Small aircraft encounters turbulence and possible icing resulting in loss of airspeed and altitude.

Attributes
ACN	271806
Time
Date	199405
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : oal
State Reference	NV
Altitude	msl bound lower : 10600 msl bound upper : 11000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Operator	general aviation : personal
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Route In Use	enroute airway : zoa
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument
Experience	flight time last 90 days : 20 flight time total : 750 flight time type : 20
ASRS Report	271806
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : weather non adherence : clearance
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	none taken : anomaly accepted other
Consequence	Other
Supplementary
Primary Problem	Weather
Air Traffic Incident	other

Narrative:

Flight encountered light to moderate turbulence from grand canyon to reno, entire route with some isolated smooth air just north of las vegas. Approximately midway between las vegas and coaldale (oal) VOR cold front was encountered. Flight was in and out of IMC with light rime ice being picked up. At time of occurrence, autoplt was set on navigation and altitude hold. Autoplt will hold plus or minus 100 ft in light turbulence. A pocket of wet heavy half rain/half snow was encountered and I was watching wings for sign of ice. Next thing I knew, airspeed showed a significant decrease and altitude was 400 ft low. (Note pilot and propeller ice on.) I initially thought pitot had froze due to heavy rain/snow. At time I noticed altitude low and corrected. Center called and reported pitot problem. Also note: airspeed returned to normal almost immediately. Though pitot plugging may have contributed to problem, I believe the actual cause was a strong downdraft, as I encountered several plus or minus 2000 FPM up- and downdrafts later in-flight. However, these did not cause altitude bust as no ice was occurring and I was watching all gauges closely. Furthermore, our aircraft is equipped with speed brakes which mitigate the effect of strong up- and downdrafts. In retrospect, had ice not been a potential and had airspeed not deviated, I would have caught deviation before 400 ft altitude loss. Lesson learned: even a good autoplt can't maintain altitude/heading in adverse conditions, and having it on is not a reason to lessen vigilance of flight instructors.

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.