A C172RG INSTRUCTOR PLT FLYING IMC AT 8000 FT ENCOUNTERED LIGHT RIME ICE AND DSNDED 400 FT DUE AIRSPD INDICATING ZERO WITH PITOT HEAT FAILURE. ATC INSTRUCTED RPTR TO RETURN TO 8000 FT DUE TO OPPOSITE DIRECTION 7000 FT TFC.

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WHILE CRUISING AT 8000 FT MSL IN IMC AND LIGHT TURB WITH A STUDENT ON A TRAINING FLT; ATC GAVE ME A HDG OF 340 DEGS FOR WX. THE ACFT BEGAN TO ACCUMULATE LIGHT RIME ICE. AFTER A FEW MINS; THE ACFT ENTERED FLT CONDITIONS BTWN 2 LAYERS OF CLOUDS. THE ICE THAT HAD ACCUMULATED BEGAN TO DISSIPATE. ATC THEN GAVE ME A HDG OF 290 DEGS AND DIRECT GOPHER WHEN ABLE. THE ACFT ENTERED IMC ONCE AGAIN AND BEGAN PICKING UP LIGHT RIME ICE AGAIN. I RPTED THIS TO ATC AND WAS TOLD WE COULD EXPECT LOWER IN ABOUT 10 MINS. CONTRIBUTING FACTORS: ALTHOUGH THE ICING CONDITIONS WERE A CONTRIBUTING FACTOR; THE ICING WAS ONLY LIGHT. I EXPECTED A DSCNT WITHIN 10 MINS WHICH WOULD PUT THE ACFT AT AN ALT WHERE ICING WOULD NOT BE A FACTOR. IN ADDITION; I KNEW AN ADDITIONAL 10 MINS IN THOSE CONDITIONS WOULD NOT BE A CONCERN BASED ON HOW SLOWLY THE ICE WAS ACCUMULATING. EVENT: WHILE CRUISING AT 8000 FT MSL; I SAW THE AIRSPD INDICATOR WAS DECREASING IN AIRSPD. I TOOK CTL OF THE ACFT AND BEGAN TO TROUBLESHOOT THIS PROB. MY FIRST REACTION WAS TO SLIGHTLY PITCH THE NOSE FORWARD TO PREVENT A STALL. SIMULTANEOUSLY; I BEGAN XCHKING THE FLT INSTS; CHKING THAT THE PITOT HEAT WAS ON; CHKING THAT ALL THE CIRCUIT BREAKERS WERE IN AND RECYCLING THE PITOT HEAT ROCKER SWITCH. I DETERMINED THAT THE PITOT HEAT WAS NOT WORKING RESULTING IN THE AIRSPD INDICATOR READING ZERO. BY THAT TIME; THE ACFT HAD DSNDED 400 FT TO 7600 FT MSL. I IMMEDIATELY RPTED THIS DSCNT TO ATC. THEY RESPONDED WITH THE INSTRUCTIONS TO CLB BACK TO 8000 FT MSL DUE TO TFC AT 12 O'CLOCK POS AND 7000 FT; 1 1/2 MI AHEAD. I IMMEDIATELY CLBED BACK TO 8000 FT MSL. REACTION: MY IMMEDIATE CONCERN WAS TO MAINTAIN POSITIVE CTL OF THE ACFT WHILE XCHKING THE INSTS. MY INSTINCT IS TO LOWER THE NOSE OF THE ACFT WHEN I OBSERVE A LOSS IN AIRSPD TO PREVENT A STALL WHILE SIMULTANEOUSLY REVIEWING THE ENTIRE SIT. WHEN I DETERMINED THAT THE AIRPLANE WAS NOT IN A STALL BUT IN FACT THE AIRSPD INDICATOR HAD FAILED; I NOTIFIED ATC OF THE DSCNT AND INITIATED THE CLB BACK TO ALT PER THEIR INSTRUCTION. IN RECYCLING THE PITOT HEAT ROCKER SWITCH; THE PITOT HEAT DID COME BACK ON AND THE AIRSPD INDICATOR DID START TO WORK AGAIN. I DID NOT DECLARE AN EMER TO ATC SINCE I WAS EVALUATING THE PROB AND BELIEVED IT WAS MORE IMPORTANT TO DETERMINE WHAT EXACTLY THE SIT WAS. BY THE TIME I RESOLVED THE PROB; THERE WAS NO EMER TO DECLARE; ONLY THE LOSS OF ALT TO RPT. HUMAN FACTORS: USING THE AVIATE; NAV; COMMUNICATE PRIORITY; I BELIEVE MY ACTIONS WERE REASONABLE. I FOCUSED ON MAINTAINING CTL OF THE ACFT AND RESOLVING THE PROB. ONCE THAT WAS ACCOMPLISHED; I RPTED THE ALT LOSS TO ATC. PERHAPS I SHOULD HAVE INFORMED THEM OF THE TEMPORARY LOSS OF AIRSPD AND THE SUBSEQUENT RETURN OF THE AIRSPD INDICATOR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT UPON CONTACTING MINNEAPOLIS APCH; SHE WAS GIVEN A PHONE NUMBER TO CALL UPON LNDG. THE SUBSEQUENT CALL TO CTR WAS CORDIAL AND THEY UNDERSTOOD HER SIT. THE ONLY COMMENT FROM THE CTLR WAS IF SHE NEEDED SPECIAL HANDLING AN EMER DECLARATION WOULD GET HER AID. SHE WAS GIVING INST INSTRUCTION AT THE TIME AND ALTHOUGH FLYING IN ICING WAS NOT IDEAL; SHE ALSO FLIES CHARTER IN THE C340 AND C402 AND IS FAMILIAR WITH ICING CONDITIONS. THE SIT IN WHICH SHE FOUND HER INST STUDENT WAS NOT SERIOUS ENOUGH TO WARRANT IMMEDIATE ACTION. SHE STATED IF THEY DID NOT GET THE AIRSPD INDICATOR BACK; SHE WOULD HAVE DECLARED AN EMER.