Narrative:

At 11;000 on the arrival we were given a descent to 7;000. We were aware of icing at 7;000 and heard another aircraft at 5;000 reporting less icing so we requested and were given 5;000. I then did a descent to 5;000 at about 2;000 FPM to reduce the time in icing conditions that started at 10;000. Before entering the clouds; level two anti-icing was selected and the icing aoa light was illuminated. Shortly after that; the ice detector reported ice and level 3 de-icing was selected. We reached 5;000 and the OAT was about -4. We checked the wings and the ice build up appeared normal and non-threatening but we asked for a lower altitude anyway. We were told to expect that in xx miles. I regularly checked out my window to make sure there was no excessive ice and there didn't appear to be. The de-ice system seemed to have knocked the ice off fairly well. We began taking vectors and were given an assigned speed of 180 so we got busy setting up and briefing the approach to ZZZ. When I looked out to check the wing again my forward side window was iced over aft of the heated portion. Up to this point we had repeatedly asked for a lower altitude. We immediately began the severe ice epc (emergency procedures checklist). When the autopilot was disengaged the aircraft felt heavy and kind of mushy. It was very shortly after that I heard a rumbling that started quiet then got progressively louder over the course of about 5-7 seconds; and with that there was a vibration in the yoke that increased with the sound increase. There was then a pop type noise and the aircraft pitched down slightly and tried to roll to the left slightly; I do not recall the degrees of pitch or roll. There was then a positive exchange of control and I; now pilot monitoring; declared an emergency and finished the epc. We were given priority handling into ZZZ and accomplished a reduced flap landing as per the epc. I only looked out my window once after the incident when we got below the cloud deck. I noted that there appeared to be a layer of ice on the de-ice boot that had not been knocked off. I was unable to see the top of the wing to note if ice had accumulated aft of the protected area; nor was I able to see the propeller spinner. What I did note was very large chunks of ice; my guess is about a foot wide; on the flap fairings.

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Original NASA ASRS Text

Title: An ATR72 crew describes encountering icing that the aircraft de-ice system is incapable of shedding and having to declare an emergency to get priority handling as the aircraft approaches a stall condition.

Narrative: At 11;000 on the arrival we were given a descent to 7;000. We were aware of icing at 7;000 and heard another aircraft at 5;000 reporting less icing so we requested and were given 5;000. I then did a descent to 5;000 at about 2;000 FPM to reduce the time in icing conditions that started at 10;000. Before entering the clouds; level two anti-icing was selected and the icing AOA light was illuminated. Shortly after that; the ice detector reported ice and level 3 de-icing was selected. We reached 5;000 and the OAT was about -4. We checked the wings and the ice build up appeared normal and non-threatening but we asked for a lower altitude anyway. We were told to expect that in XX miles. I regularly checked out my window to make sure there was no excessive ice and there didn't appear to be. The de-ice system seemed to have knocked the ice off fairly well. We began taking vectors and were given an assigned speed of 180 so we got busy setting up and briefing the approach to ZZZ. When I looked out to check the wing again my forward side window was iced over aft of the heated portion. Up to this point we had repeatedly asked for a lower altitude. We immediately began the severe ice EPC (Emergency Procedures Checklist). When the autopilot was disengaged the aircraft felt heavy and kind of mushy. It was very shortly after that I heard a rumbling that started quiet then got progressively louder over the course of about 5-7 seconds; and with that there was a vibration in the yoke that increased with the sound increase. There was then a pop type noise and the aircraft pitched down slightly and tried to roll to the left slightly; I do not recall the degrees of pitch or roll. There was then a positive exchange of control and I; now pilot monitoring; declared an emergency and finished the EPC. We were given priority handling into ZZZ and accomplished a reduced flap landing as per the EPC. I only looked out my window once after the incident when we got below the cloud deck. I noted that there appeared to be a layer of ice on the de-ice boot that had not been knocked off. I was unable to see the top of the wing to note if ice had accumulated aft of the protected area; nor was I able to see the propeller spinner. What I did note was very large chunks of ice; my guess is about a foot wide; on the flap fairings.

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.