Narrative:

While being vectored for the ILS runway 9R approach to ord, at 7000 ft MSL, chicago approach control cleared us for the approach 'to cross pratt at 5000 ft.' as PF, I commenced the descent immediately due to turbulence at that altitude and expedited our descent with a higher than normal descent rate. We were outside of the pratt fix and the GS had not started to come 'alive' when intercepting the localizer. Ord was accepting parallel approachs at this time to both runways 9L and 9R. The initial intercept altitude for wy 9L, is at 4000 ft MSL, while runway 9R (our runway) 5000 ft MSL. As we intercepted the localizer, we observed the winds aloft on our FMS indicated 180 degrees at over 50 KTS. Our assigned heading was 180 degrees. Approximately 2 mi outside of pratt, ATC told us to 'turn to 120 degrees and intercept,' which we did. While in my turn to the assigned heading of 120 degrees, we observed that we rapidly passed through the localizer. I immediately rolled wings level and corrected by turning back to a more southerly heading. My concern, of course, was the parallel approachs. In my opinion, the ATC assigned heading and our proximity to the localizer was insufficient to allow us to intercept without going through the localizer. Regardless, the aircraft was quickly corrected and stabilized on the localizer course. We were still descending rapidly and leveled at 5000 ft MSL, as assigned, when ATC asked our altitude. The captain (PNF) responded '5000 ft as assigned.' the approach controller admonished us by saying 'you're at 4500 ft...you were assigned 5000 ft.' 'huh, what's he talking about?' we had leveled at 5000 ft MSL. As we arrived over pratt at 5000 ft, the GS intersected our altitude and we followed it down to a normal landing. Later that evening, I telephoned the TRACON and spoke with the watch supervisor who assured me there had been 'no problem.' I don't know what happened here. Perhaps a combination of a couple things might have been in play here. There is a phenomenon I have experienced in the past explained to me as 'profile shadow' whereby an aircraft in a high rate of descent levels out at the assigned altitude, however, the ATC computers' detection rate cannot update quickly enough to reflect and synchronize the altitude with that superimposed on the mode C transponder signal. The ATC computers continue to 'think' the aircraft is descending at the high rate, and they show an erroneous altitude on the first couple sweeps, until all components get synchronized with the aircraft. It is my understanding that this phenomenon can even cause an 'altitude alert' on the ATC screen. Another possibility might be the fact that the controller gave us a heading that shot us through the localizer early in our turn. There may have been some confusion with us and the altitude of other aircraft flying the approach to runway 9 as he, too, descended on his GS. Regardless, we as a crew are grateful that ATC called this observed (although incorrect) excursion to our attention. Obviously a false altitude warning such as this when all is well is better than the consequences from no warning at all in a real excursion situation.

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Original NASA ASRS Text

Title: BAE146 IS QUESTIONED BY ATC REF ALT DURING PARALLEL APCHS INTO ORD.

Narrative: WHILE BEING VECTORED FOR THE ILS RWY 9R APCH TO ORD, AT 7000 FT MSL, CHICAGO APCH CTL CLRED US FOR THE APCH 'TO CROSS PRATT AT 5000 FT.' AS PF, I COMMENCED THE DSCNT IMMEDIATELY DUE TO TURB AT THAT ALT AND EXPEDITED OUR DSCNT WITH A HIGHER THAN NORMAL DSCNT RATE. WE WERE OUTSIDE OF THE PRATT FIX AND THE GS HAD NOT STARTED TO COME 'ALIVE' WHEN INTERCEPTING THE LOC. ORD WAS ACCEPTING PARALLEL APCHS AT THIS TIME TO BOTH RWYS 9L AND 9R. THE INITIAL INTERCEPT ALT FOR WY 9L, IS AT 4000 FT MSL, WHILE RWY 9R (OUR RWY) 5000 FT MSL. AS WE INTERCEPTED THE LOC, WE OBSERVED THE WINDS ALOFT ON OUR FMS INDICATED 180 DEGS AT OVER 50 KTS. OUR ASSIGNED HEADING WAS 180 DEGS. APPROX 2 MI OUTSIDE OF PRATT, ATC TOLD US TO 'TURN TO 120 DEGS AND INTERCEPT,' WHICH WE DID. WHILE IN MY TURN TO THE ASSIGNED HEADING OF 120 DEGS, WE OBSERVED THAT WE RAPIDLY PASSED THROUGH THE LOC. I IMMEDIATELY ROLLED WINGS LEVEL AND CORRECTED BY TURNING BACK TO A MORE SOUTHERLY HEADING. MY CONCERN, OF COURSE, WAS THE PARALLEL APCHS. IN MY OPINION, THE ATC ASSIGNED HEADING AND OUR PROX TO THE LOC WAS INSUFFICIENT TO ALLOW US TO INTERCEPT WITHOUT GOING THROUGH THE LOC. REGARDLESS, THE ACFT WAS QUICKLY CORRECTED AND STABILIZED ON THE LOC COURSE. WE WERE STILL DSNDING RAPIDLY AND LEVELED AT 5000 FT MSL, AS ASSIGNED, WHEN ATC ASKED OUR ALT. THE CAPT (PNF) RESPONDED '5000 FT AS ASSIGNED.' THE APCH CTLR ADMONISHED US BY SAYING 'YOU'RE AT 4500 FT...YOU WERE ASSIGNED 5000 FT.' 'HUH, WHAT'S HE TALKING ABOUT?' WE HAD LEVELED AT 5000 FT MSL. AS WE ARRIVED OVER PRATT AT 5000 FT, THE GS INTERSECTED OUR ALT AND WE FOLLOWED IT DOWN TO A NORMAL LNDG. LATER THAT EVENING, I TELEPHONED THE TRACON AND SPOKE WITH THE WATCH SUPVR WHO ASSURED ME THERE HAD BEEN 'NO PROB.' I DON'T KNOW WHAT HAPPENED HERE. PERHAPS A COMBINATION OF A COUPLE THINGS MIGHT HAVE BEEN IN PLAY HERE. THERE IS A PHENOMENON I HAVE EXPERIENCED IN THE PAST EXPLAINED TO ME AS 'PROFILE SHADOW' WHEREBY AN ACFT IN A HIGH RATE OF DSCNT LEVELS OUT AT THE ASSIGNED ALT, HOWEVER, THE ATC COMPUTERS' DETECTION RATE CANNOT UPDATE QUICKLY ENOUGH TO REFLECT AND SYNCHRONIZE THE ALT WITH THAT SUPERIMPOSED ON THE MODE C TRANSPONDER SIGNAL. THE ATC COMPUTERS CONTINUE TO 'THINK' THE ACFT IS DSNDING AT THE HIGH RATE, AND THEY SHOW AN ERRONEOUS ALT ON THE FIRST COUPLE SWEEPS, UNTIL ALL COMPONENTS GET SYNCHRONIZED WITH THE ACFT. IT IS MY UNDERSTANDING THAT THIS PHENOMENON CAN EVEN CAUSE AN 'ALT ALERT' ON THE ATC SCREEN. ANOTHER POSSIBILITY MIGHT BE THE FACT THAT THE CTLR GAVE US A HEADING THAT SHOT US THROUGH THE LOC EARLY IN OUR TURN. THERE MAY HAVE BEEN SOME CONFUSION WITH US AND THE ALT OF OTHER ACFT FLYING THE APCH TO RWY 9 AS HE, TOO, DSNDED ON HIS GS. REGARDLESS, WE AS A CREW ARE GRATEFUL THAT ATC CALLED THIS OBSERVED (ALTHOUGH INCORRECT) EXCURSION TO OUR ATTN. OBVIOUSLY A FALSE ALT WARNING SUCH AS THIS WHEN ALL IS WELL IS BETTER THAN THE CONSEQUENCES FROM NO WARNING AT ALL IN A REAL EXCURSION SIT.

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.