The real reason why you're told to put your mobile in flight mode

Are mobile phones dangerous? Depends on who you ask. Photo / 123rf

It is a plea heard by passengers at the beginning of thousands of flights around the world each day. "Please set your portable electronic devices, including any mobile phones, to flight mode."

Most people comply, even if they're not 100 per cent sure why, the Daily Telegraph reports.

The general assumption is that we must disable our mobile phones – some more begrudgingly than others – as their signal interferes with navigation instruments, and could even cause a crash. But is that really true?

One anonymous flight attendant certainly thinks not. "Betty", interviewed last week by Vice, said: "Nobody turns off their phones. I don't, even. All of those commands are really just precautionary. Keep your phone on. No one cares." We imagine her employers are grateful she remained anonymous.

Patrick Smith, a US pilot and author of Cockpit Confidential, strikes a far less blasé tone – but agrees that the rule is more an exercise in caution.

"Could a device interfere with the flight? It depends on the gadget and how and when that gadget is used," he said.

Taking the example of laptops, Smith says, though an old computer can emit harmful energy, the greater risk they pose is becoming "high-speed projectiles during a sudden deceleration or impact".

But for phones, which is at the crux of a businessman or keen tweeter's journey, Smith says: "Can cellular communications really disrupt cockpit equipment? The answer is potentially yes, but in all likelihood no, and airlines and the Federal Aviation Administration (FAA) are merely erring on the better-safe-than-sorry side."

He continues: "Aircraft electronics are designed and shielded with interference in mind. This should mitigate any ill effects, and to date there are no proven cases of a phone adversely affecting the outcome of a flight. But you never know."

A mobile phone's potential to interfere does not just exist when it is being used, but also when it is dormant, which is why the flight mode must be activated even if passengers don't intend to their phone.

But Smith reckons that despite the clear request at the beginning of each flight, "at least half of all phones, whether inadvertently or out of laziness, are left on during flight". He adds that if mobiles were that great a concern, the policy would be more actively enforced.

However, there are at least two serious incidents in which mobile phones have been implicated: first, the unsolved crash of a Crossair plane in Switzerland in 2000, when spurious transmissions confused the autopilot, and second, a fatal crash in Christchurch, New Zealand, in 2003. But these are the extremes.

A blog by a pilot on airlineupdates.net claims that the interference caused by mobile phone signals registers on the headsets of the flightdeck, in the same manner that one might have encountered on speakers affected by a nearby mobile: "dit d-dit d-dit d-dit…"

"I actually heard such noise on the radio while flying," wrote the author.

"It's not safety critical, but is annoying for sure. Of course, there is plenty of attenuation between phones in the cabin and the pilots' radio. However, if say 50 people on board are inconsiderate enough who can't be bothered to switch their cell radio off, there will be 50 phones constantly looking for cell towers at maximum power. That is a lot of radio pollution."