Narrative:

During climb out; while operating in icing conditions with level 3 icing systems engaged; the aileron/elevator horn anti-icing system faulted and was not recoverable. At this same time the afcs [auto flight control system] was not operating normally. The first officer (PF) complained of controllability issues and an unfamiliar vibration was detected. The QRH checklist directed us to leave icing conditions; so the crew decided to turn back to our departure airport. During vectoring for ILS 27 an altitude deviation occurred of approximately 500 feet. The PIC assumed control of the aircraft for the remainder of the flight. During the ILS approach; with the afcs restored; and after determining free and clear control of aircraft controls; the autopilot was engaged. The aircraft subsequently experienced an altitude loss. The PIC disconnected the autopilot; recovered to ILS profile and subsequently landed on runway 27.operating in icing conditions with horn heat inoperative probably caused control issues during the approach and airspeed decay may have led to the altitude loss. It is possible that a severe ice encounter occurred. Because of very gusty winds and icing conditions; and feeling fatigued. The PIC decided to engage the autopilot during approach because a difficult crosswind landing was imminent; with reported visibility of 1 mile due to blowing snow.

Related reports:

Google
 

Original NASA ASRS Text

Title: The loss of aileron and elevator horn anti-icing system while operating in moderate icing conditions likely contributed to an ATR-42 flight crew's failure to maintain cleared altitudes while returning to their departure airport.

Narrative: During climb out; while operating in icing conditions with level 3 icing systems engaged; the aileron/elevator horn anti-icing system faulted and was not recoverable. At this same time the AFCS [Auto Flight Control System] was not operating normally. The First Officer (PF) complained of controllability issues and an unfamiliar vibration was detected. The QRH checklist directed us to leave icing conditions; so the crew decided to turn back to our departure airport. During vectoring for ILS 27 an altitude deviation occurred of approximately 500 feet. The PIC assumed control of the aircraft for the remainder of the flight. During the ILS approach; with the AFCS restored; and after determining free and clear control of aircraft controls; the autopilot was engaged. The aircraft subsequently experienced an altitude loss. The PIC disconnected the autopilot; recovered to ILS profile and subsequently landed on Runway 27.Operating in icing conditions with horn heat inoperative probably caused control issues during the approach and airspeed decay may have led to the altitude loss. It is possible that a severe ice encounter occurred. Because of very gusty winds and icing conditions; and feeling fatigued. The PIC decided to engage the autopilot during approach because a difficult crosswind landing was imminent; with reported visibility of 1 mile due to blowing snow.

Data retrieved from NASA's ASRS site as of April 2012 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.