Narrative:

Arrive at aircraft; notice icicles 5 inches in length hanging from pitot tubes and other surfaces. Temperature -1 degree C. Do tactile check and find up to 1/8 inch in clear ice on wing surfaces. First deice using type 1 by abc ground service. Second deice. Tactile check finds clear ice still remains on horizontal stabilizer; but main wings clear. Third deice. Tactile check wings and tail clean. Rechked cedar city FSS for updated PIREPS/airmet/SIGMET on this route. Airmet for occasional moderate rime still in place. PIREP mentioned of light clear on my route/altitude north of ogden. Cleared for takeoff. Delay on ground due low visibility and traffic departing. En route: light clear accumulates 10 mi south ogden VOR 8500-10000 ft. 10000-14000 ft clear; changes to moderate rime. Light accretion aft of protected surfaces of boots. Boots do partially clear mains; but fail to clear struts. Request block altitude from 15000-17000 ft approved. On top at 16000 ft. Cruise loss of 25 KTS (maximum speed now 125 KIAS due ice load). Climb to 17000 ft to remain above clouds in clear; temperature -12 degree C. ATC assigns 10000 ft to begin descent. Begin descent 19 mi south of pih. Through first layer 17000 ft to 16000 ft light trace. Second layer at 13400 ft begin moderate clear accretion. Clear ice worsens to 10200 ft. Maximum power in descent yields 165 KTS with 1500 FPM descent. Boots moderately effective on mains. Accretion behind boots on mains visible with ridge at boot/wing junction; struts do not clear at all. Ask ATC for vectors to final approach course so I can focus on trying to clear wings and monitor ice formations. Tell ATC no longer possible to shoot pih approach due accretion rate at necessary altitudes and request climb to clear. Begin climb; but aircraft unable to climb above 10200 ft. Maximum power temperature to 740 yields 105 KIAS and 50 FPM climb in moderate clear ice. Tail begins vibration. Aircraft pitches up. Autoplt disengaged and pitches aircraft down in 1500 FPM descent. Ask ATC for vectors for terrain clearance into snake river valley and simultaneously request diversion to twf. I know from preflight check that twf terminal forecast slightly above freezing at +2 degrees C while pih 3/4 mi visibility; ceiling broken 400 ft; overcast 1900 ft; temperature; -1 degree C. 7000 ft -2 degrees C level off; windscreen ice begins to partially slide off. Descend to 6500 ft; get a bit more to slide off and leading edge of mains begin to clear with boot action and better temperatures. Tail still vibrating. Airspeed improves and ice steadily comes off. Climb back to 7000 ft to remain on radar and completed en route portion and approach to tower without further event. Need improved system to remove ice from C208B. Boots not effective enough. Need system to view tail section and visually monitor ice formation or mechanically/visually measure tail ice formation.

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Original NASA ASRS Text

Title: A C208 PLT ENCOUNTERED SEVERE ICING AND CTL PROBS DURING DSCNT FOR LNDG. FLT DIVERTED TO A WARMER ARPT.

Narrative: ARRIVE AT ACFT; NOTICE ICICLES 5 INCHES IN LENGTH HANGING FROM PITOT TUBES AND OTHER SURFACES. TEMP -1 DEG C. DO TACTILE CHK AND FIND UP TO 1/8 INCH IN CLR ICE ON WING SURFACES. FIRST DEICE USING TYPE 1 BY ABC GND SVC. SECOND DEICE. TACTILE CHK FINDS CLR ICE STILL REMAINS ON HORIZ STABILIZER; BUT MAIN WINGS CLR. THIRD DEICE. TACTILE CHK WINGS AND TAIL CLEAN. RECHKED CEDAR CITY FSS FOR UPDATED PIREPS/AIRMET/SIGMET ON THIS RTE. AIRMET FOR OCCASIONAL MODERATE RIME STILL IN PLACE. PIREP MENTIONED OF LIGHT CLR ON MY RTE/ALT N OF OGDEN. CLRED FOR TKOF. DELAY ON GND DUE LOW VISIBILITY AND TFC DEPARTING. ENRTE: LIGHT CLR ACCUMULATES 10 MI S OGDEN VOR 8500-10000 FT. 10000-14000 FT CLR; CHANGES TO MODERATE RIME. LIGHT ACCRETION AFT OF PROTECTED SURFACES OF BOOTS. BOOTS DO PARTIALLY CLR MAINS; BUT FAIL TO CLR STRUTS. REQUEST BLOCK ALT FROM 15000-17000 FT APPROVED. ON TOP AT 16000 FT. CRUISE LOSS OF 25 KTS (MAX SPD NOW 125 KIAS DUE ICE LOAD). CLB TO 17000 FT TO REMAIN ABOVE CLOUDS IN CLR; TEMP -12 DEG C. ATC ASSIGNS 10000 FT TO BEGIN DSCNT. BEGIN DSCNT 19 MI S OF PIH. THROUGH FIRST LAYER 17000 FT TO 16000 FT LIGHT TRACE. SECOND LAYER AT 13400 FT BEGIN MODERATE CLR ACCRETION. CLR ICE WORSENS TO 10200 FT. MAX PWR IN DSCNT YIELDS 165 KTS WITH 1500 FPM DSCNT. BOOTS MODERATELY EFFECTIVE ON MAINS. ACCRETION BEHIND BOOTS ON MAINS VISIBLE WITH RIDGE AT BOOT/WING JUNCTION; STRUTS DO NOT CLR AT ALL. ASK ATC FOR VECTORS TO FINAL APCH COURSE SO I CAN FOCUS ON TRYING TO CLR WINGS AND MONITOR ICE FORMATIONS. TELL ATC NO LONGER POSSIBLE TO SHOOT PIH APCH DUE ACCRETION RATE AT NECESSARY ALTS AND REQUEST CLB TO CLR. BEGIN CLB; BUT ACFT UNABLE TO CLB ABOVE 10200 FT. MAX PWR TEMP TO 740 YIELDS 105 KIAS AND 50 FPM CLB IN MODERATE CLR ICE. TAIL BEGINS VIBRATION. ACFT PITCHES UP. AUTOPLT DISENGAGED AND PITCHES ACFT DOWN IN 1500 FPM DSCNT. ASK ATC FOR VECTORS FOR TERRAIN CLRNC INTO SNAKE RIVER VALLEY AND SIMULTANEOUSLY REQUEST DIVERSION TO TWF. I KNOW FROM PREFLT CHK THAT TWF TERMINAL FORECAST SLIGHTLY ABOVE FREEZING AT +2 DEGS C WHILE PIH 3/4 MI VISIBILITY; CEILING BROKEN 400 FT; OVCST 1900 FT; TEMP; -1 DEG C. 7000 FT -2 DEGS C LEVEL OFF; WINDSCREEN ICE BEGINS TO PARTIALLY SLIDE OFF. DSND TO 6500 FT; GET A BIT MORE TO SLIDE OFF AND LEADING EDGE OF MAINS BEGIN TO CLR WITH BOOT ACTION AND BETTER TEMPS. TAIL STILL VIBRATING. AIRSPD IMPROVES AND ICE STEADILY COMES OFF. CLB BACK TO 7000 FT TO REMAIN ON RADAR AND COMPLETED ENRTE PORTION AND APCH TO TWR WITHOUT FURTHER EVENT. NEED IMPROVED SYS TO REMOVE ICE FROM C208B. BOOTS NOT EFFECTIVE ENOUGH. NEED SYS TO VIEW TAIL SECTION AND VISUALLY MONITOR ICE FORMATION OR MECHANICALLY/VISUALLY MEASURE TAIL ICE FORMATION.

Data retrieved from NASA's ASRS site as of January 2009 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.