Narrative:

While hand-flying the approach, flight director bars indicated 'on glide path.' did not cross reference raw data which would have shown that the flight director had not locked onto the ILS. Became confused when I referenced the 2 altimeters (QFE vs qnh), requiring 10-15 seconds to figure out what was going on. Had descended about 500 ft below glide path when a recovery was initiated. Landed without incident. TCASII showed no conflict. Supplemental information from acn 211135: first officer got confused by qnh and QFE altimeters. He was following the flight director v-bars with not GS capture and descended below the GS about 500 ft low before recovery was started.

Google
 

Original NASA ASRS Text

Title: AN MLG ACR CREW, IN THE WX, GOT CONFUSED WITH THEIR QFE QNH PROCS AND OVERSHOT THEIR ASSIGNED ALT ON DSCNT.

Narrative: WHILE HAND-FLYING THE APCH, FLT DIRECTOR BARS INDICATED 'ON GLIDE PATH.' DID NOT CROSS REF RAW DATA WHICH WOULD HAVE SHOWN THAT THE FLT DIRECTOR HAD NOT LOCKED ONTO THE ILS. BECAME CONFUSED WHEN I REFED THE 2 ALTIMETERS (QFE VS QNH), REQUIRING 10-15 SECONDS TO FIGURE OUT WHAT WAS GOING ON. HAD DSNDED ABOUT 500 FT BELOW GLIDE PATH WHEN A RECOVERY WAS INITIATED. LANDED WITHOUT INCIDENT. TCASII SHOWED NO CONFLICT. SUPPLEMENTAL INFO FROM ACN 211135: FO GOT CONFUSED BY QNH AND QFE ALTIMETERS. HE WAS FOLLOWING THE FLT DIRECTOR V-BARS WITH NOT GS CAPTURE AND DSNDED BELOW THE GS ABOUT 500 FT LOW BEFORE RECOVERY WAS STARTED.

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.