Narrative:

Location: final approach runway 17C, 5-7 mi out. On final approach to runway 17C at dfw. IMC with autoplt engaged, track annunciated on FMA a gradual steepening of GS was experienced. Autoplt was disengaged and localizer was tracked with a normal descent rate established. Aircraft was indicating about 2 dots high on GS at about 1000 ft AGL and 6 mi DME as we established ground and runway contact. Even though GS showed us almost full scale deflection high, we were obviously lower than a normal ILS descent profile. It looked more like a non precision type of profile. We leveled the aircraft and drove it into a more normal descent profile. GS continued to show a full scale deflection and we observed a 2 engine widebody transport airbus aircraft cargo liner holding short left of runway 17C at the end. Tower cleared him to cross and as he approached runway centerline, GS went from 2 dots bottom to show us 2 dots low. At this point we were about 2-3 mi out. The duration of GS interference was the same as his time in the hold short runway 17C position. 3 flts behind us reported full scale deviations in GS to tower also. I estimate that if we would have followed the GS it would have placed us between 400-500 ft AGL at 6 mi. WX at the field was scattered clouds 4-5 mi visibility with haze. On the approach it was overcast bases at around 1000 ft tops 4400 ft. I have seen a lot of interference from aircraft at the hold short over the yrs but never one so steady or for such a long period as this. I estimate the occurrence lasted about 2 mins. Callback conversation with reporter revealed the following information: the reporter stated he was flying an MD80. The aircraft holding at the east hold line on the most north taxiway was a widebody transport 2 engine airbus freighter (an A300). The reporter stated he was the captain of the MD80, had approximately 11000 hours total time, 220 hours in the last 90 days, and 8000 hours in the MD80. The reporter stated he did call the tower on the phone after the event. The tower supervisor said the WX was VFR and the ILS hold lines are not used when the WX is VFR at the airport. If he has a problem with that to contact ATC quality assurance and gave him 2 phone numbers. He tried numbers on numerous times and days during normal office hours but no answer. Other pilots on the approach both ahead and behind commented on the full scale deflection of their ILS GS. The reporter stated the turn onto final was normal at about 10-15 DME which would be 8-13 mi from the end of the runway. He said the rate of descent while coupled on the ILS GS had gradually increased from normal to about 1500 FPM when he disconnected the autoplt inside the OM about the time they broke out of the clouds at 6 DME. He said there was light turbulence. The reporter suggested the FAA put a freighter short of the runway hold line and fly a coupled ILS in the MD80. They may be shocked at how seemingly valid and sustained yet erroneous the false GS is.

Google
 

Original NASA ASRS Text

Title: ON AUTO PLT 'TRK TRK' ANNUNCIATED COUPLED TO THE ILS RWY 17C A GRADUAL STEEPENING OF THE GS WAS EXPERIENCED. AN AIRBUS FREIGHTER MOVED FROM THE HOLD SHORT LINE E OF RWY 17C WBOUND ACROSS THE RWY ON THE FARTHEST N TXWY AND THE ILS GS MOVED FROM AN ABNORMAL 2 DOTS HIGH TO A CORRECT 2 DOTS LOW.

Narrative: LOCATION: FINAL APCH RWY 17C, 5-7 MI OUT. ON FINAL APCH TO RWY 17C AT DFW. IMC WITH AUTOPLT ENGAGED, TRACK ANNUNCIATED ON FMA A GRADUAL STEEPENING OF GS WAS EXPERIENCED. AUTOPLT WAS DISENGAGED AND LOC WAS TRACKED WITH A NORMAL DSCNT RATE ESTABLISHED. ACFT WAS INDICATING ABOUT 2 DOTS HIGH ON GS AT ABOUT 1000 FT AGL AND 6 MI DME AS WE ESTABLISHED GND AND RWY CONTACT. EVEN THOUGH GS SHOWED US ALMOST FULL SCALE DEFLECTION HIGH, WE WERE OBVIOUSLY LOWER THAN A NORMAL ILS DSCNT PROFILE. IT LOOKED MORE LIKE A NON PRECISION TYPE OF PROFILE. WE LEVELED THE ACFT AND DROVE IT INTO A MORE NORMAL DSCNT PROFILE. GS CONTINUED TO SHOW A FULL SCALE DEFLECTION AND WE OBSERVED A 2 ENG WDB AIRBUS ACFT CARGO LINER HOLDING SHORT L OF RWY 17C AT THE END. TWR CLRED HIM TO CROSS AND AS HE APCHED RWY CTRLINE, GS WENT FROM 2 DOTS BOTTOM TO SHOW US 2 DOTS LOW. AT THIS POINT WE WERE ABOUT 2-3 MI OUT. THE DURATION OF GS INTERFERENCE WAS THE SAME AS HIS TIME IN THE HOLD SHORT RWY 17C POS. 3 FLTS BEHIND US RPTED FULL SCALE DEVS IN GS TO TWR ALSO. I ESTIMATE THAT IF WE WOULD HAVE FOLLOWED THE GS IT WOULD HAVE PLACED US BTWN 400-500 FT AGL AT 6 MI. WX AT THE FIELD WAS SCATTERED CLOUDS 4-5 MI VISIBILITY WITH HAZE. ON THE APCH IT WAS OVCST BASES AT AROUND 1000 FT TOPS 4400 FT. I HAVE SEEN A LOT OF INTERFERENCE FROM ACFT AT THE HOLD SHORT OVER THE YRS BUT NEVER ONE SO STEADY OR FOR SUCH A LONG PERIOD AS THIS. I ESTIMATE THE OCCURRENCE LASTED ABOUT 2 MINS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS FLYING AN MD80. THE ACFT HOLDING AT THE E HOLD LINE ON THE MOST N TXWY WAS A WDB 2 ENG AIRBUS FREIGHTER (AN A300). THE RPTR STATED HE WAS THE CAPT OF THE MD80, HAD APPROX 11000 HRS TOTAL TIME, 220 HRS IN THE LAST 90 DAYS, AND 8000 HRS IN THE MD80. THE RPTR STATED HE DID CALL THE TWR ON THE PHONE AFTER THE EVENT. THE TWR SUPVR SAID THE WX WAS VFR AND THE ILS HOLD LINES ARE NOT USED WHEN THE WX IS VFR AT THE ARPT. IF HE HAS A PROB WITH THAT TO CONTACT ATC QUALITY ASSURANCE AND GAVE HIM 2 PHONE NUMBERS. HE TRIED NUMBERS ON NUMEROUS TIMES AND DAYS DURING NORMAL OFFICE HRS BUT NO ANSWER. OTHER PLTS ON THE APCH BOTH AHEAD AND BEHIND COMMENTED ON THE FULL SCALE DEFLECTION OF THEIR ILS GS. THE RPTR STATED THE TURN ONTO FINAL WAS NORMAL AT ABOUT 10-15 DME WHICH WOULD BE 8-13 MI FROM THE END OF THE RWY. HE SAID THE RATE OF DSCNT WHILE COUPLED ON THE ILS GS HAD GRADUALLY INCREASED FROM NORMAL TO ABOUT 1500 FPM WHEN HE DISCONNECTED THE AUTOPLT INSIDE THE OM ABOUT THE TIME THEY BROKE OUT OF THE CLOUDS AT 6 DME. HE SAID THERE WAS LIGHT TURB. THE RPTR SUGGESTED THE FAA PUT A FREIGHTER SHORT OF THE RWY HOLD LINE AND FLY A COUPLED ILS IN THE MD80. THEY MAY BE SHOCKED AT HOW SEEMINGLY VALID AND SUSTAINED YET ERRONEOUS THE FALSE GS IS.

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.