PROX OF CLOUDS AND GND WHILE SMA PLTS PRACTICE UNAUTH SCUD RUNNING FLT TO DEST ARPT.

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FLYING WITH CFII. FLT STARTED AS AN IFR TRAINING FLT FROM STP-RST-ONA. IN VMC FROM STP-RST STEADILY PICKING UP RIME ICE BUT UNABLE TO DSND BELOW CLOUDS (CEILING 2500 FT/MEA 3000 FT). AFTER LNDG AT RST WE DECIDED TO COMPLETE THE TRIP TO ONA BELOW THE CLOUDS; VFR. (WX AT RST WAS 600 FT AND 4 MI BUT ONA IS DOWN IN THE VALLEY AND HAD 1500 FT AND 10 MI.) WE DEPARTED RST WITH A SVFR CLRNC; BUT FOUND THAT BEFORE WE REACHED THE POINT WHERE WE COULD DSND DOWN INTO THE VALLEY; CEILINGS LOWERED TO LESS THAN 500 FT. THE ONLY WAY WE COULD INSURE OBSTACLE CLRNC WAS TO FLY DIRECTLY OVER THE INTERSTATE HWY. (THIS IS A VERY SPARSELY POPULATED AREA BUT TO MAINTAIN VFR WE HAD TO BE OVER THE TFC ON THE HWY AT LESS THAN 500 FT.) WHAT CONTRIBUTED TO OUR GETTING INTO THIS SIT WAS; OF COURSE; 'GET HOMEITIS' COMBINED WITH MY INSTRUCTOR AND I BOTH RELYING ON THE OTHER'S JUDGEMENT. HE HAS EXTENSIVE EXPERIENCE FLYING IN IMC. HE KNEW THAT I HAD FLOWN EXTENSIVELY AT LOW LEVELS AND WAS VERY FAMILIAR WITH THE AREA; INCLUDING TWR LOCATIONS. WE MADE IT THROUGH; BUT BY A MUCH NARROWER MARGIN THAN EITHER OF US CARED FOR. NEXT TIME; I'LL RELY ON MY OWN JUDGEMENT A LOT MORE; AND ANYONE ELSE'S A LOT LESS.