Narrative:

I was to pick up an aircraft that had been in service at ZZZ. The chief maintenance task involved the installation of a new GPS (gtn 750) and its coupling to the already installed autopilot (stec-65). The task proved more difficult than expected; and the autopilot exhibited persistent malfunctions; which greatly extended period of maintenance (about 5 months). Immediately after completion of avionics work; the aircraft underwent annual inspection. I was then to take possession of the aircraft and fly it to ZZZ1. Before departure; I experienced delays in the readying of the aircraft for release by the company that provided the aircraft inspector. Among possible stressors was a weather system to the east of ZZZ1. The system was not expected to affect the flight to ZZZ1 but was anticipated to delay a subsequent flight out of ZZZ1. Before takeoff; I had been briefed on the standard VFR transition by ATC; and my read-back was declared 'correct'. I also reported that I possessed a paper copy of the transition. To warm up to the flight and the altered layout of the panel; I performed a lap around the traffic pattern. I gained comfort in the traffic pattern by the time I had reached the base leg. I then descended normally to about 300 MSL on final before exercising the option to overfly the runway at ZZZ1 and continue via the published transition.the initial segment of the transition was over water with a subsequent turn inland for over-flight of the tower at ZZZ. The initial climb was to 1500 MSL; at which point I engaged the altitude hold function of the autopilot while being asked to contact approach. I confirmed that I would contact approach; and as I recall; I briskly switched to approach frequency. However; the autopilot commanded am unsettling nose-down trim; which I countered with back-pressure on the yoke. Given the recent service to the avionics; I considered the possibility of an autopilot malfunction. So; I then disengaged the autopilot and trimmed nose-up. This action; perhaps in concert with a near-shore updraft; precipitated a climb above 1500 MSL. I arrested the climb at about 1750 MSL; at which point communications commenced with approach. Approach asked me to report my altitude. I was flustered and struggled to clearly articulate the altitude of 1750 [MSL]. Approach conveyed that I had made a 'late call'. I explained that I had experienced a trimming problem. The controller advised that I could have reported the problem earlier and would have been offered assistance. I am grateful for the concern demonstrated by approach. The remainder of the flight was uneventful; and all equipment performed well. I conclude that I may have trimmed the aircraft poorly before setting the altitude hold and then over-reacted to the compensatory adjustments that the autopilot commanded. The brief climb occupied my attention and disturbed my communications at a critical time. Fortunately; air traffic in the area was apparently light; and conflicts did not result.factors contributing to the incident:1. Significant flight-free period during which work on avionics and other systems was performed (about 7 hours of long cross-country during the 5-month maintenance period and 0 hours during the prior three months).2. Unfamiliarity with the appearance and functional state of avionics (some newly installed) and a sudden concern regarding a frank failure of the autopilot.3. Unfamiliarity with airspace and environmental conditions.concerning corrective/preventive actions; (at a minimum) I plan to:1. Fly with an instructor after significant aviation-free periods and/or changes in equipment critical to aircraft control.2. Practice/brief procedures in complicated unfamiliar airspaces under supervision before attempting the procedures autonomously.although ATC categorized my overt transgression as a 'late call' rather than as a 'violation;' I feel that it is important to report my experience for the benefit of the database and those whose safety it may impact. I expect that the exercise of recalling; analyzing and reporting the events will also benefit my skill-building and aeronautical decision-making.

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Original NASA ASRS Text

Title: C182 pilot reported an autopilot malfunction that led to an excursion from altitude and late call to ATC.

Narrative: I was to pick up an aircraft that had been in service at ZZZ. The chief maintenance task involved the installation of a new GPS (GTN 750) and its coupling to the already installed autopilot (STEC-65). The task proved more difficult than expected; and the autopilot exhibited persistent malfunctions; which greatly extended period of maintenance (about 5 months). Immediately after completion of avionics work; the aircraft underwent annual inspection. I was then to take possession of the aircraft and fly it to ZZZ1. Before departure; I experienced delays in the readying of the aircraft for release by the company that provided the aircraft inspector. Among possible stressors was a weather system to the east of ZZZ1. The system was not expected to affect the flight to ZZZ1 but was anticipated to delay a subsequent flight out of ZZZ1. Before takeoff; I had been briefed on the standard VFR transition by ATC; and my read-back was declared 'correct'. I also reported that I possessed a paper copy of the transition. To warm up to the flight and the altered layout of the panel; I performed a lap around the traffic pattern. I gained comfort in the traffic pattern by the time I had reached the base leg. I then descended normally to about 300 MSL on final before exercising the option to overfly the runway at ZZZ1 and continue via the published transition.The initial segment of the transition was over water with a subsequent turn inland for over-flight of the Tower at ZZZ. The initial climb was to 1500 MSL; at which point I engaged the altitude hold function of the autopilot while being asked to contact Approach. I confirmed that I would contact Approach; and as I recall; I briskly switched to Approach frequency. However; the autopilot commanded am unsettling nose-down trim; which I countered with back-pressure on the yoke. Given the recent service to the avionics; I considered the possibility of an autopilot malfunction. So; I then disengaged the autopilot and trimmed nose-up. This action; perhaps in concert with a near-shore updraft; precipitated a climb above 1500 MSL. I arrested the climb at about 1750 MSL; at which point communications commenced with Approach. Approach asked me to report my altitude. I was flustered and struggled to clearly articulate the altitude of 1750 [MSL]. Approach conveyed that I had made a 'late call'. I explained that I had experienced a trimming problem. The controller advised that I could have reported the problem earlier and would have been offered assistance. I am grateful for the concern demonstrated by Approach. The remainder of the flight was uneventful; and all equipment performed well. I conclude that I may have trimmed the aircraft poorly before setting the altitude hold and then over-reacted to the compensatory adjustments that the autopilot commanded. The brief climb occupied my attention and disturbed my communications at a critical time. Fortunately; air traffic in the area was apparently light; and conflicts did not result.Factors contributing to the incident:1. Significant flight-free period during which work on avionics and other systems was performed (about 7 hours of long cross-country during the 5-month maintenance period and 0 hours during the prior three months).2. Unfamiliarity with the appearance and functional state of avionics (some newly installed) and a sudden concern regarding a frank failure of the autopilot.3. Unfamiliarity with airspace and environmental conditions.Concerning corrective/preventive actions; (at a minimum) I plan to:1. Fly with an instructor after significant aviation-free periods and/or changes in equipment critical to aircraft control.2. Practice/brief procedures in complicated unfamiliar airspaces under supervision before attempting the procedures autonomously.Although ATC categorized my overt transgression as a 'late call' rather than as a 'violation;' I feel that it is important to report my experience for the benefit of the database and those whose safety it may impact. I expect that the exercise of recalling; analyzing and reporting the events will also benefit my skill-building and aeronautical decision-making.

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.