#1 PITOT SYSTEM FAILED ACCOUNT INADEQUATE PITOT HEAT. ALT DEVIATION 1500'.

Narrative

CLBING THROUGH FL220 THE LEFT SIDE AIRSPD DECREASED ABNORMALLY WHICH WE ATTRIBUTED TO PITOT SYS ICE EVEN THOUGH ALL THE HEAT WAS ON AND OPERATING. THE RIGHT SIDE AIRSPD WAS OPERATING NORMALLY AND WE CLBED TO FL390 ON AUTOPLT AND LEVELED AND CRUISED THERE FOR ABOUT 20 MINS. BY THIS TIME THE LEFT AIRSPD INDICATED 0 AND WE WERE OVER AN AREA OF TSTMS WITH LIGHT TURB. THE AIRPLANE BEGAN A SLOW OSCILLATION IN PITCH. I INSTRUCTED THE LEFT SEAT PLT TO WAIT AND SEE IF THE AUTOPLT WOULD DAMP THE OSCILLATION ON ITS OWN BEFORE TAKING OVER MANUALLY. WHEN IT BECAME OBVIOUS THE OSCILLATIONS WERE INCREASING HE ASKED ME TO TAKE THE AIRPLANE. I TOOK THE YOKE AND ASKED HIM TO RETARD PWR AND DISCONNECT THE AUTOPLT. BY THE TIME I HAD THE AIRPLANE BACK IN LEVEL FLT AT FL390 WE HAD DIPPED TO FL380 AND CLBED TO FL395 AND APCHED MMO. THE FAILED AIRSPD INDICATOR WAS CAUSING THE AUTOPLT TO MALFUNCTION AND ON DELAY IN DISCONNECTING IT CAUSED A 1500' ALT EXCURSION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: WHEN THE ACFT WAS DSNDED TO WARMER AIR DURING APCH THE #1 AIRSPD INDICATOR RETURNED TO NORMAL OPERATION. WATER WAS FOUND IN THE SYS. PITOT HEAT CHKED NORMAL ON THE GND. HAS RECEIVED NO FURTHER INFO FROM MAINT ON THE PROB. ANALYST ADVISED RPT HE HAD A SIMILAR EXPERIENCE AND THE RESULTING FINDING BY MAINT WAS A PLATE FOR MOUNTING THE PITOT HEAD TO THE AIRFRAME WAS CONDUCTING ELECTRICITY AWAY FROM THE HEAD AND CONSEQUENTLY NOT ENOUGH AMPERAGE WAS AVAILABLE FOR ADEQUATE HEATING UNDER CERTAIN CONDITIONS. RPTR WAS GOING TO ADVISE MAINT. SUPPLEMENTAL INFO FROM ACN 94829: THE AUTOPLT BEGAN MAKING MASSIVE CONTROL AND TRIM INPUTS CAUSING THE AIRPLANE TO OSCILLATE IN PITCH AND BECOME EXTREMELY DIFFICULT TO CONTROL. AS WE APCHED MMO I PULLED THE PWR BACK AS THE NEXT DOWN OSCILLATION BEGAN (HAND FLYING THE AIRPLANE BTWN .71 AND .765 IS NOT AUTH). CAUSE: WHEN THE AIRSPD (LEFT) IS 0 THE AUTOPLT GAINS ARE AT MAX AND NO FAILURE OF THE AUTOPLT IS INDICATED. THIS ACFT MUST HAVE A WARNING NOTE ADDED TO THE FLT MANUAL ASAP: 'ANY LOW OR NO AIRSPD INDICATION IN ERROR WILL CAUSE MASSIVE CONTROL CORRECTIONS WITH NO WARNING OR FAILURE INDICATION BY THE AUTOPLT.'