MORE than 500 Boeing 777s used by airlines around the world flew for almost seven years with a software error that could cause pilots to lose full control of a dangerously pitching plane.
The error was revealed when a Malaysian Airlines 777 travelling from Perth to Kuala Lumpur in August suddenly pitched up in a 3000-ft climb that brought it close to stalling.
Australian investigators said yesterday the error was in the operating software of the air data inertial reference unit (ADIRU), a device that supplies acceleration figures to the aircraft's flight computer.
It had not been detected during original testing of the unit by manufacturer Honeywell prior to the 777's 1995 entry into service but had been suppressed in the first three versions of the operating program.
However, a fourth version of the software deleted the functions that suppressed the error and allowed it to send the Malaysian plane on a heart-stopping roller coaster ride.
The twin-engine 777 is Boeing's flagship and is popular with Asian carriers servicing Australia.
The issue was considered serious enough for the US Federal Aviation Administration to issue an emergency airworthiness directive last August to all 777 operators requiring them to revert to version three of the operating system.
Boeing spokeswoman Liz Verdier agreed last night that the fault was an unacceptable risk, although she said other software on the Malaysian plane "mitigated the event". "It's critical software and it isn't supposed to happen and we don't like it so it needed to get fixed," she said from Seattle.
According to Australian Transport Safety Bureau investigators, the violent pitching experienced by the 177 passengers on flight MH124 resulted from a combination of the faulty software and two failed devices used for measuring acceleration.
A flight attendant began praying as the pilots fought to counter the false data being fed into the plane's autopilot system and primary flight display.
The sudden climb cut the plane's indicated airspeed from 500km/h to 292km/h and activated a "stickshaker" warning that it was approaching a stall speed where it would have insufficient lift to keep flying.
The pilot managed to lower the plane's nose to prevent the stall but his attempts to bring the plane back under control were initially thwarted by the plane's automatic system.
It climbed another 2000ft before he radioed to air traffic controllers that he was unable to maintain altitude and requested assistance to return to Perth.
He was forced to fly manually after tests revealed problems with the aircraft's two autopilot systems and the plane again tried to increase thrust and gave a windshear warning before it landed safely in Perth.
An investigation by the US National Transportation Safety Board revealed that one of six accelerometers available to the ADIRU had failed at the time of the incident and another had been inoperable since July, 2001.
The software error meant the unit did not pick up that the accelerometer that had failed in 2001 was unserviceable. It then allowed faulty data from it to be used by the flight computer.
Ms Verdier said Honeywell had investigated how the error occurred and issued new software in November.
"It's in the process of going on airplanes now if they're not completed already," she said.
