Narrative:

We were in VMC on top of a cloud layer on vectors for the ILS runway 10R into pdx. The captain was hand flying the airplane using cues from the heads-up guidance system (hgs). We were assigned to maintain 3000 ft (as I best recall) until established and were cleared for the approach. The localizer and GS were intercepted and captured by the flight director. Approximately 10-12 mi DME, the captain called for the landing gear and landing checklist. I selected the gear down, accomplished the flow and checklist. When I looked up to xchk the instruments and altitude near the FAF, I noticed the GS indicator had moved abruptly to the upper limit of full scale deflection, but the flight director still indicated a capture on the GS and was directing a further descent. Upon xchking the indicated altitude with the altitude prescribed at the FAF on the approach plate (yorky at 2011 ft), I alerted the captain and called an immediate missed approach. After alerting approach control of our missed approach, they gave us a 'low altitude alert' and told us to 'climb and maintain 2000 ft.' we accomplished the procedure all the while maintaining VFR above the lower cloud layer. I feel our following of company procedure, alertness and positive CRM allowed us to exit a potentially dangerous situation at the first indication we noticed a problem. I believe communication and periodic xchking was also essential for a safe outcome.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.