THE ACFT WAS ON AN EMERG MED SVC (EMS/MED-EVAC) FLT WITH A PLT & FLT NURSE ABOARD TO TRANSPORT A MATERNITY PATIENT FM FLAGSTAFF TO PHOENIX. DRG A NGT ARRIVAL, THE PLT BGN A VOR-A APCH IN IMC, THEN HE RPRTD A PROBLEM WITH HIS AVIONICS & ELECTED TO MAKE A MISSED APCH. DRG THE MISSED APCH, HE SAID THAT HE 'LOST' AN INVERTER, THEN RPRTD THE GYROS WERE INOP. RADAR VECTORS WERE BEING PROVIDED WHEN HE STATED 'WE HAVE BIG TROUBLE HERE.' SOON THEREAFTER, RADAR & RADIO CTC WERE LOST & THE ACFT CRASHED APRX 7 MI SE OF THE ARPT. DRG IMPACT, THE ACFT MADE A DEEP CRATER & WAS DEMOLISHED. NO PREIMPACTENG OR AIRFRAME FAILURE WAS FND. AN INV REVEALED THE #2 (COPLT'S) ATTITUDE INDCR WAS INOP ON THE PREV FLT. A DISCREPANCYRPRT WAS TAKEN TO THE AVIONICS DEPT, BUT THE REQD ENTRY WAS NOT MADE IN THE ACFT FORM-4. THE PLT TKOF BEFORE CORRECTIVE ACTION WAS TAKEN. THE OPNS MANUAL REQD 1000 HRS MULTI-ENG TIME AS PIC & TRNG BY DESIGNATED CFI'S. THE PLT HAD APRX 837 HRS MULTI-ENG TIME, RECD 9 TRNG FLTS IN N6858S WITH NON-DESIGNATED INSTRUCTORS, COMPLETED A PART 135 FLT CHK ON 2/17/88.