Can I stop a fellow passenger from using their cell phone during take off?

You don't have to be too worried about it, since the aircraft is able to cope with this. People are instructed to turn them off in order to avoid some disturbances and parasite noise in the communication between the pilot and the airport. As electronic devices using radio frequencies, they could also in theory cause some troubles to some aircraft equipment's but this is pure theory as all equipments are properly protected now. So even if the phone is turned on, it won't make the aircraft go down.

Keep in mind that a lot of people forget to switch on the flight mode (above 30% from studies) and this never caused a plane incident...

Then, if you are still worried about it, just report it to the cabin crew. They are here to make this stop...

As a cabin crew member for long time, I can tell you that your responsibility ends by notifying a crew member, that's it. Let the crew members deal with it.

This is true for all other violations, unless it's a life threatening situation that cannot wait, for example fire! Grab the extinguisher and fight the fire. But that's a whole different issue.

Regarding cell phones and out of personal experience, many people do not switch them off. In almost all my flights shortly before landings I hear one or two phones ringing, they must have been left on during the whole flight and once they get a signal they start getting messages or calls. I myself forgot mine switched on a few times, and believe it or not, sometimes when I reach my destination I find the famous welcome SMS from one or two operators along the way, how did it get the signal that high I don't know. Bottom line, I never crashed.

A few years ago I read a study about cell phone usage on U.S. domestic flights. Using RF locating equipment, they determined that the average U.S. flight has 2-3 cellphones left transmitting during the flight. Unconfirmed pilot anecdotes notwithstanding, if there were any significant risk of planes malfunctioning from cellphones, they would be falling out of the sky on a regular basis. There is no scientific basis to think that the cellphone transmissions are a risk to aircraft. I wouldn't worry about it.

Your personal safety is not affected by your fellow passenger using a cellphone.

In the US, the FCC (not the FAA!) has limited use of cellphones on aircraft because of the potential effect on cell towers on the ground.

From an empirical standpoint, if there was a significant risk, it would have been observed many times by now as people frequently leave their phones on inadvertently. And it would have been exploited by more nefarious sorts.

From a social standpoint, it is often very disturbing to have to listen to someone else talking at length on a cellphone, it makes it hard to either relax or focus (apart from the often unwelcome insight into the talkers personality or foibles).

I didn't know until I started working in the field, but the most serious problem with leaving your cellphone on is the frequent failed attempts to connect to the many cell towers you are passing over at 500 mph. Not only does this tie up the towers' ability to connect with other people, it drains your handset battery faster.

My father's an airline pilot (United Airlines) and this is something that I've asked him about at length. The idea that any kind of signal from your cellphone can interfere with a plane's electronics is purely myth (Though one that the airlines are happy to allow to propagate). There're endless such signals in the air at all times anyways, and modern planes are designed to deal with them to the extent of being a non-issue.

The reason passengers are asked to turn them off around take-off time isn't related to the actual take-off so much as it is to the safety briefings that occur at that time. By removing the #1 distraction from the passengers, the flight crew hopes that more people will pay attention to the briefings, and be better prepared for an emergency. (Obviously enforcing total attention is impossible, but they will do whatever they can to help.)

If someone next to you is not paying attention because of their phone, then they're simply choosing not to pay attention to the safety briefing, but they're not immediately dangerous.

Conceivably, this could be a problem if the offender is sitting in an exit row, but in that case, the flight crew will have a very direct one-on-one conversation with him to ensure that he is capable of fulfilling the extra duties.

opinion: You could, if you wanted to be especially safe, suggest to the individual, during the briefing, that they pay attention, since "this is pretty important, y'know," however if they're a seasoned traveler, they're likely already quire familiar with it.

Note that while EMI concerns are practically bogus considering modern airplanes and mobile phones, you are still required to turn off all electronics during takeoff and landing, for your own safety. In case your airplane has to make an RTO maneuver or catches fire, you won't see it coming if you're concentrated on your phone or laptop. You may get hurt (especially if your laptop/phone is on the unfolded tray table) or lose precious time during evacuation.

So while you can't really force other people into following the rules to the letter, I advise you to listen to crew messages yourself and do what they say.

Your Safety is Compromised if You’re an Intervener

If you feel compelled to do something about someone 'breaking the rules' you may want to consider why it's so important for others to follow them.

Primarily, if you find yourself wanting to intervene and play Sherriff you may find yourself in a compromising position if the person you are trying to 'control' verbally doesn't take kindly to your efforts.

There are all manner of mental issues humans are dealing with as well as the very situation where altitude effects an individuals judgement. A good natured person could turn on you in a delusional state if are trying to get them to what YOU want them to do and take your non-confrontational efforts as very confrontational. Reality is a subjective business and we all perceive moments in time the way we as individuals perceive it.

If you would up in a confrontation with another passenger on an airline you could find yourself in jail even though you did nothing wrong because, again, your actions will be perceived from the outside looking in and it's subjective to the viewer.

Reality Check

Imagine a scenario where you are landing in Malaysia or Singapore and your punishment for 'being aggressive' and 'disrupting an International flight' has corporal punishment associated with it. Add to that the state of prison systems in some 2nd and 3rd world countries and the potentially horrible experience you could have because of misperception from others over a cel phone issue and you can play out a very dark storyline.

I only say this because people are incorrectly perceived all day, every day, around the world and another person's testimony to an officer can carry a lot of weight if they are an excellent communicator.

Getting involved in others actions that you see as 'wrong' is a dangerous business. I will say that from own opinion I find other attempts at getting someone to adhere to a ruleset whether it's their own or an entities as someone who has issues with control and that, in terms of personal mental health, should be dealt with as a much higher priority.

As pointed out in the answers given so far, a few mobile phones are not going to cause problems. We then have a few 100 milliwatt transmitting power on frequencies that are totally different from what the plane uses to communicate with ATC, and all equipment used by the plane are well shielded from EMF interference, so it looks like we can just allow everyone to use their phones. However I see some problems here.

Without the request to shut off mobile phones or only use them in flight mode, you could well have hundreds of phones turned on, particularly during landing (people want to call home, call to arrange a taxi etc. etc.). The radio link between the phones inside the plane and the relay stations are quite poor due to the airframe, as a result the phones automatically upregulate their transmission power. All these phones then start to interfere with each other more and more causing the power to be upregulated even more, so you'll end up with hundreds of phones transmitting at the maximum power of, say, 2 Watts inside the plane.

So, it's not all that unrealistic to assume that without the rule to shut off phones, you would routinely have to deal with a total RF power inside the plane of the order of half a kilowatt during landing. Inside the plane, most of that power gets reflected in rather random ways, it can get concentrated at certain points. The phones get affected by all that power too, they can start to produce harmonics at different frequencies than they normally transmit at. After all, a phone is far less well shielded against EMF interference than the aircraft equipment. Even a weak spurious signal appearing on an ATC frequency (just a very small fraction of that half a kilowatt is needed for that) could cause disaster.

As a private pilot I can tell you cell phones do not interfere with flight systems. If that was the case, planes would go down every time they fly over a cell phone tower. A cell phone tower emits energy in the 800MHz band trillions of times stronger than a cell phone. You don't see planes crashing every time they pass over a cell tower, do you?

Don't worry about it.