Pax not setting phone to flight mode. Major issue?
 

I'm genuinely curious. I have been in this situation two days ago flying out of OTP. Was seated in 1F, guy was sitting in 1D. At some point during taxiing (after the announcement to set mobile phones to flight mode) I noticed him having a very active conversation on WhatsApp. At this point I thought he would still turn it off (we had quite a long taxi run), however when we reached take off position he was still chatting merrily. At that point I decided to say something, so I told him 'Sorry, you really need to set your phone to airplane mode'. Fortunately he complied (said 'Yes, I know'), and we took off a couple of seconds after.
I have a couple of questions regarding this:
First of all, how bad is it leaving a device on during take-off / flight? I know they can in theory produce interference with radio / controls, and I'm assuming that there is a good reason we are asked to turn them off, however I can't help but think that it happens more often than we realize (people ignoring the instructions, forgetting, etc.) with no (noticeable?) consequences.
Second - what should I have done had he not complied? Ignore it completely? Attempt to call the FA (they were strapped in at that point)? Stand up? It feels a bit like an overreaction, but yeah :)

Most of the time it's not going to cause an issue. The problem is that EMI probably can show up in funny ways, or someone with a faulty phone could leave one on and it could cause interference at low altitude where there's less margin for error and time for recovery.

As an example, a few years ago I was flying through PHX, and as we came in to land there were a whole bunch of alarms in the cabin. At first I thought they came from the aircraft, but then realized they were coming from all the phones that had not been turned to airplane mode (or off) - it was a push warning that a Haboob was about to hit, and all the phones got it when we were about 2 seconds from wheels touching the runway. There were at least half a dozen near me that I could hear.

It's still preferable that you turn your phone off, given the inconsistency of EMI compatibility problems.

It is also a problem in certain aircraft with advanced avionics operating below standard minimums which are available based on the avionics.

In those circumstances, you may hear the Captain ask passengers to physically turn off all electronics well above 10,000 feet. The reason being that while interference is rare, the rare occurence may not give the Captain time to properly react.

The real bottom line is that it is a valid requirement and therefore not up for debate because some idiot thinks that what his brother-in-law told him he read suggests that it's OK.

Probably not going to cause interference itself. The rule exists for a few reasons:

1: It COULD cause interference..even a small chance is still one you shouldn't have to take if you don't have to.

2: It discourages, although does not prevent, people from holding what amounts to a dangerous projectile during a crash.

3: It diverts passengers attention from situational awareness and crew instructions during critical phases of flight..take off and landing.

4: There may be a cumulative risk of interference if it was allowed and instead of 1 pax using doing it, you have 200 pax using doing it.

All agreed, but I'm willing to bet that a significant proportion of passengers don't turn their phones to Airplane Mode. I'm guilty of forgetting once, and I try to be diligent on these matters. Fortunately no accident has yet been caused by this issue, AFAIK

An actual big issue is if you're on a plane/carrier that provides a cell carrier connection which charges you at roaming rates. Can eat up $100 easily flying across the oceans.

I've yet to see an airplane crash related to a cell phone left on. One only needs to seek out wifi on the airplane and find the multiple smartphones that show up. Complete non issue. Just watch the FA's that still appear to text or FB while approaching the runway. Yes, it policy to shut your phone off or airplane mode, but think about it; if active phones were interfering with the flight, you would be required to relinquish your phone at the gate. No airline would allow this if it actually mattered. As a private pilot, I can create some interference with my phone, but only on the com side, even then its minor. But thats in a little 182, and I have to be within inches of the radio.

I've forgotten my phones (plural) many times on many flights and have not crashed nor have had the pilot admonish the cabin over interference issues.

Someone else can be the self-appointed phone police. If someone leaves a phone on I am not going to report them.

I can recall only one time a pilot mentioned phone use. On an SOF-LHR flight the BA pilot asked phones be turned back off while he taxied to the gate after landing. I presume all the phones going on at once caused some sort of interference with his coms.

Not that on some airlines it seems wireless works all the way down to the ground, so even if you are in Airplane mode, you still might be connected to the wireless.

That being said one major factor of the phones isn't electronic, but distraction. If god forbid something did happen while you were on the ground, the last thing you need is everyone too absorbed in their phones to notice or do anything.

As a former engineer at Boeing, there is almost nothing that a cell phone can affect. When you think of all the sources of EMI that are in the air, cell phones are the last of your worries. Airplanes these days are built to handle EMI much more severe (think solar storms etc.). There is one time that I would want to make sure that cell phones are off or in airplane mode, and that's an auto-land in CAT3 conditions (less than 100 ft vertical visibility). I have heard that its possible for phones to affect the Radio Altimeter reading which is what tells the autopilot how far off the ground you are (see the Turkish 737 crash in AMS for more info on why this is important).

Do I turn my phone on airplane mode during flight? Yes, but that's more to save battery having my phone constantly search for service.

[edited]

It is if whatever they are doing might harm me

That is an unsound generalization.
I could give many examples exposing its infirmities.