FO OF A B727-200 OVERSHOT DSCNT XING ALT DUE TO DISTR OF A FUEL IMBALANCE PROB; INOP FUEL GAUGE AND A DIRECTIONAL GYROSCOPE PROB.

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WE WERE ASSIGNED THE FOWEE 3 ARR; CROSS JUNUR AT 16000 FT. AS THE PF; I BEGAN THE DSCNT TO CROSS THE ASSIGNED FIX AT THE ASSIGNED ALT. THE CAPT WAS ENGAGED IN TROUBLESHOOTING A FUEL SYS PROB THAT COULD HAVE LED TO AN UNBALANCE SIT. I SHOULD HAVE BEEN CONCENTRATING MY EFFORTS SOLELY TO FLYING THE ACFT. INSTEAD I HAD NOTICED AN ADDITIONAL PROB THAT COULD HAVE BEEN RELATED TO THE FUEL SIT OR A DIRECTIONAL GYRO PROB. I WAS TRYING TO ASCERTAIN WHAT; IF ANY; RELATIONSHIP THERE WAS BTWN THE TWO. I WAS CAREFULLY MONITORING MY DISTANCE FROM JUNUR AND GAUGING MY DSCNT BY THE RATE AND DISTANCE FROM JUNUR. I WAS SCANNING MY ALTIMETER; BUT I WAS NOT CONCENTRATING ON IT. THE CAPT NOTICED THE ALTDEV BEFORE I DID. WHEN I RETURNED MY ATTN TO THE ALTIMETER; I WAS DISORIENTED BY NOT BEING ABLE TO READ THE ALTIMETER INSTANTLY AND THEREFORE DID NOT STOP THE RATE OF DSCNT UNTIL I HAD DSNDED 1500 FT BELOW ASSIGNED. ONCE STABILIZED; I RETURNED THE ACFT TO THE ASSIGNED ALT. NO INQUIRY BY ATC WAS MADE! CONTRIBUTING FACTORS: 1) BOTH PLTS WERE ENGAGED IN TROUBLESHOOTING EFFORTS. THE PF SHOULD NOT HAVE BEEN DOING SO; THEREBY CONCENTRATING HIS EFFORTS TO COMPLYING WITH ATC REQUIREMENTS. 2) THIS ACFT HAS SEVERAL NON STANDARD CHARACTERISTICS WHICH BOTH PLTS WERE DEALING WITH. A) THE AUTOPLT IS NOT THE SAME AS INSTALLED IN MOST OTHER FLEET ACFT. THE AUTOPLT DOES NOT AUTOMATICALLY CAPTURE ALT AS IN OTHER ACFT. B) THE ALT ALERT SYS IS DIFFERENT FROM MOST FLEET ACFT. 1000 FT BEFORE LEVELOFF; ONLY A LIGHT COMES ON. THEN AT 300 FT; A NEARLY INAUDIBLE SIGNAL. C) THE FO ALTIMETER IS DESCRIBED IN THE ACFT MANUAL AS BEING 'A CONVENTIONAL DRUM TYPE BAROMETRIC ALTIMETER.' THIS IS THE SAME TYPE OF ALTIMETER THAT WAS REMOVED FROM MIL HELIS 15 YRS AGO BECAUSE OF REPEATED ALTDEVS BY MIL PLTS. WHY DOES FAA ALLOW THEM TO BE IN SVC IN COMMERCIAL ACFT TODAY! D) FAA DOES NOT PROMOTE STANDARDIZATION IN COMMERCIAL ACFT (AIRLINES). IF STANDARDIZATION; AT LEAST AMONG THE BASIC INSTS WAS REQUIRED; THIS DEV WOULD NOT HAVE OCCURRED. SUPPLEMENTAL INFO FROM ACN 367730: I BELIEVE I SHOULD HAVE BEEN MUCH MORE AWARE AND MADE SURE THE FO WAS DIRECTING ALL HIS ATTN TO OP OF THE ACFT.