A PIPER PA46 PLT HAD UNUSUAL STATIC FLT INST INDICATIONS OVER GRV VOR.

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CRUISING E ON V92; 17000 FT MSL; 145 IAS; OUTSIDE AIR TEMP -15 DEGS C WINDS ALOFT OUT OF NW ABOUT 30 KTS. WX WAS CLR; VISIBILITY IN EXCESS OF 10 NM. GS OF 210 KTS (GPS AND RNAV). DARKNESS HAD ARRIVED AND I WAS ALMOST ON TOP OF GRANTSVILLE VOR WHERE I WAS GOING TO REQUEST A DSCNT FROM 17000 FT FROM ZOB. AT THIS POINT; I NOTICED MY AIRSPD HAD DECREASED TO 105 KIAS. I XCHKED THE OTHER INSTS AND ALL WERE NORMAL. GS ON THE GPS WAS 208. I SUSPECTED ICING AND ACTIVATED THE PITOT HEAT; TURNED ON THE OUTSIDE LIGHT AND VISUALLY CHKED THE WINGS FOR ICE AND THERE WAS NONE. THE AIRSPD CONTINUED TO DECREASE TO 90 KTS AND I WONDERED IF I WAS EXPERIENCING A MOUNTAIN WAVE; BUT REACHED FOR THE ALTERNATE STATIC SWITCH WHICH IS LOCATED ABOVE MY L KNEE. AS I ACTIVATED THE SWITCH TO ALTERNATE STATIC SOURCE; THE ACFT NOSED OVER; THE AUTOPLT DISCONNECTED AND THE WARNING HORNS SOUNDED; THE AIRSPD CAME UP TO 150 KTS; THE ALT INCREASED AND I WITHDREW MY HAND TO MANUALLY ASSURE THE AUTOPLT WAS DISCONNECTED VIA THE DISCONNECT SWITCH AND ASSUMED MANUAL CTL OF THE ACFT. NOTING THAT THE ALT WAS 700 FT HIGH; I PUSHED THE NOSE OVER AND THE AIRSPD RAPIDLY WENT UP TO 180 KTS BUT NO CORRESPONDING ALT CHANGE OCCURRED. ABOUT THIS TIME; ZOB CALLED TO TELL ME I WAS 600 FT HIGH AND I TOLD THEM I WAS EXPERIENCING A STATIC PROB AND WAS CORRECTING. I RECHKED THE STATIC ALTERNATE SWITCH AND FOUND THAT I HAD REPOSITIONED IT TO PRIMARY POS AS I HAD PULLED MY HAND OUT FROM THE SWITCH TO DISCONNECT THE AUTOPLT. THE ALTERNATE STATIC WAS AGAIN SELECTED AND THE AIRSPD CAME BACK TO CRUISE SPD AND THE ALT DROPPED TO 17000 FT. ZOB ADVISED ME I WAS BACK ON ALT AND SWITCHED ME TO ZDC. CTR STARTED ME DOWN AT THAT TIME AND THE REST OF THE FLT WAS NORMAL. AT 3000 FT; I SELECTED BACK AND FORTH FROM SECONDARY TO PRIMARY FOR THE STATIC SYS AND THE READINGS WERE OK. I LANDED ON THE PRIMARY SYS WITHOUT INCIDENT. BACK ON THE GND; I TRIED TO DETERMINE WHAT HAD OCCURRED BY REFERRING TO THE SYS MANUAL AND TALKING THIS OVER WITH OTHERS. MY BEST DETERMINATION IS THAT THERE MUST HAVE BEEN A BLOCKAGE IN THE PRIMARY STATIC SOURCE AND IT WAS PROBABLY MOISTURE SINCE THE INDICATIONS DISAPPEARED IN THE DSCNT. I HAVE MULLED OVER WHY IT DIDN'T FREEZE SOONER SINCE I HAD BEEN AT ALT FOR AROUND 2 HRS. PERHAPS IT DIDN'T REACH CRITICAL TEMPS UNTIL DARKNESS ARRIVED AND I HAD LOST THE SUN AS A HEAT SOURCE. I HAD DRAINED THE STATIC SYS THAT MORNING AND NEVER ENCOUNTERED ANY RAIN. HOWEVER; PERHAPS SITTING ON THE RAMP NEAR THE WATERS OF THE LAKE INTRODUCED SOME MOISTURE INTO THE SYS. WE FLEW THE ACFT ON OCT/XA/99; CLBED TO 10500 FT; UTILIZED BOTH STATIC SOURCES AND COULD NOT DUPLICATE THE EVENT. IN THE PAST; I HAVE EXPERIENCED PITOT BLOCKAGES DURING TKOF; AIRSPD DECREASES FROM ICING AND MOUNTAIN WAVE. HOWEVER; THIS IS MY FIRST EXPERIENCE OF A MALFUNCTION LIKE THIS. AS AN INST PLT; I HAVE TO BELIEVE MY INSTS AND IT IS NOT EASY TO DECIDE THAT THE INST MIGHT BE THE CULPRIT OR THE SOURCE FOR THE INSTS. LOGIC DOESN'T ALWAYS WORK WHEN THE REFS ARE TELLING YOU SOMETHING ELSE. THE MORAL IS BELIEVE YOUR INSTS UNTIL YOU DECIDE THEY ARE WRONG. I'M GLAD THIS DIDN'T OCCUR UNDER OTHER THAN IDEAL CONDITIONS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR EXPERIENCED 1 MORE ERRATIC FLT INST INDICATION AND HAD THE STATIC SYS CHKED. A PRESSURE CHK FOUND THE SYS TO BE IN EXCELLENT CONDITION BUT WAS BLOWN OUT AS A PRECAUTION. THERE HAVE BEEN NO FURTHER FALSE INDICATIONS SINCE THAT TIME.