INSTRUCTOR AND STUDENT PLTS ABOARD PA30 HAVE ALT AND TRACK DEVS IN IMC DUE TO MALFUNCTIONING HSI.

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I WAS ACTING AS A CFII AND THE STUDENT PLT (AND OWNER OF THE ACFT) WAS OPERATING THE ACFT. THE STUDENT PLT HAS HIS PVT PLT; SINGLE ENG VFR RATING. HE IS ALSO AN A&P MECH AND IA. DURING THE VOR/DME RWY 34 APCH; IN IMC CONDITIONS; THE STUDENT WAS HAVING DIFFICULTY HOLDING THE ASSIGNED ALT AND HDG. I DID NOT SEE THE ALT VARY MORE THAN ABOUT +/-300 FT. THE HDG WAS SOMETIMES AS MUCH AS 20/30 DEGS OFF COURSE. EACH TIME I ADVISED THE STUDENT TO CORRECT BUT SOMETHING ELSE WOULD GO WRONG. A FEW TIMES I HELD THE CTLS TO STABILIZE CONDITIONS BUT I THEN RETURNED THE CTLS TO THE STUDENT. WE HAD INTERCEPTED THE FINAL APCH COURSE AND WERE TURNED OVER TO THE TWR. I WAS VERBALLY GUIDING THE STUDENT BUT WE STILL DRIFTED OFF COURSE AND WERE TOLD TO RETURN TO THE APCH FREQ. AT THAT TIME I REALIZED THAT THE HSI WAS STICKING. THE INST WOULD FLOW FREELY; THEN LOCK UP; THEN JUMP TO THE CORRECT HDG. WE BROKE OFF THE APCH AND WERE VECTORED BACK FOR A SECOND ATTEMPT. AT SOME POINT I BEGAN FLYING THE PLANE. I INQUIRED ABOUT A VFR ARPT BUT WAS ADVISED THAT ALL ARPTS IN THE AREA WOULD REQUIRE AN IFR DSCNT. WITH SOME DIFFICULTY I WAS ABLE TO SUCCESSFULLY FLY THE APCH AND MAKE A SAFE LNDG. UPON LNDG; TWR PERSONNEL ASKED ABOUT THE CONDITIONS ON THE APCH. I ADVISED THAT THE RAIN WAS LIGHT TO MODERATE; TURB WAS LIGHT; AND THAT I BROKE OUT OF IMC CONDITIONS AT 800 FT. I BELIEVE THAT I MAY HAVE MISSPOKEN OR BEEN MISUNDERSTOOD WHEN I RPTED 800 FT BECAUSE I MEANT 800 FT AGL NOT MSL. THE MDA FOR THE APCH IS 1080 FT MSL AND THEREFORE BROKE OUT OF IMC CONDITIONS AT ABOUT 1400 FT MSL. ONCE ON THE GND WE EXAMINED THE HSI. THE STUDENT/A&P LOOKED AT THE HSI AND DETERMINED THAT THE GLASS ON THE INST WAS LOOSE AND WAS BINDING AGAINST THE ROTATING HDG INDICATOR. A TEMPORARY REPAIR WAS MADE AND WE EVENTUALLY DEPARTED WITHOUT INCIDENT. I BELIEVE THAT THERE ARE INHERENT PROBS WHILE TRAINING IN IMC CONDITIONS. I BELIEVE THAT ALTHOUGH YOU MUST LET THE STUDENT FLY THE PLANE TO GET THE EXPERIENCE HE NEEDS TO LEARN; THERE IS A POTENTIAL FOR VIOLATING ALT RESTRS OR HDG ASSIGNMENTS. YOU HAVE TO WAIT LONG ENOUGH TO SEE IF THE STUDENT CATCHES HIS MISTAKES ON HIS OWN BEFORE YOU GRAB THE CTLS. YOU CAN VERBALLY TELL THE STUDENT WHEN HE IS VIOLATING A HDG OR ALT ASSIGNMENT BUT YOU ALSO HAVE TO SEE IF HE CORRECTS THE PROB ON HIS OWN; AND WHETHER OR NOT THE CORRECTION IS IMMEDIATE AND EXACT. IN THE ABOVE MENTIONED CASE THERE WAS AN ADDITIONAL FACTOR. THE INTERMITTENT HSI WAS NOT IMMEDIATELY RECOGNIZED AS FAULTY BECAUSE IT SEEMED TO BE MOVING (WORKING) WHEN THE PLANE WAS TURNED. HAD IT FAILED COMPLETELY IT WOULD HAVE BEEN EASIER TO RECOGNIZE AND CORRECTED FOR. WHEN IT WAS RECOGNIZED AS BEING INTERMITTENT IT ALSO CREATED A DISTR IN THE COCKPIT. AN INORDINATE AMOUNT OF ATTN WAS PAID TO THE HSI AND AS A RESULT; THE PROPER HOLDING OF ALT ALSO SUFFERED. IT SEEMED THAT BOTH PLTS KEPT LOOKING AT THE HSI INSTEAD OF SCANNING ALL THE INSTS AS NECESSARY. WE ACTUALLY USED THE GARMIN 430 (EXPIRED DATABASE) TO HOLD THE HDG. IN SUM: 1) WHEN TRAINING IN ACTUAL IFR THERE IS A POTENTIAL TO VIOLATE ALT AND HDG ASSIGNMENTS; 2) INTERMITTENT INSTS CAN BE MORE PROBLEMATIC THAN A FAILED INST; 3) THE USE OF GPS FOR SECONDARY (AND PRIMARY) GUIDANCE SHOULD BE ENCOURAGED.