Narrative:

During ILS runway 23 approach to ZZZ at 4500 ft MSL and approximately 12 mi to the airport we selected engine anti-ice on as we were about to enter a cloud deck with known ice. The right engine was selected first and then the left one. A master caution sounded advising the crew that both the right and left engine anti-ice had failed. The checklist calls for the recycling of the anti-ice switch which was done on both the right and left engines. The right engine worked properly this time but the left anti-ice light remained on. At this point the checklist advises to reduce power on that engine until the light goes out. I reduced power on the left engine until the light extinguished. A moment later, the left propeller feathered. The crew decided to select the left condition lever to feather and also to advise ATC that we were declaring an emergency. We also leveled off at that point to avoid flying into the ice until we were ready to continue the approach. We contacted ATC, flight attendant, passenger and company and advised of our situation. We then elected to continue the approach with the left engine in feather in case something had been damaged. We advised ATC and they vectored us for the ILS runway 23 and we landed with no further incident. Callback conversation with reporter revealed the following information: the reporter stated the cause of the propeller going to feather was a procedural error in reducing the power slightly beyond the point where the anti-ice light went out. The airplane ground checked ok by maintenance on both the anti-ice light and the automatic feather system.

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Original NASA ASRS Text

Title: AN SF340A DURING APCH AT 4500 FT DECLARED AN EMER DUE TO L ENG ANTI-ICE FAIL LIGHT REMAINED ON. PROCS ARE TO REDUCE PWR TILL LIGHT GOES OUT. WHEN PWR REDUCED TO LIGHT OUT PROP WENT TO FEATHER.

Narrative: DURING ILS RWY 23 APCH TO ZZZ AT 4500 FT MSL AND APPROX 12 MI TO THE ARPT WE SELECTED ENG ANTI-ICE ON AS WE WERE ABOUT TO ENTER A CLOUD DECK WITH KNOWN ICE. THE R ENG WAS SELECTED FIRST AND THEN THE L ONE. A MASTER CAUTION SOUNDED ADVISING THE CREW THAT BOTH THE R AND L ENG ANTI-ICE HAD FAILED. THE CHKLIST CALLS FOR THE RECYCLING OF THE ANTI-ICE SWITCH WHICH WAS DONE ON BOTH THE R AND L ENGS. THE R ENG WORKED PROPERLY THIS TIME BUT THE L ANTI-ICE LIGHT REMAINED ON. AT THIS POINT THE CHKLIST ADVISES TO REDUCE PWR ON THAT ENG UNTIL THE LIGHT GOES OUT. I REDUCED PWR ON THE L ENG UNTIL THE LIGHT EXTINGUISHED. A MOMENT LATER, THE L PROP FEATHERED. THE CREW DECIDED TO SELECT THE L CONDITION LEVER TO FEATHER AND ALSO TO ADVISE ATC THAT WE WERE DECLARING AN EMER. WE ALSO LEVELED OFF AT THAT POINT TO AVOID FLYING INTO THE ICE UNTIL WE WERE READY TO CONTINUE THE APCH. WE CONTACTED ATC, FLT ATTENDANT, PAX AND COMPANY AND ADVISED OF OUR SIT. WE THEN ELECTED TO CONTINUE THE APCH WITH THE L ENG IN FEATHER IN CASE SOMETHING HAD BEEN DAMAGED. WE ADVISED ATC AND THEY VECTORED US FOR THE ILS RWY 23 AND WE LANDED WITH NO FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CAUSE OF THE PROP GOING TO FEATHER WAS A PROCEDURAL ERROR IN REDUCING THE PWR SLIGHTLY BEYOND THE POINT WHERE THE ANTI-ICE LIGHT WENT OUT. THE AIRPLANE GND CHKED OK BY MAINT ON BOTH THE ANTI-ICE LIGHT AND THE AUTO FEATHER SYS.

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.