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.
Synopsis
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.
Second reporter narrative
Inbound on the arrival to ZZZ; we were aware of icing reported by preceding aircraft. We were cruising above the icing layer at 11;000 FT (TAT of -2C) when we received our standard initial clearance to 7000 feet. We immediately requested and received a clearance to 5000 believing the air should warm considerably there. Ice accretion began in the descent and the TAT remained steady. We requested 3000 FT; thinking that would put us in the clear. I recall the METAR showing a 3300 FT ceiling. Moderate turbulence was present as well as strong; gusty surface winds. Ice built quickly as we were vectored on the downwind for the ILS. The rate of accumulation increased as we passed abeam of the runway and reached the threshold for primary visual cues for severe icing. We executed the EPC checklist for severe icing and told ATC we must have lower. We were being beat up by the turbulence as well and after a particularly strong bump the First Officer who was hand flying lost 300 FT. I took control of the aircraft at this point and corrected back to our altitude. Moments later; ATC gave us 4000 FT. As I flew I felt the controls were heavy and sluggish. Roll control was particularly affected. Just before we initiated the descent; we felt a strong; high frequency vibration that lasted 10 seconds or so. As we descended moments later the vibration occurred again. Not knowing the source or cause of this vibration; I decided to declare an emergency since we were still flying away from the airport. We declared our emergency and immediately got a descent to 3000 FT and a base turn. Once we descended out of the clouds; the vibrations stopped and we could visibly see ice melting off the plane. We completed the approach and landing in VFR conditions. At no time did the aircraft stall.....our speed was 180 KTS plus/minus with the windshear until slowing on short final. At no time did we lose control of the aircraft. While control seemed degraded; we had full use of the controls and they responded correctly although sluggish.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.