C172 FLT INSTRUCTOR AND STUDENT ENCOUNTER SEVERE ICING CONDITIONS. UNABLE TO MAINTAIN ALT; DECLARE EMER AND RECEIVE VECTORS TO LOWER ALT.

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MY STUDENT WAS FLYING AND WE CLBED THROUGH A FEW CLOUDS; ICE FREE; TO 10000 FT. AS WE WERE CRUISING TOWARD ZZZ WE WERE FLYING OVER WHAT APPEARED TO BE A STRATUS LAYER THAT WAS SLOWLY SLOPING UPWARD WITH CLR SKY ABOVE US. AT ZZZ THE TOPS APPEARED TO BE AROUND 9000 FT AND JUST PAST ZZZ THERE WERE A FEW AREAS IN THE STRATUS LAYER THAT HAD DEVELOPING CLOUDS PUSHING UP THROUGH THE STRATUS TO FORM WHAT LOOKED LIKE THE TOPS OF PUFFY CUMULUS CLOUDS. THE TOPS OF THE CUMULUS WERE HIGHER THAN WE WERE; BUT NOT BY MUCH. THIS MADE SENSE TO ME BECAUSE ZZZ IS OVER A MOUNTAIN RANGE THAT CONNECTS THE VALLEY WE WERE IN TO THE VALLEY WHERE WE WERE HEADING. I BELIEVED THAT THE TOPS OF THESE CLOUDS THAT WE WERE SEEING WERE THE RESULT OF THE AIR BELOW BEING FORCED UP ALONG THIS MOUNTAIN RIDGE. AS WE CROSSED ZZZ. I NOTICED THAT THERE WAS A CLOUD TOP ALONG OUR RTE THAT WAS SLIGHTLY HIGHER THAN WE WERE. JUST PAST THIS CLOUD AND THE FEW OTHER CLOUD TOPS THAT HAD FORMED WAS THE SAME FLAT STRATUS LAYER THAT WE HAD BEEN FLYING OVER PREVIOUSLY WITH NO CLOUDS PUSHING THROUGH. I THOUGHT THIS WOULD BE A GREAT OPPORTUNITY FOR MY STUDENT TO EXPERIENCE FLYING THROUGH A CLOUD AND SEE WHAT IT IS LIKE TO GO FROM VMC INTO IMC. I WANTED MY STUDENT TO EXPERIENCE THE 'WHOOSH' SENSATION AND DISORIENTATION THAT ONE CAN GET WHEN ENTERING A CLOUD AND TO TRUST THE INSTS AND NOT HER FEELINGS. I KNEW THAT THERE WAS A CHANCE THAT WE COULD ENCOUNTER ICE IN THE CLOUD; BUT I KNOW THAT WE WERE JUST AT THE TOP OF THE CLOUD AND WE COULD CLB OUT QUICKLY IF WE DID GET ICING. THE CLOUD WAS NOT VERY LARGE AND I KNEW WE WOULD PUNCH OUT THE OTHER SIDE IN A FEW MINS ANYWAY. I DIDN'T WANT TO ENCOUNTER ICE; BUT I ALSO KNEW THAT IF WE DID; WE COULD GET OUT QUICKLY; SAFELY; AND I COULD DEMONSTRATE TO MY STUDENT WHAT TO DO IF YOU DO ENCOUNTER ICING. AS WE ENTERED THE CLOUD; WE STARTED TO PICK UP TRACE AMOUNTS OF ICE ON THE WINDSCREEN. AS SOON AS I COULD SEE ICE ON THE WINDSCREEN; I CALLED APCH AND REQUESTED AN IMMEDIATE CLB DUE TO ICE AND LET HIM KNOW THAT WE COULD TAKE A HDG CHANGE. EVEN THOUGH IT WAS TRACE TO LIGHT ICE AND I KNEW WE WOULD BE OUT OF THE CLOUD SOON AT THE CURRENT ALT; I WANTED TO BE OUT OF THE ICE AS QUICKLY AS POSSIBLE AND I WANTED MY STUDENT TO SEE WHAT YOU SHOULD DO AND SAY WHEN YOU ENCOUNTER ICING. WE WERE GIVEN 12000 FT AND SHE STARTED TO CLB. WE PUNCHED OUT THE SIDE OF THE CLOUD WHILE WE WERE STILL CLBING. WE PICKED UP LESS THAN 1/8 INCH OF ICE; BUT ENOUGH ICE THAT YOU COULD SEE IT ON THE WINDSCREEN; STRUTS; AND OTHER VARIOUS PARTS OF THE ACFT. WE WERE OVER A LARGE FLAT STRATUS LAYER THAT STRETCHED AS FAR AS YOU COULD SEE ALL AROUND US WITH NOTHING BUT BLUE SKY AND SUN IN FRONT OF AND ABOVE US. THE CLOUD TOPS LOOKED TO BE AROUND 9000-10000 FT. WE WERE CRUISING DIRECT AND THE LITTLE ICE WE HAD PICKED UP WAS NOT MELTING OR BREAKING OFF. I DIDN'T THINK THE STRATUS LAYER WOULD HAVE THE ICING THREAT THAT THE CUMULUS CLOUD WE ENCOUNTERED EARLIER BY THE MOUNTAIN RANGE HAD BECAUSE I THOUGHT THAT CUMULUS CLOUDS POSSESSED A HIGHER ICING THREAT THAN STRATUS CLOUDS. THE STRATUS LAYER TOP ALONG OUR RTE SLOWLY SLOPED UP OR CLBED UP TO OUR ALT. AS SOON AS WE ENTERED THE STRATUS LAYER; ICE BEGAN BUILDING ON THE WINDSCREEN AT AN INCREDIBLE RATE. I TOLD APCH THAT I NEEDED LOWER IMMEDIATELY DUE TO ICE AND WE GOT 8000 FT. I ASKED FOR LOWER BECAUSE WE COULD NOT CLB MUCH HIGHER IN THE C172 AND THE CLB WOULD BE AT A SLOW RATE BECAUSE OF THE ALT. I ALSO THOUGHT THAT IF I CHANGED MY ALT BY 3000 FT OR MORE; I SHOULD BE OUT OF THE ICING. TURNING AROUND WAS NOT AN OPTION BECAUSE I DIDN'T KNOW IF THE CLOUD TOPS BEHIND US HAD RISEN OR NOT; AND IF THEY HAD; WE WOULD STILL BE IN ICE. A DSCNT; IN MY JUDGEMENT WAS THE BEST OPTION AND WE DSNDED FAIRLY RAPIDLY; AROUND 1500 FPM. THERE WAS A LOT OF ICE ON THE WINDSCREEN AND SOLID CONTINUOUS MODERATE TURB IN THE DSCNT. THIS WAS UNUSUAL BECAUSE WE DIDN'T HAVE ANY TURB BEFORE; AND THIS WAS A STRATUS LAYER. I COULD HEAR THE ICE HITTING THE ACFT AS IT WAS ACCUMULATING. IT SOUNDED LIKE SAND HITTING THE ACFT. I DIDN'T LOOK OUTSIDE THE ACFT AT THIS TIME TO SEE HOW MUCH ICE THERE WAS BECAUSE I WAS ENGROSSED WITH WATCHING THE INSTS AS MY STUDENT WAS DSNDING BECAUSE MY STUDENT WAS FLYING AND THE IMC TURB WAS VERY DISORIENTING. I DID LOOK UP FROM TIME-TO-TIME AT THE WINDSCREEN. PART OF THE WAY DOWN IN THE DSCNT THE ICE HITTING THE ACFT BECAME LIQUID WATER. I HAVE FLOWN IN RAIN BEFORE; BUT NOT LIKE THIS. THERE WAS A LOT OF WATER. I DON'T KNOW IF WE WERE IN FREEZING RAIN; SLD; OR JUST RAIN; BUT WE HIT SOME FORM OF SEVERE ICE. ICE HAD ACCUMULATED AT AN INCREDIBLE RATE. I DON'T KNOW IF IT WAS FROM HAVING SOME ICE ON THE ACFT THAT GAVE THE LIQUID WATER A NUCLEATION SOURCE OR IF IT WAS FROM THE ACFT THAT HAD BEEN SUPER COLD AT THE HIGHER TEMP BUT WE WERE COVERED IN ICE. I DIDN'T REALIZE THIS UNTIL WE TRIED TO LEVEL OFF AT 8000 FT. IT ONLY TOOK US BTWN 2-3 MINS TO MAKE THE DSCNT AND I ASKED FOR LOWER AS SOON AS WE ENTERED THE CLOUD; SO ADD MAYBE 30 SECONDS FOR THE CALL TO THE DSCNT TIME AND THAT IS HOW LONG IT TOOK. IN THAT TIME WE WENT FROM LESS THAN 1/8 INCH OF ICE TO OVER 1 INCH OF CLR ICE CAPPING OVER THE RIME AND MIXED ICE. I SAY I DIDN'T REALIZE IT UNTIL 8000 FT BECAUSE THAT IS WHERE WE TRIED TO LEVEL OFF. FROM HERE I DON'T REMEMBER EXACTLY THE ORDER OF THE FACTS OF HOW THEY HAPPENED OR WHAT WAS SAID BTWN ME AND CTR; BUT THIS IS MY BEST MEMORY OF WHAT HAPPENED. AS WE GOT DOWN TO 8000 FT MSL; I NOTICED THAT THE TEMP WAS AT 0 DEGS C. IT LOOKED LIKE RAIN WAS HITTING US BECAUSE WATER WAS RUNNING OVER THE ICED-OVER WINDSCREEN. I NOTICED THAT WE WERE LOSING AIRSPD WITH FULL PWR AFTER THE LEVELOFF. I TOLD CTR THAT WE NEEDED TO GET LOWER; TO A WARMER ALT FOR ICE. HE TOLD US THAT 7000 FT WAS HIS MVA. WE DSNDED FAST TO 7000 FT. AS WE TRIED TO LEVEL OFF; THE ACFT WAS LOSING AIRSPD. THE TEMP WAS STILL SHOWING 0 DEGS C AT 7000 FT. I TOLD CTR THAT I THOUGHT WE WERE GOING TO HAVE TO DECLARE AN EMER. I DON'T THINK I EVER ACTUALLY DECLARED AN EMER BY SAYING 'I'M DECLARING AN EMER;' BUT I THINK THAT CTR TOOK CARE OF THAT FOR ME. (AFTER FIRST WRITING THIS RPT MY STUDENT TOLD ME THAT AFTER TELLING CTR THAT I THOUGHT WE WERE GOING TO HAVE TO DECLARE AN EMER THAT THEY ASKED ME IF WE WERE DECLARING AN EMER AND I SAID YES). CTR GAVE US A VECTOR FOR A LOWER ALT. AS WE TURNED TO THE HDG; MY STUDENT TOLD ME THAT THE CTLS FELT FUNNY. I TOOK FULL CTL OF THE ACFT AND NOTICED THE CTLS WERE VERY SLUGGISH AND NOT VERY RESPONSIVE. WE HAD FULL PWR AND THE ACFT WAS STILL LOSING AIRSPD. AT THIS POINT; THE WHOLE ACFT BEGAN TO VIOLENTLY BUFFET. I DON'T KNOW IF IT WAS FROM TURB CAUSED BY AIR FLOWING OVER THE ICE HORNS OR IF THE PROP HAD THROWN ICE FROM 1 BLADE AND NOT THE OTHER; OR IF THE ACFT WAS ABOUT TO STALL. ALL THAT MATTERED TO ME WAS NOT STALLING; SO I PUSHED THE NOSE OVER. THE BUFFET STOPPED. I KNEW WE WERE OVER A NARROW VALLEY BTWN MOUNTAIN RANGES FROM THE GPS. I ASKED CTR HOW SOON WE COULD GET LOWER AND HE TOLD ME 7 MI. I TOLD HIM WE COULD NOT MAINTAIN ALT AND THAT WE WERE DSNDING. HE TOLD ME WE WERE BELOW HIS MVA AND ASKED WHAT ALT WE COULD HOLD. I DIDN'T KNOW HOW TO ANSWER THE QUESTION. I TRIED TO LEVEL OFF AT 6500 FT AND SAID 6500 FT. WE GOT A NASTY BUFFET AGAIN FOLLOWED BY ME PUSHING THE NOSE OVER AGAIN. THIS WENT ON FOR A WHILE. I WOULD TRY AND LEVEL OFF AND TELL HIM WE WERE TRYING TO MAINTAIN ALT. WE WOULD HOLD ALT FOR A WHILE UNTIL THE AIRSPD GOT BTWN 80-70 KTS; A NASTY BUFFET; AND ME PUSHING THE NOSE OVER. EVERY NOW AND THEN THE CTLR WOULD ASK ME A QUESTION AND I WOULD ANSWER. I DON'T REMEMBER MUCH OF WHAT WAS SAID. I DO REMEMBER HIM ASKING ME THE QUESTION OF HOW MANY SOULS WERE ON BOARD; HOW MUCH FUEL; AND THE TYPE OF ICE. AT SOME POINT I LOOKED BACK AT THE ELT ANTENNA AND SAW THAT IT HAD OVER 1 INCH OF ICE COVERING IT. THE STRUT OUT MY SIDE WINDOW HAD WATER THAT HAD ROLLED BACK AND FROZEN TO THE ACFT IN THICK LINES OF ICE AND LARGE FROZEN DROPS AT THE ENDS OF THEM. THE LEADING EDGE OF THE STRUT HAD LARGE HORNS PROTRUDING AT A 45 DEG ANGLE OUT AND UP THAT WERE OVER 1 INCH LONG. ALL OF THIS ICE HAD OCCURRED IN 3 MINS TIME. I COULD NOT SEE THE LEADING EDGE OF THE WING; BUT I COULD SEE CLR ICE THAT HAD FLOWED BACK AND FROZEN UNDER THE WING. SOMEWHERE BTWN 4500-5000 FT; I COULD SEE THE GND UNDER THE ACFT; BUT WE WERE STILL IN THE CLOUDS. WE BROKE OUT AT SOME POINT AND WE COULD FINALLY HOLD ALT AROUND 3800 FT. I TOLD CTR THAT WE WERE VMC AND THAT WE COULD REMAIN VFR. WE STILL HAD FULL PWR; SLOW AIRSPD; AND A LOT OF ICE; BUT WE COULD HOLD ALT. THE TEMP WAS 6 DEGS C. IT TOOK A WHILE AT THAT ALT FOR THE ICE TO START TO COME OFF; MAYBE AS MUCH AS 10 MINS. AFTER IT STARTED COMING OFF; IT ALL CAME OFF QUICKLY. WHEN WE GOT THE BUFFET EARLIER IN THE FLT; IT WAS SO HARD THAT I WAS SURPRISED THAT IT DIDN'T KNOCK OFF SOME OF ICE. FROM HERE WE WENT DIRECT TO ZZZ2 FOR LNG; BUT AS WE GOT CLOSER; WE REALIZED THAT WE COULD MAKE IT TO ZZZ3 VFR; SO WE DID.