Narrative:

On final approach at dfw runway 17L, first officer was hand flying the aircraft. Both VHF navigation radios were tuned to 17L ILS and idented. When cleared for approach, the first officer and I both switched to approach (app) mode on our respective flight directors. The first officer received an immediate localizer and G/south capture signal, and his F/D indicated that we were well above G/south, and commanded a significant sink rate, which was initially followed. My instruments indicated a localizer capture and a G/south armed condition. Passing 2000' MSL, the first officer realized that his indications were not correct, and I concurred. Shortly thereafter, approach control told us to maintain 2000'. Approximately 15-20 seconds later, raw data G/south data came into view on both of or ADI's, and the runway was visly sighted. We were below the cloud deck with good visibility ground contact throughout the entire event, and safety of flight was never compromised in any way. We continued the approach to a safe landing. I believe the lesson to be learned from this experience is that when the airport is not in sight, and a difference in electronic guidance indications occurs between the captain and first officer, especially in an approach environment, don't go any lower, and tell ATC that you have a problem immediately. One should never follow a F/D west/O compatible raw data indications.

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Original NASA ASRS Text

Title: FO FLYING LGT ON APCH TO DFW MISREAD HIS NAVIGATION INDICATIONS AND DESCENDED BELOW GS. APCH CTLR ADVISED A GPWS AT SAME TIME FLT CREW REALIZED THEY WERE READING NAVIGATION INSTRUMENTS ERRONEOUSLY.

Narrative: ON FINAL APCH AT DFW RWY 17L, F/O WAS HAND FLYING THE ACFT. BOTH VHF NAV RADIOS WERE TUNED TO 17L ILS AND IDENTED. WHEN CLRED FOR APCH, THE F/O AND I BOTH SWITCHED TO APCH (APP) MODE ON OUR RESPECTIVE FLT DIRECTORS. THE F/O RECEIVED AN IMMEDIATE LOC AND G/S CAPTURE SIGNAL, AND HIS F/D INDICATED THAT WE WERE WELL ABOVE G/S, AND COMMANDED A SIGNIFICANT SINK RATE, WHICH WAS INITIALLY FOLLOWED. MY INSTS INDICATED A LOC CAPTURE AND A G/S ARMED CONDITION. PASSING 2000' MSL, THE F/O REALIZED THAT HIS INDICATIONS WERE NOT CORRECT, AND I CONCURRED. SHORTLY THEREAFTER, APCH CTL TOLD US TO MAINTAIN 2000'. APPROX 15-20 SECS LATER, RAW DATA G/S DATA CAME INTO VIEW ON BOTH OF OR ADI'S, AND THE RWY WAS VISLY SIGHTED. WE WERE BELOW THE CLOUD DECK WITH GOOD VIS GND CONTACT THROUGHOUT THE ENTIRE EVENT, AND SAFETY OF FLT WAS NEVER COMPROMISED IN ANY WAY. WE CONTINUED THE APCH TO A SAFE LNDG. I BELIEVE THE LESSON TO BE LEARNED FROM THIS EXPERIENCE IS THAT WHEN THE ARPT IS NOT IN SIGHT, AND A DIFFERENCE IN ELECTRONIC GUIDANCE INDICATIONS OCCURS BTWN THE CAPT AND F/O, ESPECIALLY IN AN APCH ENVIRONMENT, DON'T GO ANY LOWER, AND TELL ATC THAT YOU HAVE A PROB IMMEDIATELY. ONE SHOULD NEVER FOLLOW A F/D W/O COMPATIBLE RAW DATA INDICATIONS.

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.