Mobile phones can work in flight while in range of towers, says CNN

Mobile phones can continue to work after a plane takes off but only while in range of a cellular tower. – AFP pic, April 16, 2014.Reports that the mobile phone of missing flight MH370's co-pilot had been switched on and made contact with a telecommunications tower in Penang have raised questions about how mobile phones work on airplanes, reported global news network CNN.

Although, an American official had told CNN that there was nothing to show that a call was made, the finding was of interest as it was reported that there were no phone calls, texts or social media postings from those on board the Malaysia Airlines flight.

This was in contrast to when hijackers took over four aircraft and attacked the United States on September 11, 2001, where passengers and crew members had scrambled to make phone calls to their loved ones, airline managers and the authorities.

Here, CNN examines how mobile phones work in an aircraft.

How high in the air can a mobile phone work?

Despite the urgings of flight crews to turn off all phones or put them in airplane mode, mobile phones can continue to work after a plane takes off, but only while in range of a cellular tower, the CNN report said.

Mobile phones communicate through towers, which are located on the ground but can stretch hundreds of metres into the air.

As an airplane rises, it gets further from these towers and eventually moves out of contact range.

Most towers were designed to direct their signal where it was most useful: on the ground, not into the sky above, said CNN.

The maximum distance at which a phone can still make calls and send texts varies depending on the type of tower and transmitter, but an airplane would have to be no more than 3,000m in the air for any mobile phones on board to still have a signal, CNN quoted Bill Rojas, director of telecom research at IDC Asia Pacific, as saying.

It's not just altitude that can limit mobile phone service.

When flying over sparsely populated rural areas, nearby towers are fewer and further between. Over the open ocean, there are no towers at all with which to communicate, making phone calls, texts and social media posts impossible.

Speed could also make maintaining a cellular connection difficult, as a device has to switch from tower to tower to maintain a connection, the CNN report said.

Rojas told CNN that a plane would have to be going 250kph or less for phones in cell-tower range to make or receive calls.

The metal body of an airplane could also impede cellular service, CNN said.

What is airplane mode?

Airplane mode is a setting on smartphones and tablets that turns off most or all of a device's wireless communication features. This includes shutting down cellular connections, Wi-Fi, GPS and Bluetooth so that no calls, messages or data can be sent or received.

This setting has different names depending on the device and operating system, but is present on all smartphones.

Can a phone in airplane mode still communicate?

It is not possible to send or receive communications while in full airplane mode.

On some phones, it is possible to turn Wi-Fi back on while in airplane mode to take advantage of any on-board wireless networks, or to reactivate Bluetooth to hook up to any compatible devices.

Neither setting would communicate with a cell tower on the ground, though.

Are all phones on flights in airplane mode?

Up to 30% of all passengers reported forgetting to power down their devices when asked on airplanes, according to a survey conducted last year by the Consumer Electronics Association.

It's likely that many passengers also forget to turn their phone to airplane mode when taking off and that all flights have a few cellular devices that can still search for towers, reported CNN.

Do phones communicate with towers when not in use?

A mobile phone not set to airplane mode could still automatically connect to cellular base stations when in the proper range, the CNN report said.

Background phone tasks, such as location services, email and other things that push through notifications, are always attempting to connect and send and receive information.

Any attempted connections or pings would be recorded by mobile operators.

Don't many airlines offer inflight Wi-Fi?

Some airplanes offer inflight wireless connections that can be used to connect to the Internet and make calls and send texts using services such as Skype and WhatsApp.

There are different technologies for bringing Wi-Fi to airplanes, reported CNN.

Some use special mobile broadband cell towers on the ground flipped to point into the air. Satellite technology can be used to get signals where there are no ground towers.

If the inflight entertainment system is turned off, phones and other devices cannot use these wireless connections, the report said.

Aren't some flights going to allow calls in the air?

Recently, the US Federal Aviation Administration (FAA) loosened its rules for using mobile devices on airplanes.

Many airlines now allow you to use tablets and smartphones during take-off and landing as long as they are in airplane mode. It is also considering allowing mobile phone calls in the air, reported CNN.

To make that possible, airplanes would need to be equipped with specialised miniature base stations called picocells.

These base stations would relay any cellular signals through satellites or cell towers on the ground that are already set up to work with airplanes.