Shortly after reaching cruise at 36,000 feet, the commander’s attitude indicator malfunctioned affecting numerous aircraft systems, and the aircraft climbed 600 feet. After a significant time delay an inertial reference system (IRS) caution was displayed. The IRS calculates an aircraft’s position. The crew followed the actions detailed in the quick reference handbook, but erroneous information continued to be displayed to the pilot in command and other systems were also affected. The aircraft was flown manually to Edinburgh where it landed safely.
The investigation found that the left IRS suffered a transient fault which had led to an erroneous calculation of position. False position information led to the incorrect attitude information on the commander’s primary flight display and the autopilot responded by initiating a slow climb.
One Safety Recommendation is made to Boeing and safety action has been taken as a result of the investigation.