SA-227 AC Metro III
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 80; Flight Crew Total 3400; Flight Crew Type 500
Workload
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control
Aircraft
When I entered the approach environment at ZZZ; Approach Control had me increase speed to 210 knots. I was then told to slow to 180 knots or less on the downwind segment. Shortly thereafter; I was given an altitude change (9000 feet to 8000 feet) and a 180 degree heading change. As I inputted all those into the altitude select and OBS; the airplane started the turn and began diving for the ground at a high rate. I disconnected the autopilot and began a recovery but not before I descended to around 7000 feet. During this time; the sink rate and pull up visual and aural warnings were going off.After landing and securing the aircraft; I called my mechanic and explained what happened. He said that when the plane was going 210 knots at that low altitude; the autopilot trimmed the airplane for that speed; which is trimmed pitched down significantly to maintain level attitude. When I slowed the plane to below 180 knots by reducing power and deploying 1/4 flaps; it further pitched the nose forward. So; when I commanded the 180 degree turn and descend; the airplane configuration along with the turbulence in the area at the time probably overloaded the autopilot servos; causing it to be unresponsive.I have never flown the plane at 210 knots in the approach environment in this airplane. I've been advised not to do that again due to the above situation possibly reoccurring.
SA-227 pilot reported entering speed changes; altitude descent; and 180 degree heading changes in rapid succession causing aircraft to nose dive and activate aural warnings.