PLT TRAINING CREW OF A C172; LOST CORRECT INDICATIONS FROM THE AIRSPD; VERT SPD AND ALTIMETER CAUSING DSCNT BELOW ASSIGNED ALT DUE TO PITOT STATIC TUBE ICING. ALL INDICATIONS CAME BACK CORRECTLY WITHIN A COUPLE OF MINS AFTER THE PITOT TUBE HEAT WAS APPLIED.

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REGARDING: ALT EXCURSION BELOW ASSIGNED ALT. CONDITIONS OF FLT: 1 HR OF CONSTANT FLT IN VERY WET CLOUDS AND INTERMITTENT RAIN. OUTSIDE AIR TEMP APPROX 10 DEGS C. STUDENT IFR INSTRUCTIONAL FLT. TYPE: C172 1997 MODEL. DSNDING FROM 6000-5000 FT VECTORS TO JOIN THE LOC FOR AN ILS APCH. RPTED WX WAS ABOUT 300 FT ABOVE DECISION ALT AND 3 MI. ACTUAL VISIBILITY TURNED OUT TO BE ABOUT 1-1/2 MI. DURING THE DSCNT WE EXPERIENCED COMPLETE FAILURE OF THE PITOT STATIC SYS. THE AIRSPD INDICATOR WAS INDICATING ABOVE RED LINE. THE ALTIMETER WAS FROZEN AT 5600 FT. THE VERT SPD INDICATOR WAS RETURNING TO A LEVEL FLT ATTITUDE. WE IMMEDIATELY PULLED THE ALTERNATE AIR CTL AND PUT ON PITOT HEAT EVEN THOUGH THE OUTSIDE AIR TEMP WAS AT 10 DEGS C. IT HAD NO IMMEDIATE EFFECT ON THE PITOT STATIC FAILURE. AS WE STARTED TO TROUBLESHOOT THE CONDITION AND WE LEVELED THE ACFT USING THE ATTITUDE INDICATOR AND NOTIFIED ATC THAT WE HAD A MAJOR PROB. WE WERE NOW BELOW 5000 FT. THE CTLR THEN CLRED US TO 4100 FT. WE WERE ABLE TO APPROXIMATE OUR ALT USING THE VISUAL READING OF PRESSURE ALT SHOWN ON THE XPONDER (BENDIX KING). AFTER ABOUT 2 MINS ALL INSTS RETURNED TO NORMAL. ATC CONFIRMED OUR ALT AT 4100 FT. WHAT PROBABLY HAPPENED WAS THAT THE STATIC LINE HAD FILLED WITH WATER. AFTER WE OPENED THE ALTERNATE STATIC PORT THE WATER PROBABLY STARTED TO DRAIN WITH THE HELP OF THE CHANGING PITCH OF THE ACFT. AFTER THE FLT WE NOTICED THAT THE CARPET ON THE FLOOR WAS SOAKING WET. AFTER THIS INCIDENT I RECALLED; WHILE FLYING OLDER MODELS OF C172'S; THAT THERE HAD BEEN TIMES WHEN I HAD BEEN FLYING IN WET CLOUDS THAT THE PITOT STATIC INSTS WOULD OCCASIONALLY JUMP. I ATTRIBUTED THAT TO THE STATIC PORT BEING COVERED WITH WATER. I PROBABLY WAS WRONG. IT PROBABLY WAS AN INGESTION OF WATER INTO THE STATIC LINE. I AM NOT A MECH AND I'M ONLY GUESSING BASED ON MY OBSERVATIONS. WE FLEW BACK WITH THE ALTERNATE STATIC PORT OPEN AND THE PITOT HEAT ON. THE INSTS OPERATED CORRECTLY.