Narrative:

On aug/sat/93 at approximately XX45 alaska time, I was flying a twin small transport from anc to pdx. I was on the first leg of the trip to yakutat and had filed IFR. I had been vectored around severe (reported) turbulence areas around anc and then cleared to johnstone point (no severe tub was encountered). At approximately 100 NM from yakutat I noticed the cylinder head temperatures getting very low. I was then at 11000 ft MSL and asked for lower. Zan gave me 10000 ft which is the MEA for V319. I leaned the mixtures to get more heat, but was only partially successful and didn't want to run the engines too lean and burn a valve or piston. The induction heats were on hot and I noticed we were in very heavy rain with big droplets that glistened in the landing light's beams when I turned them on. The outside air temperature was minus 10 degrees C. At approximately 70-80 NM from yakutat, the airplane slowed up to maintain altitude. I noticed ice buildup on the lower protrusions of the nacelles of approximately 3 inches. I juggled the hot/cold induction levers to obtain maximum engine power and began losing the engines in the cold position so engaged full hot. The airplane felt it was approaching the stall so I had to descend to maintain flying speed. The zan controller wanted me to do a 180 and vector me back to cordova, but I didn't want to turn lest I stall. He informed me he had exercised his option of declaring an emergency. I deployed approximately 1/8 flap to bring the center of pressure rearward and let the nose down. This succeeded and the airplane (now at 9000 ft) flew much better and increased speed about 5 KTS indicated. Meanwhile, the cylinder head indicators went to the bottom of the green and the right one then went to zero degrees. The left load meter (ammeter) read very high (about 80 amps) and when I turned the left generator off, the meter still read about 25 amps, so, as it (the meter) was not over maximum, I turned the left generator back on. I had been reporting only light rime icing conditions to center up until I encountered the severe conditions described and just before having to descend, I cracked a load off the wing leading edges with the boots. But, there was still a 'shelf' of ice left about and below the boots. From then on, the ice accretion was clear ice. At approximately 50 mi from yakutat, I began losing the ice from under the nacelles (and no doubt, the underbelly) and the airplane felt and flew a lot better. I had cracked ice from the boots 3 times by then and was just taking very light rime ice again. I had to leave the induction heats on 'hot' until landing at yakutat, but the cylinder head temperatures came back to normal and each generator load meter finally normalized. I retracted the flaps and was able to regain the MEA without any trouble. After landing at yakutat, I uncowled the engines and withdrew a solid chunk of ice from each intake side that was very heavy to hold. After the trip and this incident, I was sure that the occurrence was caused by an overrunning warm moist layer precipitating super cooled water on an already cold-soaked aircraft which became the nucleus for intense ice accretion. I consider that the incident (and 1 other incident 21 yrs ago) was quite beyond the engine and airframe capabilities of any cabin class twin. The only way this could be avoided is recognition of the ingredients (cold wet air at around the MEA plus an overrunning wet warmer layer) -- by FSS personnel and briefing that the possibility of severe icing exists.

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Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION IN TWIN SMT AFTER INFLT ENCOUNTER WITH HVY ICING.

Narrative: ON AUG/SAT/93 AT APPROX XX45 ALASKA TIME, I WAS FLYING A TWIN SMT FROM ANC TO PDX. I WAS ON THE FIRST LEG OF THE TRIP TO YAKUTAT AND HAD FILED IFR. I HAD BEEN VECTORED AROUND SEVERE (RPTED) TURB AREAS AROUND ANC AND THEN CLRED TO JOHNSTONE POINT (NO SEVERE TUB WAS ENCOUNTERED). AT APPROX 100 NM FROM YAKUTAT I NOTICED THE CYLINDER HEAD TEMPS GETTING VERY LOW. I WAS THEN AT 11000 FT MSL AND ASKED FOR LOWER. ZAN GAVE ME 10000 FT WHICH IS THE MEA FOR V319. I LEANED THE MIXTURES TO GET MORE HEAT, BUT WAS ONLY PARTIALLY SUCCESSFUL AND DIDN'T WANT TO RUN THE ENGS TOO LEAN AND BURN A VALVE OR PISTON. THE INDUCTION HEATS WERE ON HOT AND I NOTICED WE WERE IN VERY HVY RAIN WITH BIG DROPLETS THAT GLISTENED IN THE LNDG LIGHT'S BEAMS WHEN I TURNED THEM ON. THE OUTSIDE AIR TEMP WAS MINUS 10 DEGS C. AT APPROX 70-80 NM FROM YAKUTAT, THE AIRPLANE SLOWED UP TO MAINTAIN ALT. I NOTICED ICE BUILDUP ON THE LOWER PROTRUSIONS OF THE NACELLES OF APPROX 3 INCHES. I JUGGLED THE HOT/COLD INDUCTION LEVERS TO OBTAIN MAX ENG PWR AND BEGAN LOSING THE ENGS IN THE COLD POS SO ENGAGED FULL HOT. THE AIRPLANE FELT IT WAS APCHING THE STALL SO I HAD TO DSND TO MAINTAIN FLYING SPD. THE ZAN CTLR WANTED ME TO DO A 180 AND VECTOR ME BACK TO CORDOVA, BUT I DIDN'T WANT TO TURN LEST I STALL. HE INFORMED ME HE HAD EXERCISED HIS OPTION OF DECLARING AN EMER. I DEPLOYED APPROX 1/8 FLAP TO BRING THE CTR OF PRESSURE REARWARD AND LET THE NOSE DOWN. THIS SUCCEEDED AND THE AIRPLANE (NOW AT 9000 FT) FLEW MUCH BETTER AND INCREASED SPD ABOUT 5 KTS INDICATED. MEANWHILE, THE CYLINDER HEAD INDICATORS WENT TO THE BOTTOM OF THE GREEN AND THE R ONE THEN WENT TO ZERO DEGS. THE L LOAD METER (AMMETER) READ VERY HIGH (ABOUT 80 AMPS) AND WHEN I TURNED THE L GENERATOR OFF, THE METER STILL READ ABOUT 25 AMPS, SO, AS IT (THE METER) WAS NOT OVER MAX, I TURNED THE L GENERATOR BACK ON. I HAD BEEN RPTING ONLY LIGHT RIME ICING CONDITIONS TO CTR UP UNTIL I ENCOUNTERED THE SEVERE CONDITIONS DESCRIBED AND JUST BEFORE HAVING TO DSND, I CRACKED A LOAD OFF THE WING LEADING EDGES WITH THE BOOTS. BUT, THERE WAS STILL A 'SHELF' OF ICE LEFT ABOUT AND BELOW THE BOOTS. FROM THEN ON, THE ICE ACCRETION WAS CLR ICE. AT APPROX 50 MI FROM YAKUTAT, I BEGAN LOSING THE ICE FROM UNDER THE NACELLES (AND NO DOUBT, THE UNDERBELLY) AND THE AIRPLANE FELT AND FLEW A LOT BETTER. I HAD CRACKED ICE FROM THE BOOTS 3 TIMES BY THEN AND WAS JUST TAKING VERY LIGHT RIME ICE AGAIN. I HAD TO LEAVE THE INDUCTION HEATS ON 'HOT' UNTIL LNDG AT YAKUTAT, BUT THE CYLINDER HEAD TEMPS CAME BACK TO NORMAL AND EACH GENERATOR LOAD METER FINALLY NORMALIZED. I RETRACTED THE FLAPS AND WAS ABLE TO REGAIN THE MEA WITHOUT ANY TROUBLE. AFTER LNDG AT YAKUTAT, I UNCOWLED THE ENGS AND WITHDREW A SOLID CHUNK OF ICE FROM EACH INTAKE SIDE THAT WAS VERY HVY TO HOLD. AFTER THE TRIP AND THIS INCIDENT, I WAS SURE THAT THE OCCURRENCE WAS CAUSED BY AN OVERRUNNING WARM MOIST LAYER PRECIPITATING SUPER COOLED WATER ON AN ALREADY COLD-SOAKED ACFT WHICH BECAME THE NUCLEUS FOR INTENSE ICE ACCRETION. I CONSIDER THAT THE INCIDENT (AND 1 OTHER INCIDENT 21 YRS AGO) WAS QUITE BEYOND THE ENG AND AIRFRAME CAPABILITIES OF ANY CABIN CLASS TWIN. THE ONLY WAY THIS COULD BE AVOIDED IS RECOGNITION OF THE INGREDIENTS (COLD WET AIR AT AROUND THE MEA PLUS AN OVERRUNNING WET WARMER LAYER) -- BY FSS PERSONNEL AND BRIEFING THAT THE POSSIBILITY OF SEVERE ICING EXISTS.

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.