The real reason why you’re told to put your mobile in Flight Mode

Two people using phones on a plane - Getty
Two people using phones on a plane - Getty

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.

But not all of us. In a Twitter poll this week, three quarters of Telegraph Travel followers said they turned on Flight Mode when asked, leaving one in four who claim not to. This created some ire, with one user asking: “Do those who have voted no actually realise it’s for their own safety? Staggering.”

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Such debates might soon be a thing of the past, however, with the EU recently announcing that airlines may soon be expected to provide 5G to passengers.

The ruling threatens to put an end to the relatively blissful, phone-free aesthetic of the plane cabin; access to 5G would make in-flight telephone calls, messaging and video streaming all possible.

It has also raised questions about why – if it is now deemed safe – we have been prevented from using our mobile phones for so long. Could a mobile phone really bring down a jet?

The Flight Mode rule does seem, in part, to be an exercise in overcaution. Patrick Smith, a US pilot and author of Cockpit Confidential, said: “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.

“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.”

He added that the greater risk devices such as laptops pose is becoming “high-speed projectiles during a sudden deceleration or impact.”

A person switching on Airplane Mode - Getty
A person switching on Airplane Mode - Getty

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 cabin crew ask passengers to use Flight Mode.

Smith estimated 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”. But he added that if mobiles were that great a concern, the policy would be more actively enforced.

Mobile phone use had only been raised as a possible factor in the aftermath of one major air accident: a Crossair crash in Switzerland in 2000. But investigators pointed to numerous other factors and concluded that there were “no indications that aircraft systems were negatively affected by electromagnetic interference (EMI)” from mobile phones.

Even if mobile use is not safety critical, however, it may well annoy your pilot. And when the cockpit should ideally be a place of calm and serenity, that is not ideal. Discussing the issue, the website The Points Guy quotes a private pilot, Nikita Schmidt:

“Your phone will probably annoy a few pilots and air traffic controllers. But, most likely, not badly enough for them to take action against you, if that’s what you want to know.

“You may have heard that unpleasant noise from an audio system that occasionally happens when a mobile phone is nearby. A phone’s radio emissions can be very strong, up to 8W; they cause this noise due to parasitic demodulation. I actually heard such noise on the radio while flying. It is 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.

“When in-flight cellular service is provided, there is a cell station right beside those phones. They communicate at very low power without causing any disturbance. The Wi-Fi signal is much weaker (100mW) than GSM at its peak, and I never heard of it causing any problems.”

So under the current system, phones do have the potential to interfere with the flight crew, but only to a small, if annoying degree. And when airlines offer in-flight 5G, this should not be a problem.

Nevertheless, as Smith points out, there will be social implications.

“The minute it can be proven beyond reasonable doubt that phones are safe, a percentage of flyers will demand the right to use them, pitting one angry group of travellers against another, with carriers stuck in the middle,” he said.

“The airplane cabin is a last refuge of relative silence. Let’s keep it that way.” We’re inclined to agree.