In the United States, both the Federal Aviation Administration and the Federal Communications Commission regulate the use of mobile phones on aircraft.

Safety concerns exist for both the cell networks and the airplane when traditional "terrestrial" cell phones are used in flight.

Because cell phones emit radio signals, government agencies worry that they can interfere with sensitive aviation electronics like communications equipment or the navigation system. Both of those systems rely on radio signals.

In the mid-1980s, when the first commercial cell phone networks began operating, the U.S. government adopted rules prohibiting their use onboard commercial flights. The rules were based on the above concerns.

On Sept. 11, passengers disregarded those rules when they called loved ones and law enforcement officials using both Airfone-type and traditional cell phone technology. Another safety concern, however, is that terrorists might use the signal from a cell phone to detonate an onboard bomb.

In spring 2007, the U.S. government decided to continue its ban on in-flight mobile phone use. An FAA spokesman said the agency was reacting to public fears that the signals given off by cell phone might interfere with plane controls.

Those fears might be well founded. The Telegraph reported that the British Civil Aviation Authority found up to 20 incidents between 2000 and 2005 where aircraft malfunction was related to mobile phone use.

Still, the European Aviation Safety Agency recently gave approval to OnAir to begin providing in-flight mobile phone service. The company's system uses phones that are less powerful than traditional cell phones aimed at avoiding such control interference.

Many European air carriers plan to use mobile phones in aircraft. But the topic remains controversial, according to the Telegraph article.

The FCC bans the use of cell phones using the common 800 megahertz frequency, as well as other wireless devices, because of potential interference with the wireless network on the ground. This interference happens as the planes, traveling several hundred miles per hour, leave one "cell" of mobile phone towers and enter a new one quickly.

The phones -- as well as any portable electronic devices such as iPods or portable radios -- also might interfere with the aircraft's navigation and communication systems. Therefore, the FCC bans their use during a flight.

Cell phones are a special concern in that they're specifically designed to send out signals strong enough to be received at a distance.

The FCC, however, has approved rules that allow in-flight voice and data services, such as broadband Web service, that use dedicated frequencies previously used by services such as Airfone.

An FAA fact sheet on the subject stated the agency's main fear is the unknown. The signals cell phones and other devices give off might effect critical aircraft systems over time. The FAA regulations prohibit most devices, though they specifically exempt voice recorders, hearing aids, heart pacemakers and a few others.

In general, however, the FAA advises airlines that most personal electronic devices are safe to operate over 10,000 feet. This is mainly because any potential interference would be a more serious problem at a lower altitude.

In some cell phones, users can operate in "airplane mode," which disables the phone's transmitter but enables the passenger to use other functions, such as its calendar or games.(from howstuffworks)