It is confusing, and sometimes irritating, but there is no doubt, according to the regulatory authorities, that passengers' electronic devices can - and do - interfere with critical plane systems.
And in Australia, Europe and the US, aviation watchdogs have warned passengers that they face severe penalties if they do not follow crew instructions on the use of mobile phones and other electronic devices, such as laptops, on planes.
A study by the engineering department of the Carnegie Mellon University in the US found mobile phones and other portable electronic devices could interfere with the normal operation of critical components, even more so than previously thought.
Researcher Bill Strauss said that the study "found that the risk posed by these portable devices is higher than previously believed".
The study was conducted over three months in late 2003 with the co-operation of the US Federal Aviation Authority, three airlines and the US Transportation Security Administration. Another study by the British Civil Aviation Authority in 2003 found a range of electronic anomalies that could be produced by mobile phones operating at maximum power.
The signal from a mobile phone increases the further it is from the base station.
The Australian Transport Safety Bureau has reported 100 incidents over 10 years relating to mobile phones and portable electronic devices. All equipment installed on an aircraft, including seat-back videos, has been designed to eliminate interference.
The confusion on the use of such items as mobiles is because a few airlines have installed mobile base stations - called picocells - on board which allow passengers to use their own phone.
One air crash has been linked to the use of mobile phones.
Investigators of a crash in New Zealand in 2003 which killed eight people found that "the pilot's own mobile phone may have caused erroneous indications" on the navigational aid.
In Australia, mobile phones and other electronic devices have been proved to have disrupted aircraft navigation systems many times.
Some incidents include:
·Within 30 seconds of a captain announcing a landing delay on a Perth-Sydney flight, his navigation instruments were disrupted. A flight attendant found a passenger using his mobile phone to tell a friend of the delay.
·Near Brisbane, two passengers using laptop computers caused a plane to roll from side to side.
·A mysterious navigation problem disappeared after a passenger was asked to switch off a CD player.
·A mobile phone signal which opened the pressurisation outflow valves in a Boeing 747-400, causing the cabin pressure to rise was traced to a passenger on the upper deck using a satellite mobile telephone.
·A Slovenian airliner made an emergency landing in Ljubljana after a mobile phone stowed in passenger's luggage in the cargo hold caused the electronics system to malfunction and falsely indicate a fire on board.
The British CAA study said many pilot reports on the problem included false cockpit warnings, which increased the workload for the crew and reduced their confidence in important warning systems, malfunction of aircraft systems and avionics and interference in pilots' headsets.
Another danger comes from copied electronic equipment, typically made in China, which does not have the correct shielding.
Passengers are also required to stow all electronic devices and baggage for take-off and landing, including the climb and descent, because this is the most critical part of flight and more likely to be subject to turbulence.
Passengers must be ready to evacuate the plane in an emergency and laptops and the like will only interfere with that process.Passengers' electronic devices can - and do - interfere with critical plane systems.