Narrative:

Prior to departure, aircraft was deiced with type I and type iv fluid. During climb out, ailerons felt very stiff. We entered IMC approximately 3000-4000 ft and were in IMC conditions for approximately 60-90 seconds. Once on top and when the aircraft was properly trimmed, I engaged the autoplt while still in the climb. Several seconds later we received an anti-ice horn (right side) fault indication. After completing the appropriate checklist in the aircraft QRH, the captain discussed our options with company dispatch/maintenance and we decided to return to ord. The aircraft had accumulated very light rime ice in the climb. After turning back wbound approximately 3-4 mins later, we experienced an uncommanded roll to the left about 20 degrees with the autoplt engaged. We had just descended from 9000 ft to 7000 ft and we were level. During the occurrence, we were slowing from 240 KTS to 210 KTS. We lost approximately 200 ft of altitude. I recovered manually and hand flew the remainder of the flight. The controls felt normal from the recovery until landing. Callback conversation with reporter revealed the following information: the flight crew received an anti-ice horn fault. Flight crew received the fault by noting the fault light was on, chime sound, and written message from the message prompter that anti-ice had failed. This fault meant the heat to the right ailerons or rudder had failed. Flight crew contacted their dispatch and determined the best course of action would be to return to land at their departure airport. When turned back to the airport 3-4 mins later, the aircraft experienced a roll of 20 degree bank to the left. The first officer disengaged the autoplt and quickly stopped the roll. Autoplt was never re-engaged for the remainder of the flight. Manual control of aircraft was fine, there were no problems using manual flight controls. The reporter felt the icing fault light was delayed. That ice was forming before the icing fault indications illuminated. Therefore, ice had been accumulating for the previous 10- 20 min period. Pilot was asked if this could have been caused by wake turbulence. Pilot stated that it didn't feel like wake turbulence. The flight crew did ask approach who they were following. The reply was 'no one.' the cockpit voice recorder and flight data recorder were pulled from the aircraft for the following investigation with alpa, FAA, NTSB and company representatives.

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Original NASA ASRS Text

Title: AN ATR72 DEPARTING ORD, FLIES THROUGH CLOUDS TO VFR CONDITIONS. WHEN THE AUTOPLT IS ENGAGED, THE ACFT ENCOUNTERS A NON COMMANDED 20 DEG ROLL.

Narrative: PRIOR TO DEP, ACFT WAS DEICED WITH TYPE I AND TYPE IV FLUID. DURING CLBOUT, AILERONS FELT VERY STIFF. WE ENTERED IMC APPROX 3000-4000 FT AND WERE IN IMC CONDITIONS FOR APPROX 60-90 SECONDS. ONCE ON TOP AND WHEN THE ACFT WAS PROPERLY TRIMMED, I ENGAGED THE AUTOPLT WHILE STILL IN THE CLB. SEVERAL SECONDS LATER WE RECEIVED AN ANTI-ICE HORN (R SIDE) FAULT INDICATION. AFTER COMPLETING THE APPROPRIATE CHKLIST IN THE ACFT QRH, THE CAPT DISCUSSED OUR OPTIONS WITH COMPANY DISPATCH/MAINT AND WE DECIDED TO RETURN TO ORD. THE ACFT HAD ACCUMULATED VERY LIGHT RIME ICE IN THE CLB. AFTER TURNING BACK WBOUND APPROX 3-4 MINS LATER, WE EXPERIENCED AN UNCOMMANDED ROLL TO THE L ABOUT 20 DEGS WITH THE AUTOPLT ENGAGED. WE HAD JUST DSNDED FROM 9000 FT TO 7000 FT AND WE WERE LEVEL. DURING THE OCCURRENCE, WE WERE SLOWING FROM 240 KTS TO 210 KTS. WE LOST APPROX 200 FT OF ALT. I RECOVERED MANUALLY AND HAND FLEW THE REMAINDER OF THE FLT. THE CTLS FELT NORMAL FROM THE RECOVERY UNTIL LNDG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FLC RECEIVED AN ANTI-ICE HORN FAULT. FLC RECEIVED THE FAULT BY NOTING THE FAULT LIGHT WAS ON, CHIME SOUND, AND WRITTEN MESSAGE FROM THE MESSAGE PROMPTER THAT ANTI-ICE HAD FAILED. THIS FAULT MEANT THE HEAT TO THE R AILERONS OR RUDDER HAD FAILED. FLC CONTACTED THEIR DISPATCH AND DETERMINED THE BEST COURSE OF ACTION WOULD BE TO RETURN TO LAND AT THEIR DEP ARPT. WHEN TURNED BACK TO THE ARPT 3-4 MINS LATER, THE ACFT EXPERIENCED A ROLL OF 20 DEG BANK TO THE L. THE FO DISENGAGED THE AUTOPLT AND QUICKLY STOPPED THE ROLL. AUTOPLT WAS NEVER RE-ENGAGED FOR THE REMAINDER OF THE FLT. MANUAL CTL OF ACFT WAS FINE, THERE WERE NO PROBS USING MANUAL FLT CTLS. THE RPTR FELT THE ICING FAULT LIGHT WAS DELAYED. THAT ICE WAS FORMING BEFORE THE ICING FAULT INDICATIONS ILLUMINATED. THEREFORE, ICE HAD BEEN ACCUMULATING FOR THE PREVIOUS 10- 20 MIN PERIOD. PLT WAS ASKED IF THIS COULD HAVE BEEN CAUSED BY WAKE TURB. PLT STATED THAT IT DIDN'T FEEL LIKE WAKE TURB. THE FLC DID ASK APCH WHO THEY WERE FOLLOWING. THE REPLY WAS 'NO ONE.' THE COCKPIT VOICE RECORDER AND FLT DATA RECORDER WERE PULLED FROM THE ACFT FOR THE FOLLOWING INVESTIGATION WITH ALPA, FAA, NTSB AND COMPANY REPRESENTATIVES.

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.