September 19, 2005 21:10 PM
Glitch Blamed For Out-Of-Control MAS Aircraft
From Neville D’Cruz
MELBOURNE, Sept 19 (Bernama) -- Pilots on a Malaysia Airlines Boeing 777 from Perth to Kuala Lumpur battled to gain control of the plane last month after an unknown computer error caused the aircraft to pitch violently and brought it close to stalling, an Australian Transport Safety Bureau report said.
It said the pilots fought to counter false information being fed into the aircraft's autopilot system and primary flight display, according to a report in The Australian newspaper.
Flight MH124 was about an hour out of Perth when the aircraft began behaving erratically.
The incorrect data from a supposedly fail-safe device caused the plane to pitch up and climb 3,000 feet (914 metres), cutting its indicated airspeed from 500km/h to 292km/h and activating a stall warning and a "stickshaker", the report said.
A stickshaker vibrates the aircraft's controls to warn the pilot he is approaching a speed at which the plane will have insufficient lift to keep flying.
The bureau report released last Friday revealed the pilot in command disconnected the autopilot and lowered the plane's nose to prevent the stall but the aircraft's automatic throttle responded by increasing the power.
The pilot countered by pushing the thrust levers to the idle position but the aircraft pitched up again and climbed 2,000 feet.
He notified air traffic control that the plane, carrying 177 passengers, could not maintain altitude and requested a descent and radar assistance for a return to Perth.
As the plane descended through 20,000 feet, the pilot reported the primary flight display appeared normal and separately tested the aircraft's two autopilot systems.
But he was forced to keep flying manually when the plane banked to the right and the nose pitched down during both tests. The pilot reported no difficulties flying the plane but noted that the automatic throttles remained armed.
As the aircraft was positioned to approach Perth, however, the flight display again gave a low airspeed warning and the auto-throttle responded by increasing thrust.
The report said the aircraft's warning system also indicated a dangerous windshear but the crew continued the approach and landed safely.
Shaken passengers remained in Perth overnight and were offered alternative flights the next day.
Investigations are focusing on faulty acceleration figures supplied by a device called the air data inertial reference unit.
The device, which was sent to manufacturer Honeywell and to US investigators for examination, collates aircraft navigation and performance data from other systems and passes the information to the primary flight computer.
The glitch prompted plane manufacturer Boeing to issue a global notice to all 777 operators alerting them to the problem.
Boeing spokesman Ken Morton told The Australian newspaper the incident had not occurred before or since. Operators have since been told to load a previous software version.
"There is a very simple test to do before you take off and that will tell you if your system has that problem or not," he is quoted as saying.
"To this point we haven't had any people coming back saying they've had faults," he said.
Morton said there were 525 777s in service and they had accumulated more than 10 million flight hours and two million landings.
"All incidents are thoroughly investigated and appropriate steps are taken to ensure the continuing safety of the in-service fleet," he said.
"Nothing is more important than that," he added.
-- BERNAMA
You're saying that after a violent pitch-up and approach to stall with the A/P, you'd try it again so that you'd have a more concise write-up for the MX folks? If my A/P shat on me like theirs apparently did, there's no way I'd re-engage it!
