AIRLINES that prosecute passengers who insist on using mobile phones in the
air can now back their case with hard evidence. In tests aboard two airliners,
Britain’s Civil Aviation Authority has confirmed that cellphone radiation
interferes with flight-critical electronics.
“Until now, we’ve only had pretty strong anecdotal evidence that mobile phone
emissions have set off warning systems in cockpits and disrupted signals in
instrument landing systems,” says CAA spokesman Jonathan Nicholson. “We carried
out the tests to get scientific proof that there is a threat from
interference—and we’ve now done that.” Last July, a passenger who
persistently refused to switch off his mobile phone was jailed for a year by a
British court for endangering the aircraft. But until now, there’s been little
science to support the mobile ban.
To find out how an aircraft’s control systems react to microwave signals from
a cellphone on board, a team led by Dan Hawkes, head of avionics at the CAA’s
Safety Regulation Group, borrowed a Virgin Atlantic Boeing 747 and a British
Airways Boeing 737. In tests carried out on the ground at London’s Gatwick
Airport earlier this year, they generated signals at freqencies ranging from 380
to 1700 megahertz at various points within the aircraft. They measured the
strength of these signals on the flight deck and in the avionics equipment bay
beneath the galley.
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In a report on the tests, the CAA says its results show “interference levels
that exceed demonstrated susceptibility levels for aircraft equipment approved
against earlier standards”. Avionics systems such as instrument landing systems
that came into service before 1989 were never tested for resistance to mobile
phone emissions. Some of these systems are still being installed in aircraft, so
the CAA cannot risk lifting the ban on the use of cellphones in flight. Newer
equipment has been designed to withstand higher levels of interference, using
screened cable and Faraday cages, says Hawkes—especially on fly-by-wire
planes like the Airbus.
The CAA says cellphone interference on aircraft potentially reduces pilots’
faith in warning systems, by causing false alarms. Audible interference can
distract the crew so that they make mistakes, such as not flying at the
designated height. “Altitude busts are often caused by crew distraction,” Hawkes
says. A key factor the tests uncovered was that the varying power output of a
cellphone could lead to fast-changing interference patterns: “A cellphone emits
more power the further it is from a base station. So as an aircraft climbs, the
mobile signal increases in power, boosting the interference level at a critical
time in a flight,” says Hawkes.
Les Barclay, a radio engineer on the British government’s Independent Expert
Group on Mobile Phones, says: “I understand the aviation industry’s
difficulties, but it has been slow in responding to the growing demands for
spectrum use and to new electromagnetic compatibility problems. With better
design and implementation, it should be possible to eliminate direct signal
entry into aircraft audio and control circuitry.”
