What happened
On 9 October 2018, a Boeing 737-8AS, registration EI-GJT, was operating a commercial passenger flight from Porto, Portugal, to Edinburgh, Scotland. While cruising at FL360, the aircraft began an uncommanded pitch-up, climbing to approximately FL3/66. This maneuver caused the airspeed to drop below minimum maneuvering speeds, triggering the autothrottle to increase thrust to 95% N1.
During this period, the commander's Primary Flight Display (PFD) began showing incorrect attitude information, including a 10-degree nose-up pitch and later a 60-degree left bank. The crew observed yellow pitch and roll comparator annunciations on both PFDs. To maintain control, the crew disconnected the autopilot and autothrottle, transitioning to manual flight using standby instruments.
Approximately ten minutes later, a Master Caution was triggered by a left IRS fault. Following the Quick Reference Handbook (QRH), the crew switched the system to ATT mode. However, the commander's PFD continued to display erroneous data. Despite the technical difficulties and unexpected handling characteristics in roll, the crew managed the approach to Edinburgh safely, landing without any injuries to the 177 passengers or 6 crew members.
The investigation
The AAIB examined the flight data recorder and cockpit voice recorder, which revealed that the left Inertial Reference System (IRS) experienced a longitudinal acceleration offset of 1.6g. This transient anomaly caused the system to calculate an incorrect position, eventually drifting east of the North Pole. This positional error was the direct cause of the erroneous attitude and heading information presented to the pilot.
Engineering analysis by the manufacturer confirmed that while a specific hardware failure could not be permanently replicated, simulating a 1.6g input to the Y-axis accelerometer produced the same drift angle test fault. The investigation also noted that the crew's ability to resolve the display issue was hindered by the existing QRH, which provided no guidance for the specific comparator annunciations encountered.