A sa-227 with the stec-50 autopilot made an uncommanded climb and turn with heading and altitude mod...

Narrative:

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 'heading' and 'altitude' 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1301054
Time
Date	201510
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	SA-227 AC Metro III
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Autopilot
Person 1
Function	Captain Pilot Flying
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural FAR Deviation - Track / Heading All Types

A sa-227 with the stec-50 autopilot made an uncommanded climb and turn with heading and altitude mode engaged annunciated. Later; the autopilot would deviate ± 100' without a trim up or trim down alert.