FLT CREW OF DHC8; UTILIZING HGS FOR AN ILS APCH TO PDX; DISCOVER IT IS FOLLOWING A FALSE GS WELL BELOW THE PROPER VERT PATH.

Narrative

WE WERE IN VMC ON TOP OF A CLOUD LAYER ON VECTORS FOR THE ILS RWY 10R INTO PDX. THE CAPT WAS HAND FLYING THE AIRPLANE USING CUES FROM THE HEADS-UP GUIDANCE SYS (HGS). WE WERE ASSIGNED TO MAINTAIN 3000 FT (AS I BEST RECALL) UNTIL ESTABLISHED AND WERE CLRED FOR THE APCH. THE LOC AND GS WERE INTERCEPTED AND CAPTURED BY THE FLT DIRECTOR. APPROX 10-12 MI DME; THE CAPT CALLED FOR THE LNDG GEAR AND LNDG CHKLIST. I SELECTED THE GEAR DOWN; ACCOMPLISHED THE FLOW AND CHKLIST. WHEN I LOOKED UP TO XCHK THE INSTS AND ALT NEAR THE FAF; I NOTICED THE GS INDICATOR HAD MOVED ABRUPTLY TO THE UPPER LIMIT OF FULL SCALE DEFLECTION; BUT THE FLT DIRECTOR STILL INDICATED A CAPTURE ON THE GS AND WAS DIRECTING A FURTHER DSCNT. UPON XCHKING THE INDICATED ALT WITH THE ALT PRESCRIBED AT THE FAF ON THE APCH PLATE (YORKY AT 2011 FT); I ALERTED THE CAPT AND CALLED AN IMMEDIATE MISSED APCH. AFTER ALERTING APCH CTL OF OUR MISSED APCH; THEY GAVE US A 'LOW ALT ALERT' AND TOLD US TO 'CLB AND MAINTAIN 2000 FT.' WE ACCOMPLISHED THE PROC ALL THE WHILE MAINTAINING VFR ABOVE THE LOWER CLOUD LAYER. I FEEL OUR FOLLOWING OF COMPANY PROC; ALERTNESS AND POSITIVE CRM ALLOWED US TO EXIT A POTENTIALLY DANGEROUS SIT AT THE FIRST INDICATION WE NOTICED A PROB. I BELIEVE COM AND PERIODIC XCHKING WAS ALSO ESSENTIAL FOR A SAFE OUTCOME.