PLT OF AN SMA SEL DEVIATED FROM ASSIGNED ALT DUE TO A FROZEN OUTSIDE STATIC SOURCE CAUSING ERRONEOUS ALT AND AIRSPD INDICATIONS.

Narrative

ENRTE FROM RICHMOND; VA (RIC) TO NASHUA; NH (ASH) AT FL200 RECEIVED AN AMENDED ROUTING FROM CTR AND WAS HEAD DOWN FOR A FEW MINS REPROGRAMMING THE NAV RADIOS AND LOOKING OVER THE RTE. I PERCEIVED A SOUND QUALITY CHANGE IN THE ACFT AND LOOKED UP TO DISCOVER AN INDICATED AIRSPD AROUND 60 KTS AND A 5 DEG PITCH UP ON THE ADI. PWR AND ENG GAUGES WERE NORMAL. I DISENGAGED THE AUTOPLT; LEVELLED THE ACFT OFF. AIRSPD SLOWLY INCREASED (BUT NOT MUCH) AND INDICATED ALT WAS STILL 20000 FT. SUSPECTING STATIC BLOCKAGE; I SELECTED ALTERNATE STATIC AND IMMEDIATELY THE AIRSPD JUMPED TO 140 KTS AND THE ALTIMETER SPUN UP TO 21000 FT. INITIATED AN IMMEDIATE DSCNT AND BEFORE I COULD CALL CTR; THEY CALLED ME. I EXPLAINED THE PROB AND HEARD NOTHING MORE ABOUT IT. I BELIEVE MOISTURE CONDENSED IN THE STATIC LINES AS I CLBED; AND FROZE AT ALT. THE OUTSIDE AIR TEMP AT THE TIME WAS MINUS 25 DEGS C. THE PREFLT STATIC DRAIN HAD RESULTED IN NO WATER. HAD THE BLOCKAGE NOT OCCURRED DURING MY 'HEAD DOWN' PERIOD I THINK I WOULD HAVE NOTICED THE PROB PRIOR TO AN ALTDEV. INTERESTINGLY; THIS FLT WAS THE SECOND LEG OF A 6 HR FLT. THE FIRST LEG THAT DAY WAS 3.5 HRS AT FL200 WITH NO PROBS OF ANY KIND. THE STATIC BLOCKAGE OCCURRED ABOUT 1.5 HOURS INTO THE SECOND LEG; IN CLR AIR; WITH THE ONLY WX ENCOUNTERED BEING A LAYER FROM 8000 FT TO 13000 FT DURING CLBOUT FROM RICHMOND.