Tuesday, July 20, 1999 Published at 22:11 GMT 23:11 UK
Can mobile phones bring down planes?
Computer-controlled equipment is vulnerable to electronic interference
Any pilot who flies passenger jets can tell stories of mobile phones jeopardising passenger flights.
Dan Hawkes, head of avionic systems at the Civil Aviation Authority (CAA), cites a typical incident.
"He heard the interference in his headphones, and at the same time his automatic landing system was showing anomalous behaviour. He decided it was unsafe to continue with the automatic approach."
In another incident, the stick-shaker in the cockpit, a warning signal that the plane is about to stall, started to operate. The plane was on the ground at the time. A passenger at the back was found to be using a mobile phone.
But most of the evidence is circumstantial and anecdotal. There is no absolute proof mobile phones are hazardous. But, as Mr Hawkes explains, even the possibility of interference by mobile phones has serious consequences.
"There's an industry consensus, throughout the world, that mobile phones are a potential hazard to aircraft and must be switched off.
"The computer may shut down, which would affect the aircraft's navigation, which in turn would affect the signals sent to the auto pilot, and the way the aircraft is automatically flown.
"The aircraft might go off course, and even might change height."
Carolyn Hawkins, who advises on safety for the pilots' union, Balpa, says absolute proof is not needed.
"If you've got navigational systems being affected, by whatever it is being affected by, and you turn off the mobile phones, the laptop, the CD player, whatever, and it corrects it - let's stick with that, let's be safe in the air."
Part of the problem is that mobile phones emit signals even when they are not being used, if users leave them switched on. They are hunting for their nearest ground station to make a connection.
David Learmount, of Flight International, agrees. "As a result of all this anecdotal evidence which they haven't been able to prove, they are justifiably nervous. It is a serious matter.
"Although it has not been proven that these machines can make aeroplanes go off course, they really don't want to prove it."
It may be that mobile phones are not hazardous to all aircraft.
'Older models more vulnerable'
Neil Whitehouse, found guilty in June of endangering an aircraft because he refused to switch off his mobile phone, was on board an older model.
Mr Learmount says: "This fellow happened to be using his mobile phone on board a Boeing 737.
"The 737 was designed in the early 1960s, and that aeroplane has never had its electronic equipment screened against the kind of emissions that mobile phones put out.
"Whereas in the more modern aeroplanes, like the fly-by-wire, computer controlled Airbus A-320s, A330s, A340s, they have been screened.
"So he could actually say if this aeroplane had been an Airbus he might well have walked away from the court."
But even if you use a mobile phone on an aircraft which has been screened, it is still against the Wireless and Telegraphy Act.
That will be a great relief for people to whom aeroplanes are about the last place left where you can eat, sleep, think and relax without being disturbed by electronic warbling.