GA SMA EXPERIENCED RUNAWAY STABILIZER TRIM AND INCURRED ALT EXCURSION.

Narrative

I WAS ASSIGNED 8000' BY ATC. I EXPERIENCED A RAPID DSCNT (GREATER THAN 1500 FPM). ATC WAS INFORMED IMMEDIATELY AND CORRECTIVE ACTION WAS TAKEN. THE PROB WAS A RUNAWAY TRIM SWITCH WHICH TRIMMED THE PLANE TO FULL NOSE DOWN IN A FEW SECS OVERRIDING THE ALT HOLD AND CAUSING THE CONDITION. THE PLANE NOSED OVER AND ABOUT 600' (AT LEAST; I WAS TOO BUSY TO NOTICE) OF ALT WAS LOST BEFORE THE PLANE WAS RETRIMMED AND PROCEEDED TO CLB BACK TO 8000' (ASSIGNED). ATC SAID 'NO PROB' AND ASKED TO BE INFORMED WHEN 8000' WAS REACHED. DURING THOSE FEW MOMENTS (20 SECS APPROX) I DISCONNECTED THE ALT SYS AND SCANNED FOR THE PROB. WITH FULL DOWN TRIM IT WAS IMPOSSIBLE TO STOP THE DSCNT BY PULLING BACK ON THE YOKE. UPON SPOTTING THE POS ON THE TRIM INDICATOR I TRIED USING THE ELECTRIC TRIM UP; WHICH WAS NOW OPERATIONAL; SUCCESSFULLY. THE FLT PROCEEDED NORMALLY AFTER THIS EVENT AND THE TRIM WAS OPERATING PROPERLY. THE TRIM INDICATOR IS BELOW KNEE LEVEL AND IS NOT USUALLY PART OF THE REGULAR SCAN OF THE INSTRUMENTS; MAKING THIS CONDITION HARD TO SPOT INITIALLY; EVEN THOUGH I SAW THE PLANE DSNDING AND TRIED TO PULL UP.