FROZEN STATIC LINE CAUSES SMT TO CLB 600 FT ABOVE ASSIGNED ALT WITH THE FLC UNAWARE.

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ON THE GND I HAD A FROZEN SPOT SOMEWHERE IN THE STATIC SYS. I TURNED ON THE PITOT HEAT WHICH SOLVED THE PROB TEMPORARILY ENRTE AT 8000 FT; I THOUGHT I WOULD TRY THE ALTERNATE STATIC TO CHK OUT THE SYS. AT THE TIME I CHKED IT EVERYTHING (ALTIMETER; AIRSPD; AND VERT SPD INDICATOR) WERE NORMAL. ON THE ALTERNATE STATIC (ALTIMETER; AIRSPD; AND VERT SPD INDICATOR) JUMPED LIKE NORMAL WHEN SWITCHING. THEN I WENT BACK TO THE NORMAL STATIC. A FEW SECONDS LATER; THE AIRSPD WAS 40 KTS SLOWER. THEN CTR ADVISED ME I WAS 300 FT LOW. I ASKED THEM IF THEY WANTED ME TO STOP MY ALT SQUAWK. THEY REPLIED NO. AT THE TIME I WAS SHOWING 80 KTS EXACTLY. THEN I SWITCHED TO THE ALTERNATE STATIC AND THE AIRSPD WENT BACK TO NORMAL; BUT THE ALTIMETER JUMPED TO 8600 FT. I ASKED CTR WHAT ALT THEY SHOWED; BECAUSE I DIDN'T KNOW WHICH ONE TO BELIEVE. CTR REPLIED 8400 FT. THEN I TOLD THEM ABOUT MY STATIC PROB. THEY WERE NOT TOO CONCERNED ABOUT ALT PROB AND DID NOT ASK ME TO DO ANYTHING DIFFERENT. I LEARNED THAT IF THE STATIC SYS IS WORKING JUST FINE; BUT YOU HAD A PROB ON THE GND BEFORE TKOF; 'DON'T TOUCH IT.'