HSI COMPASS SYSTEM FAILURE ON IMC APCH. EMERGENCY DECLARED. VECTORED TO VFR CONDITIONS.

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ADVISED BY CENTER MEDFORD NOW 1800 OVCST; 900 BROKEN RAIN; 1 1/2 MI VIS. TOPS RISING MODERATE TO SEVERE TURB. NON FLYING PAX GETTING FLT SICKNESS. ASKED FOR HIGHER ALT; GIVEN 22000'. PLT WAS AWARE THAT HE WAS ABOUT TO BE TURNED OVER TO MEDFORD APCH. PAX CONDITION WAS THE CONCERN. CENTER ASSIGNED 18000' DSCNT STARTED AT 500 FPM; INCREASED TO 1000 FPM; UP AND DOWN DRAFTS 2000-3000 FPM. HSI SET ON LOC BACK CRUISE DME 110.3; 320 DEGS INBND AT ASHIE D18.0; ACFT HDG 270 DEGS TO INTERCEPT. ASHIE DSNDING THROUGH 18000; UP AND DOWN DRAFTS 1000-2000'. MEDFORD APCH CORRECTED HDG FOR LOC BACK COURSE. INBND ON BACK COURSE STILL DSNDING; UP AND DOWN DRAFTS. 14 DME INBND HSI SLAVE COMP SYSTEM WENT OUT. CHANGED OVER TO SECONDARY SYS. LOC-G/S SET ON OED VOR AND CALLED FOR MISSED APCH. APCH CTL QUESTIONED THE MISSED APCH AND WAS ADVISED OF LOSS OF HSI SLAVE COMP SYS. ADVISED ACFT TO CLB TO 10000' CONTACT CENTER. CONTACTED CENTER TO ADVISE LOSS OF EQUIP. VACUUM SYS ON COPLTS SIDE WAS RESET TO MAG COMP. RIGHT SIDE VACUUM COMP TUMBLED WITH TURNS AND TURB. MAG COMP SPINNED AND DIPPED IN SEVERE TURB. CENTER GAVE GYRO OUT APCH TO ILS. PLT STILL TRYING EMER PROC TO GET HSI AND LOST EQUIP BACK ON LINE. CENTER TURNED ON ILS DME RWY 14; APPROX SAMIE D14.4; TURNED OVER TO MEDFORD APCH; ACFT TRACKING INBND ON ILS ON SECOND ILS G/S. DSNDING TO 8000' APCH ASKED WHAT RADIAL WE WERE ON AND SAID THAT CENTER STILL HAD ACFT ON RADAR AND WE WERE HEADED 180 DEGS FROM THE 140 DEGS INBND. IN EVALUATING THE LOSS OF EQUIP I ADVISED APCH I WAS DECLARING AN EMER AND WAS CLBING BACK TO 10000'. APCH ASKED WHAT TYPE OF EMER WE WERE DECLARING. I ADVISED OF THE EQUIP FAILURE AND THE ERROR IN THE DIRECTION BTWN APCH; CENTER AND ACFT. CENTER ADVISED THAT ROSEBURG RBG WAS VFR AND ASKED IF I COULD TRACK THE RBG AND CENTER WOULD GIVE MVA FOR THE DSCNT. ACFT WAS VFR AT RBG AND RETURNED TO MEDFORD VFR UNDER THE WX. SYNOPSIS: BACK COURSE STARTED FROM TOO HIGH ALT. DG ON COPLT SIDE BADLY LOCATED AND OUT OF REACH OF PLT IN SEVERE TURB. WHEN QUESTIONED BY APCH OF HDG WHILE SHOOTING THE ILS; THERE WAS A QUESTION IN THE PLT'S MIND OF WHAT RADIAL THE APCH WAS SPEAKING OF BECAUSE ACFT WAS ON ILS AND NOT A VOR APCH. WHEN APCH ADVISED CENTER HAD ACFT HDG 180 DEGS FROM THE INBND COURSE; PLT BECAME CONCERNED AND FEELS THE ACTIONS TAKEN IN DECLARING THE EMER; BEFORE MORE EQUIP FAILED; WAS IN BEST JUDGEMENT AT THE TIME CONDITIONS. HOLDING AN ASSIGNED ALT WAS IMPOSSIBLE DUE TO THE SEVERE TURB. PLT WAS MAKING CONSTANT PWR CHANGES AND PITCH CHANGES. HSI AND SLAVE COMP FOUND TO BE FAULTY AND HAS BEEN REPLACED. COPLT DG HAS BEEN REPLACED SO PLT CAN REACH AND RESET FROM THE LEFT SEAT. MEDFORD; OR; IS SURROUNDED BY MOUNTAINS. APCH DOES NOT HAVE RADAR AND AFTER ACFT ARE TURNED OVER FROM SEA CENTER THE MEDFORD APCH IS RELYING ON OLD METHODS. MFR NEEDS APCH RADAR. APCH HAD ASSIGNED A HOLDING PATTERN AT SAMIE AT 10000' AFTER THE MISSED APCH. TWO OTHER ACFT WERE ALSO HOLDING (ONE AT 9000') DUE TO THE UP AND DOWN DRAFTS AND THE LOSS OF THE SLAVE COMP; AND PROB WITH THE VACUUM COMP; IT WAS IMPOSSIBLE TO ENTER AND MAINTAIN A PATTERN.