Why Jumbo gets nervous round mice

Contrary to advertisements showing executives hard at work at 39,000 feet, airlines are becoming increasingly belligerent about notebook PCs. Some ban their use in flight altogether; others will not allow passengers to boot up for 15 minutes either side of takeoff and landing.

The problem is radio-wave emission. Everything electric, from a Magimix upwards, produces radio waves. Broadcasting souffls poses no threat to anyone, as you're unlikely to scramble the navigational equipment of another kitchen coming at you at 500mph. But in an aeroplane it could be a different matter.

"I was flying my Beechcraft Bonanza, and I turned on my Toshiba T1950 to test a navigational program," says Thatcher Stone, a pilot. "Suddenly, there was a 35-degree swing in the visual omni range, and the aircraft, on autopilot, swung and banked right into restricted airspace."

Larger planes might be just as susceptible. US newspapers reported a recent case involving a United Airlines 757 attempting a night landing. Everything was fine, until one of the laptop-equipped passengers tried saving his file to hard disk.

A crew member recalls: "The VOR and ILS [instrument landing system] needles started swinging like crazy. The captain had to go into manual. We were lucky no serious damage was done."

How can this be, given all the international regulations governing electronic equipment and potential interference? In the United States all laptops, by law, must meet Federal Communications Commission limits on their permitted radio-wave emissions. Most pass even more stringent guidelines. In fact, it's peripherals that could send you into a tail-spin.

"Laptops with certain combinations of batteries and peripherals cards can, on occasion, generate an amazing amount of radio frequency across a wide spectrum," says a spokesman for one of the US domestic carriers. "The problem is in trying to identify the exact circumstances in which these phenomena can occur.

"Where you're seated could be a factor. If a passenger sits over the wing, radio interference from his notebook could go out the window and bounce off the jet's antenna. Another seat might be fine. Aircraft with exteriors made from composite materials, as opposed to all-metal, seem more at risk, as they don't reflect internal radio emissions as well."

On the other hand, internally reflected radio waves could pose a problem if you're flying in a more modern aircraft. Unlike the elderly Boeings and their siblings, whose flaps and rudder are operated mostly mechanically, Airbuses are controlled electronically by a wire running down the middle of the aircraft. The potential for interference here is greater.

Hence the airlines' ban on "radial mice" - those attached to the computer's serial port by a wire. While trackballs behave well, the wire on radial mice acts as an aerial. The longer it is, the more possibility of signals being amplified. So try clicking on the "save" icon and the next thing you know, your 747 could be jettisoning fuel, mid-Atlantic.

No one really believes an Alicante-bound passenger playing "Chuck Yeager: Test Pilot" on his notebook will suddenly cause the aircraft to behave like a sub-orbital rocket. But in a safety-conscious industry caution must be the watchword.