SMA ENCOUNTERS TURB AND POSSIBLE ICING RESULTING IN LOSS OF AIRSPD AND ALT.

Narrative

FLT ENCOUNTERED LIGHT TO MODERATE TURB FROM GRAND CANYON TO RENO; ENTIRE RTE WITH SOME ISOLATED SMOOTH AIR JUST N OF LAS VEGAS. APPROX MIDWAY BTWN LAS VEGAS AND COALDALE (OAL) VOR COLD FRONT WAS ENCOUNTERED. FLT WAS IN AND OUT OF IMC WITH LIGHT RIME ICE BEING PICKED UP. AT TIME OF OCCURRENCE; AUTOPLT WAS SET ON NAV AND ALT HOLD. AUTOPLT WILL HOLD PLUS OR MINUS 100 FT IN LIGHT TURB. A POCKET OF WET HVY HALF RAIN/HALF SNOW WAS ENCOUNTERED AND I WAS WATCHING WINGS FOR SIGN OF ICE. NEXT THING I KNEW; AIRSPD SHOWED A SIGNIFICANT DECREASE AND ALT WAS 400 FT LOW. (NOTE PLT AND PROP ICE ON.) I INITIALLY THOUGHT PITOT HAD FROZE DUE TO HVY RAIN/SNOW. AT TIME I NOTICED ALT LOW AND CORRECTED. CTR CALLED AND RPTED PITOT PROB. ALSO NOTE: AIRSPD RETURNED TO NORMAL ALMOST IMMEDIATELY. THOUGH PITOT PLUGGING MAY HAVE CONTRIBUTED TO PROB; I BELIEVE THE ACTUAL CAUSE WAS A STRONG DOWNDRAFT; AS I ENCOUNTERED SEVERAL PLUS OR MINUS 2000 FPM UP- AND DOWNDRAFTS LATER INFLT. HOWEVER; THESE DID NOT CAUSE ALT BUST AS NO ICE WAS OCCURRING AND I WAS WATCHING ALL GAUGES CLOSELY. FURTHERMORE; OUR ACFT IS EQUIPPED WITH SPD BRAKES WHICH MITIGATE THE EFFECT OF STRONG UP- AND DOWNDRAFTS. IN RETROSPECT; HAD ICE NOT BEEN A POTENTIAL AND HAD AIRSPD NOT DEVIATED; I WOULD HAVE CAUGHT DEV BEFORE 400 FT ALT LOSS. LESSON LEARNED: EVEN A GOOD AUTOPLT CAN'T MAINTAIN ALT/HDG IN ADVERSE CONDITIONS; AND HAVING IT ON IS NOT A REASON TO LESSEN VIGILANCE OF FLT INSTRUCTORS.