Small transport ices up, autoplt goes into oscillation and aircraft loses 300'.

Attributes
ACN	91929
Time
Date	198808
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : odi
State Reference	MN
Altitude	msl bound lower : 18700 msl bound upper : 19000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zmp
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 1700 flight time type : 960
ASRS Report	91929
Person 2
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude inflight encounter : weather
Independent Detector	other flight crewa
Resolutory Action	flight crew : exited adverse environment
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Was flying at FL190. Entered clouds, ragged clear ice formed rapidly. As I booted ice off autoplt (and aircraft) started to oscillate. I asked center for lower, met with 'stand by,' asked for an immediate descent (had lost about 200-300' by this time). Center cleared me to 15000'. Ice melted rapidly through 16000'. I was not aware of the problem with the autoplt until I disengaged it for the descent. Autoplt worked fine after ice melted. At the time the aircraft oscillation began, I had pitot, static and propeller deice on. I was a bit unnerved by the aircraft's action, so I didn't think to uncouple the autoplt when the motion started. First time I've ever experienced this type of event. A good lesson for the future is ice forms as fast in the summer at altitude as it does in the winter.

Original NASA ASRS Text

Title: SMT ICES UP, AUTOPLT GOES INTO OSCILLATION AND ACFT LOSES 300'.

Narrative: WAS FLYING AT FL190. ENTERED CLOUDS, RAGGED CLEAR ICE FORMED RAPIDLY. AS I BOOTED ICE OFF AUTOPLT (AND ACFT) STARTED TO OSCILLATE. I ASKED CENTER FOR LOWER, MET WITH 'STAND BY,' ASKED FOR AN IMMEDIATE DSCNT (HAD LOST ABOUT 200-300' BY THIS TIME). CENTER CLRED ME TO 15000'. ICE MELTED RAPIDLY THROUGH 16000'. I WAS NOT AWARE OF THE PROB WITH THE AUTOPLT UNTIL I DISENGAGED IT FOR THE DSCNT. AUTOPLT WORKED FINE AFTER ICE MELTED. AT THE TIME THE ACFT OSCILLATION BEGAN, I HAD PITOT, STATIC AND PROP DEICE ON. I WAS A BIT UNNERVED BY THE ACFT'S ACTION, SO I DIDN'T THINK TO UNCOUPLE THE AUTOPLT WHEN THE MOTION STARTED. FIRST TIME I'VE EVER EXPERIENCED THIS TYPE OF EVENT. A GOOD LESSON FOR THE FUTURE IS ICE FORMS AS FAST IN THE SUMMER AT ALT AS IT DOES IN THE WINTER.

Data retrieved from NASA's ASRS site as of August 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.