HELICOPTER PILOT ENCOUNTERS ICING CONDITIONS DURING NIGHT IFR FLIGHT AND CONTINUES TO DESTINATION.

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I ACCEPTED A VFR FLT FROM THE BASE ARPT TO AN ACCIDENT SCENE ALONG THE LAKE'S SHORELINE. FORECAST AND RPTED CONDITIONS WERE VFR WITH VISIBILITIES P6SM AND THE LOWEST CEILING APPROX 2000 FT AGL WITH MOST HIGHER. 5 MI FROM THE SHORELINE AND 10 NM FROM THE ACCIDENT SCENE; I ENCOUNTERED AN UNFORECASTED SCATTERED LAYER WITH BASES AT 500 FT AND TOPS NEAR 1000 FT AGL. I EASILY AVOIDED THE CLOUDS. FORTUNATELY; A NEWS HELI ENCOUNTERED THE SAME SCATTERED LAYER AND REMARKED ON HELI COMMON TFC 123.025 THAT IT BECAME A BROKEN LAYER ALONG THE LAKE SHORE. I QUERIED THE PLT AND BASED UPON HIS OBSERVATION; DETERMINED THE BEST DEST HOSPITAL TO AVOID WX. ON THE GND; A COMPANY PLT CONFIRMED THE RTE FROM THE ACCIDENT SCENE TO THE HOSPITAL AS VFR AND THAT METARS AND FORECASTS STILL SUPPORTED EACH OTHER. THE PART 135 FLT WITH THE PATIENT WAS UNEVENTFUL AND THE SCATTERED LAYER WAS LOCALIZED TO WITHIN 5 SM OF THE LAKE'S SHORELINE. AFTER REFUELING I CONDUCTED ANOTHER WX REVIEW WITH OBSERVATIONS AND FORECASTS SHOWING VFR FOR THE RTE HOME. ONE STATION; APPROX 30 NM W OF MY DEST; RPTED AND FORECASTED 3 SM VISIBILITY -- STILL WITHIN COMPANY NIGHTTIME VFR MINIMUMS; AND IT WAS THE ONLY STATION RPTING SUCH NEAR MY RTE. DEST AWOS MIN WX RPTED 007 SCATTERED; 21 BROKEN WITH 10 SM VISIBILITY. THE NEWS COPTER PIREP AND THE 007 SCATTERED CONCERNED ME ENOUGH TO CHK FOR A DIRECT IFR FLT BACK TO BASE; THINKING THE LAKE SHORE LAYER MIGHT MOVE INLAND; BUT I STILL HAD PLENTY OF VFR OPTIONS. IFR PLANNING SHOWED AN AIRMET FOR POTENTIAL LIGHT ICING. PIREPS SHOWED 3 ACFT RPTING TRACE AND/OR 'BARELY LIGHT' ICING WITH THE LOWEST ICING RPTED AT 4000 FT BY A C172 SLIGHTLY W OF MY DEST; AND THE 2 LATEST PIREPS SHOWED NO ICING NEAR MY RTE AND DEST. WIDESPREAD CEILINGS RPTED 3000 FT AND HIGHER. OFFICIAL SUNSET GETTING CLOSE; I CHOSE A VFR FLT WITH COMPANY FLT FOLLOWING BASED UPON THE WIDESPREAD VFR; BUT FILED (BUT NOT ACTIVATED) AN IFR FLT PLAN AS MY BACK-UP PLAN. I WAS CONCERNED ABOUT THE LAKE SHORE LAYER THICKENING TO A CEILING AND MOVING INLAND; AND DIDN'T WANT AN EMER CLB NEAR CLASS B AIRSPACE. THINKING ICING PROBABILITY AND ICING RATE A LOWER RISK IF WX BECAME QUESTIONABLE; I WOULD ACTIVATE MY FLT PLAN AS MY BAILOUT. THE SUN SET BY THE TIME I PICKED UP THE MEDICAL CREW AND DEPARTED THE HOSPITAL FOR THE RETURN FLT. ALL FORECASTS TO THE CONTRARY; A NAGGING FEELING TOLD ME I WAS GOING TO BE DODGING WX IN AN UNFAMILIAR AREA IF I REMAINED VFR. I TRUSTED MY GUT FEELING 5 MINS INTO THE VFR FLT; I ACTIVATED MY IFR FLT PLAN WITH APCH. THE CLB TO 4000 FT WAS IN VMC. WITHIN 5 MINS OF THE 35 MIN IFR FLT THE CONDITIONS GRADUALLY CHANGED TO IMC. AT ABOUT 22 MINS INTO THE IFR FLT I ENCOUNTERED LIGHT ICING; WHICH I FIRST THOUGHT WAS CONDENSATION AROUND THE WINDSHIELD WIPER. AT ABOUT 25 MINS; I REALIZED IT WAS ICING; BUT WAS BEGINNING MY APCH AND FELT THIS WAS THE BEST OPTION FOR GETTING OUT OF THE ICING. I BROKE OUT AT 2100 FT AS THE AWOS RPTED WITH VISIBLE GND LIGHTS; BUT COULD NOT SEE THE ARPT LIGHTS. WITHIN 3 MI OF THE ARPT; I ENCOUNTERED ANOTHER LAYER WHILE LEVELED AT MDA. AT 3/4 MI I WAS ABLE TO SEE THE RWY LIGHTS AND LANDED UNEVENTFULLY. I CALLED A PIREP FOR ICING AND UNFORECASTED LOW CEILINGS DURING MY IFR CLOSEOUT. 10 MINS LATER THE AWOS RPTED 006 BROKEN. IN HINDSIGHT; I COULD HAVE TURNED AROUND FOR VMC AND OUT OF THE ICING. SOME 'GET HOMEITIS' PROBABLY CAME INTO PLAY. IF CONDITIONS HAD WORSENED AND A MISSED APCH NEEDED; I COULD HAVE BEEN FLYING LONGER IN ICING CONDITIONS. BECAUSE I ENCOUNTERED NO ASSOCIATED ICING PROBS WITH THE ACFT AND I WAS ALREADY BEGINNING THE APCH; I DID NOT DECLARE AN EMER FOR ICING. PLUS; I FELT I WAS OPENING MYSELF UP FOR A VIOLATION; WHY DID YOU CONDUCT A PLANNED IFR FLT IN KNOWN ICING CONDITIONS? IT WAS MY BACK UP PLAN; BUT HOW COULD I PROVE THAT. USUALLY; ANY ONE OF ICING AIRMETS; TEMP; OR PIREPS IMMEDIATELY DISQUALIFIES AN IFR FLT FOR ME. INITIALLY I FELT A VFR FLT WAS SAFE WITH A LOW RISK IFR FLT PLAN IN MY POCKET AS BACKUP. HOWEVER; I COULD HAVE DELAYEDTHE RETURN FLT TO SEE IF MY HUNCH PLAYED OUT. BUT THAT WOULD HAVE PUT ME PAST MY 14 HR DUTY DAY WITHOUT SUPPORTING DATA EVEN THOUGH MY COMPANY WOULD HAVE SUPPORTED MY DECISION. PERHAPS THIS WAS A SECOND CASE OF GET HOMEITIS.