Narrative:

A local IFR flight was planned to demonstrate 'actual' conditions to student. Airmet for IFR covered the area, but none for icing. PIREPS contained no reports of icing in area. A phone call to approach control (on our field) reported no one picking up ice around our airport. Departed field, entered clouds at 1000 ft AGL. At around 1800 ft AGL, traces of rime ice began to appear. Immediately requested lower altitude and return to field for ILS approach. Approximately 1/4 inch ice accumulated on leading edges, struts and tires during vectoring for approach. Broke out on approach at 1000 ft AGL. Ice began melting and blowing off airplane. Student continued approach to landing. Wind was quartering headwind from right, so student set up for crosswind landing. During flare, the left wing dropped well above stall speed and the airplane began a corresponding turn to the left. I called for power for a go around before we departed the runway, but airplane would not climb. Airplane departed runway to the left. I pulled off the power and stopped the aircraft. No damage was done to the plane or field (lights, etc). In retrospect, I misjudged the performance degradation from the remaining ice on the airframe. I knew from past reading that a higher than normal landing speed, without the use of flaps, was the proper approach with an iced-up airplane, and I did halt my student from extending them, but I was under the false assumption that the ice had exited the plane, when in fact it had not entirely. I believe that a combination of a higher than normal stall speed, coupled with a small blanketing of the left wing due to the crosswind sideslip, caused the left wing to stall. I have learned from this incident an even more healthy respect for ice and the degraded performance that it can cause. I have learned to land with plenty of extra speed should I ever encounter unforecast ice again, and to flare as low as possible. I have often read, and now seen, that any ice -- even a 'small' accumulation -- can cause completely unpredictable aerodynamic consequences.

Related reports:

Google
 

Original NASA ASRS Text

Title: C172 INSTRUCTOR TAKES STUDENT UP FOR AN ACTUAL INST CONDITIONS FLT RESULTING IN PICKING UP ACFT ICING AND LOSING CTL OF THE ACFT DURING A XWIND LNDG.

Narrative: A LCL IFR FLT WAS PLANNED TO DEMONSTRATE 'ACTUAL' CONDITIONS TO STUDENT. AIRMET FOR IFR COVERED THE AREA, BUT NONE FOR ICING. PIREPS CONTAINED NO RPTS OF ICING IN AREA. A PHONE CALL TO APCH CTL (ON OUR FIELD) RPTED NO ONE PICKING UP ICE AROUND OUR ARPT. DEPARTED FIELD, ENTERED CLOUDS AT 1000 FT AGL. AT AROUND 1800 FT AGL, TRACES OF RIME ICE BEGAN TO APPEAR. IMMEDIATELY REQUESTED LOWER ALT AND RETURN TO FIELD FOR ILS APCH. APPROX 1/4 INCH ICE ACCUMULATED ON LEADING EDGES, STRUTS AND TIRES DURING VECTORING FOR APCH. BROKE OUT ON APCH AT 1000 FT AGL. ICE BEGAN MELTING AND BLOWING OFF AIRPLANE. STUDENT CONTINUED APCH TO LNDG. WIND WAS QUARTERING HEADWIND FROM R, SO STUDENT SET UP FOR XWIND LNDG. DURING FLARE, THE L WING DROPPED WELL ABOVE STALL SPD AND THE AIRPLANE BEGAN A CORRESPONDING TURN TO THE L. I CALLED FOR PWR FOR A GAR BEFORE WE DEPARTED THE RWY, BUT AIRPLANE WOULD NOT CLB. AIRPLANE DEPARTED RWY TO THE L. I PULLED OFF THE PWR AND STOPPED THE ACFT. NO DAMAGE WAS DONE TO THE PLANE OR FIELD (LIGHTS, ETC). IN RETROSPECT, I MISJUDGED THE PERFORMANCE DEGRADATION FROM THE REMAINING ICE ON THE AIRFRAME. I KNEW FROM PAST READING THAT A HIGHER THAN NORMAL LNDG SPD, WITHOUT THE USE OF FLAPS, WAS THE PROPER APCH WITH AN ICED-UP AIRPLANE, AND I DID HALT MY STUDENT FROM EXTENDING THEM, BUT I WAS UNDER THE FALSE ASSUMPTION THAT THE ICE HAD EXITED THE PLANE, WHEN IN FACT IT HAD NOT ENTIRELY. I BELIEVE THAT A COMBINATION OF A HIGHER THAN NORMAL STALL SPD, COUPLED WITH A SMALL BLANKETING OF THE L WING DUE TO THE XWIND SIDESLIP, CAUSED THE L WING TO STALL. I HAVE LEARNED FROM THIS INCIDENT AN EVEN MORE HEALTHY RESPECT FOR ICE AND THE DEGRADED PERFORMANCE THAT IT CAN CAUSE. I HAVE LEARNED TO LAND WITH PLENTY OF EXTRA SPD SHOULD I EVER ENCOUNTER UNFORECAST ICE AGAIN, AND TO FLARE AS LOW AS POSSIBLE. I HAVE OFTEN READ, AND NOW SEEN, THAT ANY ICE -- EVEN A 'SMALL' ACCUMULATION -- CAN CAUSE COMPLETELY UNPREDICTABLE AERODYNAMIC CONSEQUENCES.

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.