Those who forget to turn on the feature have seen hundreds of pounds added to their monthly phone bill, as their phone could still connect to the in-flight roaming network.
Passengers are advised to switch their phone to airplane mode when on board, or they could face being landed with in-flight roaming charges.
One traveller was recently charged $300 (£245) when he forgot to turn on flight mode during an Aer Lingus flight, according to the Financial Review. The man stored his phone in the overhead compartment without realising signal and roaming had not been turned off.
His phone then connected to the aircraft's in-flight network Aeromobile, which provider AT&T explained was the result of "antennas installed on the plane that operate outside an unlimited international roaming plan... automatically connect[ing] with phones that are not in flight mode."
Due to the way the network operates on planes, users are not warned with an explanatory text messages, as is typically the case with data roaming.
Aer Lingus is just one of 23 airlines that use Aeromobile's in-flight connectivity, with Virgin Atlantic, Emirates and Lufthansa among the carriers who also use the network.
Aer Lingus confirmed passengers who fail to switch off their phones during a flight could be faced with charges and advised that they should put their devices on airplane mode.
A spokesperson for the airline told the Financial Reviews that phones that are left on and not switched to airplane mode "may connect to the in-flight roaming network and the guest will be billed by their home operator for any usage".
Virgin Atlantic warns passengers of such charges on its website, noting the service is available on all aircraft based at Gatwick and selected planes at Heathrow and Manchester.
The airline states, "The charges are almost the same as they would be if you were roaming from another country.
"The calls and texts will just be added to your mobile after you've flown."
Emirates also has a warning on its website, stating that usage during a flight will be "charged to your service provider in line with international roaming rates."
Avoiding the charges
To avoid being caught out by such charges, passengers should ensure they put their phone in airplane mode as soon as they board the plane, or turn off their device completely.
Airplane mode should be kept on for the entirety of the flight.
If the aircraft has free WiFi on board, this can be used safely without passengers being landed with any additional charges.