usually not issued by the aviation regulation body but by the radio frequency regulation body. E.g. in the US, it is the FCC, not the FAA, that forbids cellphone use.

The reason is that a cellphone that is this high in the air but still “relatively” close to the ground has a large number of cell towers in line-of-sight. However, there is a speed limit built into various cellphone technologies (e.g. GSM only works for relative speeds less than 250kph between tower and handset, otherwise the packets are “stretched” or “squished” too much to fit into their designated time slots). Also, while you might have line-of-sight to many towers, you will only have enough power to reach a few of them.

Your phone will still try, though, and thus block frequencies and time slots for many more other users than what would happen if you were on the ground and trying to connect to only one or two towers.

This is also why using Bluetooth is allowed, for example, and why using your WiFi is allowed but only when the airplane’s WiFi is active: Bluetooth has much lower power (it is designed to work over only a couple of meters), and both Bluetooth and WiFi devices have power regulation that will lower the transmit power to only the minimum necessary to achieve a stable connection. So, if you connect to the plane’s WiFi or your Bluetooth headset, the power will be set such that your transmission never leaves the airplane.

The same applies to airplanes that offer in-flight cellphone service: the way it works is that the plane has a cellphone base station on board, and since that station will always have a better signal than any station on the ground, your phone will never try to connect to them and in fact never even increase its power to a point where it could connect.

The real answer to your question is thus:

What could have actually happened?

In many jurisdictions failure to comply with signs on the aircraft and instructions from the crew is illegal, so your travel partner broke the law. While the possibility is remote, your partner could be prosecuted.

There is probably little danger to the aircraft systems. However the cellular system capacity is based on re-using frequencies; cellular handsets communicate line-of-sight to nearby towers and then the frequency is used by other cell sites that are out of range. When you have a cellular phone operating in an aircraft that phone has line-of-sight to many more cells and occupies bandwidth on them, thus reducing the overall system call capacity.

This isn’t “dangerous” per se, but it is a bit rude.

While there’s little risk to the plane equipment, which needs to be able to handle interference, you’re risking draining your battery. Out of airplane mode, the phone will keep trying to connect to cell towers, but most likely without success. This will drain your battery. Finding yourself with a dead cellphone after landing sounds highly undesirable.

This applies to other situations where your phone is unable to connect – in underground places (like clubs/bars) or when hiking. It’s best to turn on airplane mode if you don’t need it.

My phone has personally connected to a mobile network of a country I was flying over at least on one occasion, as I have received roaming welcome text messages upon landing to my destination welcoming me to the country I never actually landed to!

I was flying from Greece to the UK and was welcomed to Germany, Luxembourg and Belgium. And I’ve never visited any of those countries at that time.

Now this was in Europe where roaming is free for me but if it wasn’t, I’m not so sure if I wouldn’t have incurred any charges. So even if you’re not turning airplane mode on, I guess it might be safe to turn the data off / roaming data off.

Also, this newspaper article will let you to believe that your phone might be able to connect to a satellite network (which comes with a significant price tag). I am not sure that’s what happened to me, but thought would mention it too.

Tests have shown that the average commercial flight has around 40 cell phones still active. Obviously, there is no impact (aside from your battery being badly drained).

Most experts agree that there is probably zero impact, but nobody wants to take the risk to officially claim so (and be held responsible), and nobody wants to pay for the huge effort of verifying it formally, so the rule will stay.

Most if not all commercial airplanes have extremely high quality parts that are very well isolated against interference – nothing could’ve happened.

Say the pilot accidentially disconnects the headset while receiving instructions then the pilot would ask to have them repeated and wouldn’t act on a guess.

From my experience, you shouldn’t worry too much and most likely your partner was not the only one on board who didn’t put their phone into Airplane Mode.

The question “what could have happened?” Not much either. Electronics you are allowed to bring on board don’t cause direct danger to the systems in an aircraft. If phones would put aircrafts in an unsafe situation, then safety regulations would most likely ban passengers from bringing their phones onboard.

The pilot or co-pilot however could have potentially heard an unpleasant noise through his headphones as phones are trying to find network connectivity. It might interfere, not block radio contact. They won’t take any (legal) action against you or your partner but be aware that pilots should be focused at all times, but especially during take off and landing.

Also note that some aircrafts do allow you to use network connectivity (Bluetooth, LTE or WiFi above the height of 10,000ft).