AN ACR HVT FREIGHTER CREW TURNED ITS INS TO 'OFF.'

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DC-8 DEPART BRU. ON CLBOUT; INS FAILURE. 3 ON ACFT. #3 PROBABLY HAD NOT COUNTED DOWN BEFORE WE STARTED TAXI. THIS WAS THE 4TH LEG FOR THE CAPT AFTER IOE -- BRAND NEW; NO HVY TURBO JET TIME. THE FO HAS HAD 5 DAYS OFF IN THE LAST 90 DAYS -- COMPLETELY WORN OUT. THE CAPT HAD NO DIFFERENCES TRAINING. THIS ACFT IS DIFFERENT FROM THE REST OF FLEET. THE FO AND MYSELF HAD INITIAL TOGETHER IN JAN-FEB 1994 WE HAD NO INS TRAINING AT ALL. ON THIS ACFT; WHEN INS FAIL; FLT INSTS ALSO FAIL. WE DID NOT KNOW THAT THE INS CAN BE TURNED TO THE 'ATT' POS AND THAT LETS THE FLT INST SWITCH TO GYROSCOPE. WE WERE IN IMC CONDITIONS ABOUT 1700 FT. THE ACFT WAS IN UNUSUAL ATTITUDES. THE STANDBY GYROSCOPE ALSO WAS ACTING UP. WE DECLARED EMER. BRU GAVE US A SEPARATE FREQ WHILE THEY WORKED US. WE GOT THE FLT INSTS BACK; CANCELED THE EMER. THE INS WAS NEVER RECOVERED. WHEN THE INS FAILED; THE CAPT AND FO TURNED OFF 1 AND 2 INS. THEY WERE NEVER RIGHT. WE WERE IN A VERY; VERY BAD WAY FOR A FEW MINS. I WAS VERY CONCERNED FOR US. WE CONTINUED TO DEST WITHOUT INS AND NO MORE EVENTS. SUPPLEMENTAL INFO FROM ACN 282860: I WAS NEW IN TYPE (ONLY 50 HRS) AND UNFAMILIAR WITH THIS INDIVIDUAL ACFT AND ITS GYROSCOPE ARRANGEMENT (IT'S UNIQUE IN OUR FLEET). I AGREED TO THE FO'S SUGGESTION THAT WE TURN OFF THE INS SINCE WE FELT THEM ALL TO BE UNRELIABLE. A LACK OF ADEQUATE DIFFERENCES TRAINING. BOTH THE FO AND MYSELF WERE NOT SUFFICIENTLY AWARE OF THE UNIQUE FEATURES OF THIS AIRPLANE. FURTHERMORE; THE FO WAS FATIGUED FROM CONTINUOUS INTL OPS AND HAD PREVIOUSLY REQUESTED TO BE RELIEVED; CITING AN AVERAGE OF ONLY 4 OR 5 DAYS OFF A MONTH FOR THE PAST 6 MONTHS. SINCE HE WAS MORE EXPERIENCED IN THE AIRPLANE; HIS INPUT WAS VERY IMPORTANT AND I BELIEVE HIS RUN DOWN CONDITION WAS A FACTOR.