Autopilot Disconnect During Cruise
A Boeing 787 experiences an uncommanded autopilot disconnect during cruise at FL350. The flight crew reports that the autopilot disengaged and the aircraft pitched up slightly. The crew re-engaged the autopilot successfully. Maintenance downloads the fault codes and finds a message indicating a discrepancy between the two air data computers (ADCs) for altitude. The aircraft has three ADCs. The AME must troubleshoot the issue.
1. What are the possible causes of the autopilot disconnect and how should the AME proceed?
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Possible causes: faulty ADC, wiring issue between ADC and autopilot, or a software anomaly. Steps: 1) Review fault codes and compare ADC outputs using built-in test equipment. 2) Perform a pitot-static system leak check. 3) Swap ADCs to see if fault follows. 4) Inspect wiring and connectors. 5) If no fault found, update software if applicable. 6) Perform autopilot system test.
2. Explain how the autopilot uses ADC data and why a discrepancy could cause a disconnect.
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The autopilot uses ADC data for altitude, airspeed, and vertical speed to maintain flight path. If ADCs disagree, the autopilot may disconnect to prevent unsafe control inputs. The system compares data from multiple ADCs and if the difference exceeds a threshold, it triggers a disconnect. This is a fail-safe design.