mobile phones interfering with planes

[filthiestfis]

How many mobile phones does it take to cause an interference with planes if they are all switched on onboard an aircraft? I know that they always tell you to switch your phone off but some idiot always has theirs on. Have there been any cases of mobile phones or other electronic equipment interfering with the aircraft?

[mushroom25]

I don't know the answer to your question, but if there was the slightest chance of mobile phones seriously interfering with the aircraft, I believe they would be confiscated before you got any where near to boarding.

[Buenchico]

(2-part post):

There have been several cases where aircraft navigation systems have experienced short-term problems for no obvious reason. It's suspected that these may have been due to mobile phone transmissions but I'm not aware of a situation where this has actually been proved.

The problem is that a person doesn't actually have to be using a mobile phone for it to present a risk to aircraft electronics. Anyone who has ever left a moblie phone next to a radio set knows that, every so often, the radio will pick up interference from the phone even though no call is in progress. Similarly, regular train travellers may have noticed interference from the conductor's mobile phone being heard over the speakers when he makes an announcement.

This occurs because of the way the cellular system works. Imagine that person A, on Vodafone, is in Croydon when someone calls him on his mobile. The telephone call is routed across the country to Croydon. The signal to A's mobile phone isn't sent out from every transmitter in the country (if that hapened the system would quickly get overloaded); it's only transmitted from one or two transmitters in Croydon. For this to happen, Vodafone's system has to know where A's mobile phone is located. It's for this reason that, every few minutes, A's mobile phone sends out a signal which effectively says 'Here I am'.

It's these 'locator' signals which cause the interence to radios and PA systems and it's these signals which pose risks to aircraft electronics.

[Buenchico]

In order to conserve battery life, the normal 'locator' signals sent out by mobile phones are at very low power. (Typically, not more than a few thousandths of a Watt). If there's a transmitter nearby, the signal is picked up and the transmitter sends back an acknowledgemet which effectively means "Message received. We know where you are". If, however, there's no transmitter in the immediate locality (so no acknowledgement is received), the phone tries again to establish contact, but this time using slightly increased power. Again, if there's no acknowledgement, the phone tries with yet more power. The maximum power that most phones will use is 5W, which is at least 100 times the normal power levels.

If someone is sitting in an aircraft, 6 miles above the Atlantic ocean, a phone will be unable to establish contact with any transmitter and will therefore continue to transmit 5W locator signal until the battery runs out. Although normal power' mobile phone signals aren't strong enough to interfere with aircraft electronics, 5W (in radio terms) is quite a high-powered signal. (Radio amateurs can transmit from the UK to the USA with 5W of power).

So, even though the owner of the phone isn't trying to make calls, a switched-on phone can still interfere with a plane's sensitive electronic circuits.

Chris

[filthiestfis]

Yeah thanks. Well the whole point of it is that everyone should keep their phone switched off. I dont just mean 'not in use' but OFF! I suspect that phones dont cause major interference even if every passenger on the plane had theirs switched on.

[captken]

it takes one to ring for the altimar to be read incorrectly