Pilots of Boeing 757s and 767s have been told to exercise special vigilance by the US aviation authorities following a series of incidents in which an autopilot has engaged, disengaged or given faulty readings. Eight British airlines, including British Airways, Britannia, Air 2000 and Air UK, operate one or both of these aircraft.
The warning stems from an incident on 15 June at Frankfurt when a United Airlines 767 unaccountably left the runway after landing. Investigators found that its rudder had moved without instruction from the pilot, pushing the plane to the right on to a grassy verge. Only corrective action by the pilots prevented an accident.
The rudder had locked into position as if it had been instructed by the autopilot, but the exact cause is unknown. The pilots had turned off the autopilot in preparation for a manual landing.
During subsequent testing, information on the autopilot display panel changed without any action by the pilot and the plane turned in response to the new instruction.
On checking United's records, the US National Transportation Safety Board found that there had been 29 instances - all but one since 1985 - in which autopilots on 757s and 767s had behaved unpredictably. Boeing says that most of these incidents involved faulty readings during flight which were corrected by the crew. A spokeswoman said: 'We know it's a problem and we are working to fix it.'
Investigators are mystified by the faults since the autopilot is a triple system, including two back-ups to ensure there is no failure.
The transportation board has asked the Federal Aviation Administration, the equivalent of the British Civil Aviation Authority, to warn pilots of the risk from the autopilots. It also wants Boeing and Rockwell Collins Avionics of Iowa, the autopilot's maker, to 'seek corrective action'. The FAA has 90 days in which to respond to the request from the board, which has advisory powers only, and if it agrees, all airlines will be told of the potential danger. A spokesman for the NTSB said: 'United, which operates 72 757s and 34 767s, is known to be very concerned about the large number of incidents.'
Other airlines have also been affected. In June last year an American Airlines 767 had a locked rudder without instruction from the pilot and despite a year-long monitoring of the autopilot, the fault was never found.
The US pilots' association is also worried that the autopilot system is so complex that an attempt to remedy the fault may exacerbate the problem.
There are 557 757s and 500 767s flying, of which 83 757s and 33 767s are registered with British airlines. The Collins autopilots are installed on all 757s, 767s and some 747s and Fokker 100s. The company refused to comment last night.Reuse content