Narrative:

Aircraft was being vectored for an approach into sbn in flight conditions that included light rain and temperatures near freezing. Anti-icing and deicing equipment was in use at the time. No ice was visible on the windows, wipers , or ice evidence probe mounted just outside the captain's window. Also, no ice was seen on the leading edge of the wing. While being vectored to intercept the localizer, a buffet was noted at 170-180 KTS. Power was advanced briskly, and within 2 to 3 seconds, the buffet stopped. Closer inspection of the wing showed that a ridge of ice about 1/2 inch thick had formed on top of the wing just aft of the boot. Since I had not experienced ice buildup that looked like this in the past, the decision was made not to change confign until the ice melted off during approach (surface temperature was 37 degrees F). An uneventful landing was made at sbn. This event mainly emphasizes that, even though ice buildup may not be seen on the parts of the aircraft where it is usually seen, it may still be building in significant quantities. Callback conversation with reporter revealed the following information: reporter stated that normal way to determine if ice is forming is an ice probe outside the cockpit window. When ice forms on the probe, it is probably forming on the wing. However, on this flight, ice was forming on the wing, but not on the probe. Flight crew increased airspeed to fly out of buffet range and kept airspeed high until descending to lower altitude and they could see the ice start to melt off. Temperature at lower altitude was 37 degrees F so they felt ice would leave the aircraft. They were able to reduce airspeed, extend gear and flaps, and make a normal approach. Reporter stated that aircraft flies well in rime icing but not well when clear has formed. From the cockpit the flight crew can see about 10 percent of the wing behind the leading edge. This is their best indication of icing. At night time, as in this case, the aircraft has a strong light that illuminates the top of the wing that originates from the side of the fuselage. This is how they determined the buffeting was caused by the formation of ice. Deicing boots were effective as far as they cover the wing. Where the boot does not cover is where the ice formed. Deicing is controled by the flight crew turning on the switch and setting the outside temperature in the control panel. This determines the frequency the boot cycles to break off any ice. This incident took place about 50 mi and 2 hours from where another ATR crashed that day.

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Original NASA ASRS Text

Title: BUFFETING FROM ICE ACCUMULATION.

Narrative: ACFT WAS BEING VECTORED FOR AN APCH INTO SBN IN FLT CONDITIONS THAT INCLUDED LIGHT RAIN AND TEMPS NEAR FREEZING. ANTI-ICING AND DEICING EQUIP WAS IN USE AT THE TIME. NO ICE WAS VISIBLE ON THE WINDOWS, WIPERS , OR ICE EVIDENCE PROBE MOUNTED JUST OUTSIDE THE CAPT'S WINDOW. ALSO, NO ICE WAS SEEN ON THE LEADING EDGE OF THE WING. WHILE BEING VECTORED TO INTERCEPT THE LOC, A BUFFET WAS NOTED AT 170-180 KTS. PWR WAS ADVANCED BRISKLY, AND WITHIN 2 TO 3 SECONDS, THE BUFFET STOPPED. CLOSER INSPECTION OF THE WING SHOWED THAT A RIDGE OF ICE ABOUT 1/2 INCH THICK HAD FORMED ON TOP OF THE WING JUST AFT OF THE BOOT. SINCE I HAD NOT EXPERIENCED ICE BUILDUP THAT LOOKED LIKE THIS IN THE PAST, THE DECISION WAS MADE NOT TO CHANGE CONFIGN UNTIL THE ICE MELTED OFF DURING APCH (SURFACE TEMP WAS 37 DEGS F). AN UNEVENTFUL LNDG WAS MADE AT SBN. THIS EVENT MAINLY EMPHASIZES THAT, EVEN THOUGH ICE BUILDUP MAY NOT BE SEEN ON THE PARTS OF THE ACFT WHERE IT IS USUALLY SEEN, IT MAY STILL BE BUILDING IN SIGNIFICANT QUANTITIES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT NORMAL WAY TO DETERMINE IF ICE IS FORMING IS AN ICE PROBE OUTSIDE THE COCKPIT WINDOW. WHEN ICE FORMS ON THE PROBE, IT IS PROBABLY FORMING ON THE WING. HOWEVER, ON THIS FLT, ICE WAS FORMING ON THE WING, BUT NOT ON THE PROBE. FLC INCREASED AIRSPD TO FLY OUT OF BUFFET RANGE AND KEPT AIRSPD HIGH UNTIL DSNDING TO LOWER ALT AND THEY COULD SEE THE ICE START TO MELT OFF. TEMP AT LOWER ALT WAS 37 DEGS F SO THEY FELT ICE WOULD LEAVE THE ACFT. THEY WERE ABLE TO REDUCE AIRSPD, EXTEND GEAR AND FLAPS, AND MAKE A NORMAL APCH. RPTR STATED THAT ACFT FLIES WELL IN RIME ICING BUT NOT WELL WHEN CLR HAS FORMED. FROM THE COCKPIT THE FLC CAN SEE ABOUT 10 PERCENT OF THE WING BEHIND THE LEADING EDGE. THIS IS THEIR BEST INDICATION OF ICING. AT NIGHT TIME, AS IN THIS CASE, THE ACFT HAS A STRONG LIGHT THAT ILLUMINATES THE TOP OF THE WING THAT ORIGINATES FROM THE SIDE OF THE FUSELAGE. THIS IS HOW THEY DETERMINED THE BUFFETING WAS CAUSED BY THE FORMATION OF ICE. DEICING BOOTS WERE EFFECTIVE AS FAR AS THEY COVER THE WING. WHERE THE BOOT DOES NOT COVER IS WHERE THE ICE FORMED. DEICING IS CTLED BY THE FLC TURNING ON THE SWITCH AND SETTING THE OUTSIDE TEMP IN THE CTL PANEL. THIS DETERMINES THE FREQ THE BOOT CYCLES TO BREAK OFF ANY ICE. THIS INCIDENT TOOK PLACE ABOUT 50 MI AND 2 HRS FROM WHERE ANOTHER ATR CRASHED THAT DAY.

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.