Confusion; Workload; Troubleshooting; Distraction
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural FAR
Air Traffic Control Issued Advisory / Alert; Aircraft Automation Overrode Flight Crew; Flight Crew FLC Overrode Automation; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Human Factors; Procedure; Aircraft
While cruising in flight with the autopilot on (STEC-50 model); heading and altitude mode were selected. I pulled out the aircraft AML to perform the trend checks; in addition to organizing all of my respective paperwork/jepp charts. Only a minute or two had passed (as I was just completing my paperwork); but when I glanced up to fully rescan my instruments; the aircraft was in an usual attitude: the nose was slightly highly; and the aircraft was banking to the right approximately 30 degrees. The aircraft had deviated 200 feet higher than my assigned altitude of 12;000ft; and had deviated course by 160 degrees. I immediately disconnected the autopilot via the control yoke button; and corrected the unusual attitude with pitch and bank. While correcting; I notified ATC of the deviation: Center re-cleared me to a heading of 320; heading direct when able to my next Nav-Aid; and re-assigned 12;000ft for my altitude.One thing that I saw upon recognizing the unusual attitude; was also that the autopilot status lights still showed the unit was engaged: The green active light was on; in addition to the 'HDG' and 'ALT' modes engaged (via their amber annunciators). In monitoring the autopilot for the remainder of the flight; I noted that the altitude would not uncommonly deviate by as much as 100ft; yet the autopilot would not give the 'trim up' or 'trim down' annunciations. When completing paperwork and organizing charts; devote more time to quick instrument scans to avoid automation.
A SA-227 with the STEC-50 autopilot made an uncommanded climb and turn with HDG and ALT mode engaged annunciated. Later; the autopilot would deviate ± 100' without a TRIM UP or TRIM DOWN alert.