Switching to flight mode has become part of the routine for travellers – but leaving it on accidentally could have expensive consequences.
Depending on the aircraft, leaving your phone on without switching it to flight mode can rack up hefty roaming charges if it automatically connects to the plane's antenna.
A traveller on Irish carrier Aer Lingus recently found this out the hard way that when he was charged almost US$300 ($435) in roaming chargers from provider AT&T weeks after flying.
The passenger said he hadn't even used the phone - it had been stored in the overhead compartment for the duration of the flight from Ireland to the US, but had not been switched to flight mode.
AT&T told the Irish Times that the charges were due to "antennas installed on the plane that operate outside an unlimited international roaming plan", which can automatically connect to phones that are not in flight mode.
This can result in hefty charges – even when the phone is not being used.
Aer Lingus confirmed to the Times that this could happen – but said it could not be held accountable if passengers did not follow clear instructions.
"For safety reasons, before every flight, Aer Lingus cabin crew advise guests to switch their phones to airplane mode," a spokeswoman told the Times.
She also said that passengers could use in-flight wifi on transatlantic flights if they paid for it, but if a phone was left on it "may connect to the in-flight roaming network and the guest will be billed by their home operator for any usage".
Generally, when passengers are asked to switch to flight mode, they are warned that it could interfere with the plane's navigational equipment.
Writing on Quora, a pilot said it was more of an annoyance than a serious danger – as radio emissions sent by a phone can cause an annoying "dit-dit-dit" sound on aircraft radios.
"I actually heard such noise on the radio while flying. It is not safety critical, but is annoying for sure," the pilot said.
While GSM phones were the worst offenders for interference, the problem has become less common as phone technology advances.
When passengers are told to "switch devices to airplane mode", it could be as much in the interest of their phone bill to comply as keeping their pilots' happy.