David Learmount/LONDON

Mobile telephones used on aircraft can "disrupt" avionics and systems, a report published by the UK Civil Aviation Authority has confirmed.

The widely believed theory has been difficult to prove until now, the CAA says, despite considerable anecdotal and circumstantial evidence gathered through pilot reports over the years.

In February, however, the CAA completed tests on a British Airways Boeing 737-200 and Virgin Atlantic Boeing 747-200, "classic" aircraft certificated before the rules on avionics interference protection were tightened. Even now, the CAA warns, many current production avionics systems have components which are manufactured to old standards.

Although post-1989 equipment is less susceptible, it is not always immune from mobile telephone signal interference.

The tests were conducted on the ground for safety. Head of the CAA's Safety Regulation Group avionics section Dan Hawkes says the tests showed:

• interference levels varied with relatively small changes in the location of the telephone in the cabin [which has made it difficult to reproduce recorded events];

• internal doors made of composite material did not block signals;

• passengers in the path of the signal attenuate it, so the number of passengers can influence an aircraft's vulnerability to interference.

Hawkes refers to an event last December when mobile telephone signals opened the pressurisation outflow valves in a Boeing 747-400, causing the cabin altitude to climb. This was traced to a passenger on the upper deck using an Iridium satellite mobile telephone.

The CAA study summarises pilot reports which had been filed as not proven. They include:

• false cockpit warnings which increase the workload for the crew and reduce their confidence in important warning systems;

• malfunction of aircraft systems and avionics;

• interference in pilot's headsets.

Hawkes says he is worried by the fact that some airlines allow passengers to use mobile telephones while the aircraft is parked. During this phase, he points out, navigation systems can be updating the flight management system and mobile signals could corrupt this process as well as interfering with voice and data communications.

Source: Flight International