Narrative:

Deviation from GS. This report is documentation of a descent below actual GS occurred to air carrier flight on dec/xx/95 at dfw while on ILS approach to runway 17L. After numerous approach control delays we were finally vectored onto an intercept heading for runway 17L at 4000 ft. First officer was flying. Autoplt captured localizer high on indicated GS. I used vertical speed mode to descend to capture GS. I noticed the slope did not initially appear correct for the afl altitude, but due to numerous other distractions including low fuel, tight spacing and changing aircraft confign, I did not xchk afl altitude versus DME as frequently as I should have. Localizer appeared normal. We could tell the actual WX was about 1000/8 broken or better rather than the ATIS WX of 600/4 overcast. I was therefore not overly concerned with the electronic GS until, while following the GS, the autoplt pitched the nose down. We descended below the actual GS plane but remained coupled with a GS track and localizer track on the pma. I disconnected the autoplt, leveled the aircraft and flew visually to pick up a normal visual GS to a normal landing. A flight behind us called on ATC frequency that the GS was apparently in error and we verbally concurred on 119.4. On landing, we noticed an unmarked cargo DC8 aircraft slowly taxiing to cross runway 17L on taxiway north. It was between taxiway P and runway 17L. I suspect this aircraft may have disrupted the electronic GS signal.

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Original NASA ASRS Text

Title: FLC OF AN MLG DSNDED BELOW THE GS DUE TO SUSPECTED INTERFERENCE OF ANOTHER ACFT TAXIING, OR GND HOLDING, BEYOND THE ILS HOLD LINE.

Narrative: DEV FROM GS. THIS RPT IS DOCUMENTATION OF A DSCNT BELOW ACTUAL GS OCCURRED TO ACR FLT ON DEC/XX/95 AT DFW WHILE ON ILS APCH TO RWY 17L. AFTER NUMEROUS APCH CTL DELAYS WE WERE FINALLY VECTORED ONTO AN INTERCEPT HEADING FOR RWY 17L AT 4000 FT. FO WAS FLYING. AUTOPLT CAPTURED LOC HIGH ON INDICATED GS. I USED VERT SPD MODE TO DSND TO CAPTURE GS. I NOTICED THE SLOPE DID NOT INITIALLY APPEAR CORRECT FOR THE AFL ALT, BUT DUE TO NUMEROUS OTHER DISTRACTIONS INCLUDING LOW FUEL, TIGHT SPACING AND CHANGING ACFT CONFIGN, I DID NOT XCHK AFL ALT VERSUS DME AS FREQUENTLY AS I SHOULD HAVE. LOC APPEARED NORMAL. WE COULD TELL THE ACTUAL WX WAS ABOUT 1000/8 BROKEN OR BETTER RATHER THAN THE ATIS WX OF 600/4 OVCST. I WAS THEREFORE NOT OVERLY CONCERNED WITH THE ELECTRONIC GS UNTIL, WHILE FOLLOWING THE GS, THE AUTOPLT PITCHED THE NOSE DOWN. WE DSNDED BELOW THE ACTUAL GS PLANE BUT REMAINED COUPLED WITH A GS TRACK AND LOC TRACK ON THE PMA. I DISCONNECTED THE AUTOPLT, LEVELED THE ACFT AND FLEW VISUALLY TO PICK UP A NORMAL VISUAL GS TO A NORMAL LNDG. A FLT BEHIND US CALLED ON ATC FREQ THAT THE GS WAS APPARENTLY IN ERROR AND WE VERBALLY CONCURRED ON 119.4. ON LNDG, WE NOTICED AN UNMARKED CARGO DC8 ACFT SLOWLY TAXIING TO CROSS RWY 17L ON TXWY N. IT WAS BTWN TXWY P AND RWY 17L. I SUSPECT THIS ACFT MAY HAVE DISRUPTED THE ELECTRONIC GS SIGNAL.

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.