Flights require plane passengers to follow numerous rules, be it wearing a seatbelt or keeping the window blind open at certain times.
One regulation, in particular, has many travellers flummoxed – why they need to turn their mobile phone onto airplane mode.
Switching a phone to flight mode means any radio-frequency signal transmission by the device is suspended.
Consequently, phone users are unable to send or receive calls and text messages or use Bluetooth and Wi-Fi.
Although it might seem frustrating to cut off all access to the outside world there is a very good reason behind the rule.
A mobile phone could serve to interfere with cockpit equipment, even if it’s not actually being used by a passenger.
According to the Civil Aviation Authority (CAA): “Modern passenger aircraft are heavily reliant on electronic communication and navigation systems which work on the basis of electromagnetic energy transfer and thus they can be susceptible to electromagnetic interference.”
Mobile phones and many electronic gadgets will transmit electromagnetic signals when not in flight mode as they try to connect with a communications network.
The CAA said: “The cumulative effect of a large number of mobile phones or transmitting electronic devices being used simultaneously when not in Flight Mode, particularly during the critical phases of flight, such as take-off and landing, remains a serious concern.”
If airlines want to permit their passenger to use a phone in flight mode (compared to completely switched-off) they need to seek permission.
A spokesman for the CAA told Express.co.uk: “Any UK airline which wants to allow passengers the use of electronic devices during a flight must conduct a risk and safety assessment, in line with regulations set out by the European Aviation Safety Agency.
“Most UK carriers have now completed this safety testing, which means passengers can use smartphones and tablets in flight mode.
“In addition, certain aircraft are now equipped with WiFi and/or voice call capability, however, notwithstanding this, passengers should still ensure any device remains in flight mode at all times.
“At the beginning of the flight, cabin crew will make clear when and what electronic devices can be used.
“All passengers must obey the instructions of cabin and flight crew, which includes information provided during in-flight briefings. Anyone who fails to do so may be committing an offence and could be prosecuted.”
The likelihood of a problem arising from mobile phone interference is, in reality, pretty low and authorities are erring on the side of caution.
Aircraft electronics are made with interference in mind and, what’s more, the policy isn’t actively enforced – passengers are simply trusted to turn their phones to the flight mode setting.
However, there have been two aviation incidents when a mobile phone was implicated, but both are extreme examples.
One was in 2000 when a Crossair plane crashed in Switzerland when fake transmissions confused the autopilot.
A second incident was a fatal crash in Christchurch, New Zealand, in 2003.
Passengers also need to be very careful if they drop their mobile phone down their seat during a flight as it could cause a fire.