What was found
THE RT SIDE PITOT SYSTEM BECOMES BLOCKED, WITH THE SYMPTOM BEING THE RIGHT PFD (AND STANDBY) AIRSPEED SLOWLY DROPS TO 0. MAINTENANCE HAS CHECKED THE PROBE HEAT REPEATEDLY AND FOUND NO DEFECTS. THIS HAS HAPPENED TO ME ON AT LEAST TWO DIFFERENT PC12S. MY THEORY IS THAT WATER ENTERS THE PITOT SYSTEM AND FREEZES BEFORE IT CAN DRAIN OUT, AS THE SYSTEM RECOVERS WHEN DESCENDING THROUGH THE FREEZING LEVEL.