Here’s an excellent reason why you should use flight mode on a plane

We’re always warned to switch our phones to aeroplane mode on a plane. Here's a good reason why you should definitely listen.

Before every flight, passengers are advised to switch off their mobile devices or turn on aeroplane mode.

While safety is part of it, doing this could also save you from being hit with hefty roaming charges.

Depending on the plane and your device, a phone that hasn’t been put in aeroplane mode could automatically connect to the aircraft’s antenna and rack up the charges without you realising it, The Sun reports.

This is because some phones will automatically connect to a roaming network — often using satellite, where charges are much higher — as one traveller flying with Irish flag carrier Aer Lingus recently found out.

The unnamed passenger told The Irish Times he left his phone on in the overhead compartment while travelling to the United States and ended up with almost $US300 ($AU409) in charges from his provider AT&T — the bill for which he only received weeks later.

A passenger got $400 in charges from his telco after he didn't put his phone on aeroplane mode during his flight.

AT&T said the phone had connected to the plane’s antenna and used data that was “outside an unlimited international roaming plan”, which incurred the extra charges.

Although it doesn’t always happen, the airline confirmed that passengers’ devices “may connect to the in-flight roaming network” without them opting into the fee-paying Wi-Fi network.

Instead, the money is directly billed to the provider, as was the case in this instance.

But it’s not just roaming on flights you need to worry about — leaving your phone on while at sea might incur sizeable fees too.

Aer Lingus said passengers’ devices ‘may connect to the in-flight roaming network’ without them opting into the Wi-Fi network.

In 2016, British man Mark Stokes received a $590 bill from UK telco O2 after roaming while on a ferry between England and France.

He had paid for a bolt-on package with the service provider so he could use his phone as normal while abroad so was surprised to receive the extra charges.

According to the BBC, the extra charges were as a result of the ferry’s own mobile network, which is operated via satellite.

The satellite network falls outside the usual mobile networks meaning that normal phone tariffs don’t apply.

This article originally appeared on The Sun and was reproduced with permission