PLT'S ACFT UNABLE TO MAINTAIN ASSIGNED ALT DUE TO UNFORECAST ICING.

Narrative

MY WIFE AND I AND 3 OF OUR 5 CHILDREN WERE RETURNING FROM COLEVILLE; WA; IN OUR CESSNA 206. WE HAD RETREATED BEFORE 50 KT HEADWINDS; MODERATE TO SEVERE TURB AND ICING ON THE VICTOR 2 AIRWAY BTWN ELLENSBURG AND SEATTLE THE AFTERNOON BEFORE AND SPENT THE NIGHT AT ELLENSBURG. WITH A MUCH MORE FAVORABLE WX RPT (FREEZING LEVEL AND CEILINGS UP FROM GND LEVEL TO 5000 FT; MODEST TURB; NO RPTED ICING BELOW 13000 FT; AND 'ONLY' 40 KTS OF HEADWIND) WE HAD ANOTHER GO AT IT SUNDAY MORNING. BEFORE I TELL YOU WHAT FOLLOWED; I'LL REMIND YOU THAT I FLY THIS RTE ON THE AVERAGE OF EVERY OTHER WK AND I AM FAMILIAR WITH THE NOTORIOUS ICE CORRIDOR ON THE W SLOPE OF THE CASCADES. IN ANY CASE; WE CLBED TO 10000 FT ON AN IFR FLT PLAN AND PROCEEDED W ON VICTOR 2 UNTIL APPROX 40 MI E OF SEA. WE WERE PRETTY MUCH ON TOP BUT OCCASIONALLY IN AND OUT OF THE STRATUS AND PICKING UP A VERY LIGHT FROSTING OF ICE ON THE LEADING EDGES AND WINDSCREEN; NOT ANY WORSE THAN I HAD ENCOUNTERED ON NUMEROUS OTHER OCCASIONS. AS WE PROGRESSED W; THE STRATUS BECAME SOLID; NO MORE IN AND OUT; AND THE GRADUAL FROST ACCUMULATION CONTINUED. KNOWING THERE WAS A CEILING AND FORECAST WARM AIR AT 5000 FT; I REQUESTED AND RECEIVED A DSCNT TO 8000 FT WHICH WAS THE MEA AT THAT POINT. DURING THAT DSCNT AND AT THAT ALT; THE ACFT BEGAN TO ACCUMULATE RIME ICE AT A FRIGHTENING RATE. WITHIN 5 MINS THE ICE HAD BUILT A 1 1/2 - 2 INCH LAYER ON THE LEADING EDGES OF WINGS; HORIZ STABILIZER; STRUTS; WINDSCREEN AND PROBABLY A LOT OF OTHER PLACES HANGING OUT THERE I COULD NO LONGER SEE. LIKE MOST AVIATION RELATED CRISES YOU READ ABOUT IN THE NTSB; OTHER THINGS BEGAN TO GO WRONG. PROP ICE (WHICH I HAD NEVER KNOWINGLY EXPERIENCED) FORMED AND THE ENG; ALTHOUGH RUNNING SMOOTH; BEGAN TO NOTICEABLY LABOR. MY WIFE PRAYED; AND I SWEATED. WE WERE LOSING ALT FROM LOSS OF PWR AND THE SHEER WT OF ICE. SEATTLE COULD NOT READ MY XMISSIONS. SO MUCH ICE HAD ACCUMULATED ON THE ANTENNAE THAT I COULD NO LONGER XMIT. WE COULD STILL RECEIVE. OUR LORAN; XPONDER; DME AND VORS TOLD US AND CTR WHERE WE WERE; IT WAS JUST A MATTER OF WHETHER WE WOULD CLR THE W SLOPE WITH ENOUGH ALT TO MAKE THE PUGET SOUND BASIN. CTR KNEW WE WERE IN TROUBLE AND INTERROGATED US BY ASKING QUESTIONS TO WHICH WE COULD RESPOND POSITIVELY BY 'IDENTING.' WE ALSO HEARD THEM ATTEMPTING CONTACT THROUGH ACRS BOTH BY RADIO AND VISUALLY. WE WERE CAUGHT IN THE CLASSIC SUPER COOLED ACFT (10 DEGS BELOW FREEZING AT 10000 FT) DSNDING THROUGH THE RISING UP-SLOPE SUPERCOOLED WATER DROPLETS. AN INTERESTING NOTE HERE WHICH JUST SHOWS AGAIN THAT EXPERIENCED PLTS WILL 'BLANK' IN SOME CRISES - - I HAVE KNOWN FOR 25 YEARS THAT EMER SQUAWK IS 7700 BUT I SET XXXX! EXPLAIN THAT ONE. THROUGH NO FAULT OF THEIRS; CTR/APCH NEVER REALLY REALIZED THE EXTENT OF OUR CRISIS. ALL THEY REALLY KNEW WAS THAT WE WERE COMING DOWN AND COULDN'T XMIT. FORTUNATELY A CTLR HAD ENOUGH WISDOM TO FIGURE IT OUT. WELL; I'M WRITING TO YOU SO YOU KNOW THAT BY THE GRACE OF GOD; WE CLRED THE W SLOPE; BROKE OUT INTO CLR AND WARM AIR AT 4000 FT AND THE CESSNA 206 CONTINUED TO MAINTAIN CTL RESPONSE WHICH WAS A MIRACLE IF YOU COULD HAVE SEEN THE ICE! THE WT OF ICE IS ONE THING; THE DEFORMING OF THE AIRFOIL IS PROBABLY MORE CRITICAL. SEATTLE HAD CLRED US TO LAND 'ANYWHERE' BUT; NOT KNOWING WHAT OUR CTL SIT WOULD BE AT LOWER SPD; WE ELECTED TO LET THINGS MELT. IF YOU'VE NEVER SEEN A PIECE OF ICE AN INCH THICK SLIDE OFF YOUR WINDSCREEN; I DON'T RECOMMEND THE EXPERIENCE. FORTUNATELY; THE PROP CLRED ITS LOAD; THE ENG REGAINED ITS FULL STRENGTH AND THERE WAS ONLY MINIMAL DAMAGE TO THE R HORIZ STABILIZER WHICH WAS STRUCK BY (YOU GUESSED IT) FLYING ICE! WE HAD BEEN DEALT WITH MOST GRACIOUSLY AND KNOWING WE HAD BEEN SPARED A MUCH WORSE FATE; A GREAT OCCASION FOR TEARS OF JOY.