Date: 202202
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Small Transport; Low Wing; 2 Turboprop Eng
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Final Approach
Airspace: Class D ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Autopilot
Component problem: Malfunctioning
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Distraction; Confusion
Second reporter function: Single Pilot; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Overshoot; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Overcame Equipment Problem; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Executed Go Around / Missed Approach; Flight Crew Became Reoriented; Flight Crew Overrode Automation; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Airspace Structure; Human Factors; Procedure; Software and Automation
Primary problem: Aircraft

Narrative: Aircraft X was being vectored for the RNAV Y Runway XXL approach in the downwind. I descended them to 032 and told them to turn right heading 250; they descended but didn't turn; and I had to reissue the turn immediately to avoid a higher MVA. Then when being told to turn right and maintain 032 cleared for the approach; Aircraft X began a descent; and I immediately issued a low altitude alert and told them to climb back to 3200 ft. I believe they got all the way down to 2400 ft. in a 032 MVA. When asked what happened Aircraft X said they were having avionics issues; some new function on his GPS wasn't working correctly. After they climbed back to 032; I recleared them with a turn and told them again to maintain 3200 ft. which they again descended below the MVA so I issued another low altitude alert. This happened 1 or 2 more times before I decided to just climb them to 5000 ft.; and began handing off the position to a new Controller; while briefing the new controller I noticed Aircraft X climbed through 5000 ft. so I issued them 6000 ft. The pilot was issued a possible pilot deviation once safely on the ground. I can't think of any [recommendations] since it seems to have been an aircraft equipment problem.


Second narrative: A new advanced digital autopilot was recently installed. Test flights had been done with a safety pilot and technician aboard; and all issues resolved. After the paperwork had been signed off; I was informed that Garmin had enabled a new functionality for this aircraft. VNAV; designed to fly a vertical profile from cruise to glideslope or path intercept; including stepdowns. This function was not test flown; but I later did read the STC (Supplemental Type Certificate) and pilot's handbook descriptions of the procedures.On the day in question; the flight had gone smoothly; with the new autopilot functioning perfectly. The conditions aloft were CAVU; but with a thin stratus layer over the destination in the Willamette Valley. RNAV Y XXL was in use; with VMC below the stratus. Given the conditions; I decided to try the new VNAV functionality of the autopilot. Approach gave me vectors and stepdown altitudes; and all seemed well at first; however; the autopilot did not capture the expected altitude at the turn to base. I attempted to re-select the correct altitude; but the autopilot continued a descent. When the Approach Controller called about the altitude; I used the TOGA (takeoff/go-around) switch to command an immediate climb; but the response was rather sluggish. When the Controller then issued a low-altitude alert I disconnected the autopilot; and climbed manually. All this in VMC above the stratus layer over the airport environment; with terrain in sight around the valley perimeter at all times; and the TAWS never alerted. After asking for a delay vector; I attempted the procedure again; with the same results. I then flew the procedure manually; and landed uneventfully. Ground Control asked me to call the Supervisor; which I did; explaining the situation.In hindsight; it would have been better to first fly the new functionality in a training environment; and not at the end of a cross-country on an IFR flight plan. Also; after the autopilot unexpectedly did not capture the stepdown altitude; I should have just taken over manually and entirely immediately; instead of continuing to attempt to adjust the autopilot; which just snowballed. My rationale for persisting was that the new functionality would have to be figured out sometime; and considering the weather; this was a good time to do it (VMC until the FAF; plenty of fuel left). I would decide differently in the future.


Synopsis: A TRACON Controller and pilot reported the aircraft's new auto pilot malfunctioned while being vectored for an approach causing the aircraft to fly below the minimum vectoring altitude.