The use of mobile phones on aircraft is generally forbidden during flight. One reason given for this is that the mobile phone could interfere with the sensitive equipment on the aircraft. This could be restated as "during development these aircraft were not designed to accept signals from mobile phones and there has not been sufficient testing to be sure that they could" as can be seen from plans to improve certification [1].

Some level of electromagnetic interference is theoretically possible from active radio transmitters such as mobile phones on aircraft. Exactly how much and in what way is dependent on the particular phone system in use and the plane component in question. Whether that level of interference should have any influence on electronic systems which should be designed to fly through lightning storms without falling out of the sky is an entirely different question.

One area in which interference would be most likely is in the radio-based audio equipment used for voice communications between the aeroplane and the ground. The mobile phone transmitter is much closer to the receiver on the aircraft than the ground station, but operates at a lower power than the ground station.

Some mobile phone systems such as GSM may cause an irritating buzz (explained in the TDMA article) which would certainly disrupt communications from the pilot to ground. The high speed of air travel may make interference more likely than it would otherwise be. The maximum speed of travel in a mobile phone system is limited by several factors, frequency changes, rate of change of timing offset, etc. The speed of an aeroplane often exceeds these (typically phones are designed for use in a fast car) which means the mobile will fail to register to the network and retry registration repeatedly.

Older analogue systems simply broadcast at a high power of up to several watts. This has the potention to cause more general interference, and since the voice signal is not encoded there may be direct crosstalk into the communication systems of the plane.

But perhaps the greatest issue is with the principle of frequency reuse. Mobile phones are designed to transmit only as far as the closest cell tower. This allows the same frequencies to be used by different phones in any non-adjacent cells, a key component in allowing tens or hundreds of thousands of people to use their phones at the same time in a given metropolitan area. From an altitude, distant cells are visible to the mobile with no line-of-sight attenuation from intervening obstacles. Because the cells are in some cases several tens of thousands of feet below the aircraft, the phone will transmit at its maximum power (also increasing the risk of interference with electronic equipment on the aircraft). Since the phone is occupying its frequency and channel in all of the cells its signal reaches to, that frequency and channel cannot be used by any other phones in any of those cells because of interference, resulting an an overall decrease in the cellular system's capacity. This is the primary reason the U.S. Federal Communications Commission prohibits the use of mobile phones in the air. (The U.S. Federal Aviation Administration is more concerned with interference to aircraft systems.)

All of the above having been said, according to the BBC "most of the evidence is circumstantial and anecdotal. There is no absolute proof mobile phones are hazardous." [2] Some airlines do allow use of mobiles phones in flight, only restricting their use (and use of all other electronic devices) during take off and landing when communications with the ground are most critical. Meanwhile the passenger aircraft manufacturers, such as Boeing and Airbus, have begun to introduce wireless services on their planes (e.g. WLAN) and radio-based satellite phones are a standard installation on aeroplanes. Clearly there is a direct airline industry advantage in having control over communication systems from within an aeroplane with no clear way for potential competitors to certify their systems as safe for use on board. Some articles have even gone so far as to accuse the airline industry of pushing the ban on mobile phones in order to increase revenue from on-board telephones [3]. A number of new phones have an "aeroplane mode" feature that presumably stops all incoming and outgoing communications while still allowing the user to play games, type notes etc.

A few U.S. airlines have announced plans ([4] and [5]) to allow mobile phones to be used on aircraft, pending approval by the FCC and the FAA. The idea is similar to that used in some cars on the German ICE train: the aircraft will contain a mobile signal repeater that will then transmit the signals to a terrestrial-based system on separate frequencies that do not interfere with the cellular system. Since the repeater is literally right next to the passengers, the phones' output power would be reduced to the lowest level possible, reducing interference with cells on the ground.

That leaves one final concern: mobile-phone etiquette. Many have brought up concerns about being forced to sit next to a loud mobile phone user for a several-hour flight, adding airlines to the list of establishments trying to figure out how to enforce common courtesy.