[Robinson (OP)]

So I was flying home the other day. And as usual - before the flight took off we were asked to turn our cell phones off. Why are we asked to do this? And does it really make a difference to the safety of the flight? The man next to me didn't turn his phone off - he just turned the ringer off. I spent much of the flight trying to decided if I should say something to him. In the end, I did not say anything and we landed safely. But for the future I'd like to know. Thank you.

[another_someone]

It is very unlikely to have a major impact on safety.  In fact, it cannot have a significant impact on safety because, as you have seen, it cannot be enforced, and it has never knowingly caused a plane crash.

It is likely that it would not be easy to receive a signal inside the metal body of the plane, but if you could receive a signal, it may well cause problems for the ground based networks, since both your high speed, and the fact that it would be difficult to identify which cell you were in, would make it difficult for the network to track you and assign you to a cell.

That having been said, it is clear that passengers were using cell phones successfully on some of the hijacked planes involved in the 9/11 attacks (although in New York, they were probably at low altitude, whereas the plane in Pennsylvania was in a rural area with more widely spaced mobile phone cells).

[Pumblechook]

It is unlikely that they could cause interference to any of the plane's vital systems but there have been incidents which may have been caused by a phone.   

[daveshorts]

The other issue is that using phones on planes is really bad for the mobile networks. If you are on the ground your signal only travels a few km until it hits a hill, so it only uses up its bit of the spectrum over a small area. If on the other hand you use the phone on a plane the signal will cover a 30-40km radius interfering with lots of other people's calls, and reducing the capacity of the phone networks. I don't think this is why they are banned, but it will be a major headache when they stop being so.

[Alandriel]

Thanks Dave, that is very interesting. Never knew that before  []
I always knew the safety issues were phoney

[Carol-A]

Can you imagine being in a plane with all those irritating ring tones going off every minute or so! I think they are banned to save the sanity of other plane users!

[techmind]

As has already been pointed out, there are two sides to this:
1) Risk of interference to aircraft electronics/communications
2) Problems to the mobile phone operators' networks

We've all held an operating mobile phone near a radio or stereo and heard it go blip-de-blip-de-blip-brrrrrrrrrrrrrrrr-brrrrrrrrrrrrrrr etc. If you hold an operating phone near a CRT computer monitor it causes "interesting" effects on the picture too.
Because of the sheilding effect of a plane, a phone would probably tend to operate at a higher power than it typically would on the ground, and because the signal will be "contained", the radiofrequency field strength inside the metal plane could get fairly high. Just because a plane hasn't (knowingly) crashed doesn't mean things haven't been made more difficult for a pilot. I have heard of pilots hearing the brrrrrrrp-de-brrrrrrp breakthrough on their headsets. If the interference was causing an instrument to mis-read, then that could be a problem too.
Sure, we'd know about it if planes fell out of the sky as soon as anyone made a call, but I'd still suggest the interference risk is more than just hypothetical.

A phone high in the sky will spread its signal far and wide, "blocking" that
frequency/channel across a far far larger ground area than it would on the ground.
This is a real issue, especially if very many passengers on planes were talking on their phones. I also believe the GSM specification only supports phones travelling at up to 500km/h (although it may work above this, it's out-of-spec and not guaranteed).


When they permit phones to be used on planes (in future), they will install a mini-base station within the plane. The handsets will then connect at very low power to this "pico-cell", and the calls will be trunked back to ground via a dedicated (satellite?) link. Because the handsets will only operate at extremely low power (only a few 10's metres range) the risk of interference to both the planes and the mobile networks will be very much reduced. The engineers have been looking at techniques to jam the phones to prevent them from connecting directly to ground base-stations.

[lyner]

When they permit phones to be used on planes (in future), they will install a mini-base station within the plane. The handsets will then connect at very low power to this "pico-cell", and the calls will be trunked back to ground via a dedicated (satellite?) link.

This is bound to happen. Just watch this space!

[techmind]

When they permit phones to be used on planes (in future), they will install a mini base-station within the plane. The handsets will then connect at very low power to this "pico-cell", and the calls will be trunked back to ground via a dedicated (satellite?) link.

This is bound to happen. Just watch this space!

It is happening. I believe engineering trials are in progress.

[another_someone]

When they permit phones to be used on planes (in future), they will install a mini base-station within the plane. The handsets will then connect at very low power to this "pico-cell", and the calls will be trunked back to ground via a dedicated (satellite?) link.

This is bound to happen. Just watch this space!

It is happening. I believe engineering trials are in progress.

Indeed, and guess what the mark-up the airlines are going to put on the calls for the privilege of routing them to the terrestrial networks is going to be like.