Narrative:

On 22R ILS at ord, ATC assigned 170 KTS to ridge OM. Just prior to saltz intersection (about 2 DME from ridge), visual contact with the aircraft ahead indicated we were closing and spacing had become marginal. TCASII indicated distance as just under 3 NM. Spacing was marginal and deteriorating, in accordance with safety we slowed to 140 KTS. About 1/2 mi from ridge, approach control asked our speed. We stated 140 KTS. Approach controller instructed us to go right back to 170 KTS and hold that until ridge. We began accelerating and noted that spacing had become a concern and that TCASII had showed less than 3 NM spacing at one point. Controller emphatically told us that spacing would be done with his radar and not with TCASII and that our spacing was now 5 NM. At that point TCASII showed 3.5 NM spacing which agreed with visual picture. Passed ridge, contacted tower. Just prior to touchdown, the in trail aircraft called 'ridge inbound' so spacing about 5 NM and no problem. We slowed based on visual contact and confirmed by TCASII. We were unable to advise ATC prior to his inquiry due to frequency congestion.

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Original NASA ASRS Text

Title: ACR LGT FLC USES TCASII TO PROVIDE THEIR OWN SPACING WHILE ON APCH AT ORD WHICH MADE THE APCH CTLR VERY UNHAPPY.

Narrative: ON 22R ILS AT ORD, ATC ASSIGNED 170 KTS TO RIDGE OM. JUST PRIOR TO SALTZ INTXN (ABOUT 2 DME FROM RIDGE), VISUAL CONTACT WITH THE ACFT AHEAD INDICATED WE WERE CLOSING AND SPACING HAD BECOME MARGINAL. TCASII INDICATED DISTANCE AS JUST UNDER 3 NM. SPACING WAS MARGINAL AND DETERIORATING, IN ACCORDANCE WITH SAFETY WE SLOWED TO 140 KTS. ABOUT 1/2 MI FROM RIDGE, APCH CTL ASKED OUR SPD. WE STATED 140 KTS. APCH CTLR INSTRUCTED US TO GO R BACK TO 170 KTS AND HOLD THAT UNTIL RIDGE. WE BEGAN ACCELERATING AND NOTED THAT SPACING HAD BECOME A CONCERN AND THAT TCASII HAD SHOWED LESS THAN 3 NM SPACING AT ONE POINT. CTLR EMPHATICALLY TOLD US THAT SPACING WOULD BE DONE WITH HIS RADAR AND NOT WITH TCASII AND THAT OUR SPACING WAS NOW 5 NM. AT THAT POINT TCASII SHOWED 3.5 NM SPACING WHICH AGREED WITH VISUAL PICTURE. PASSED RIDGE, CONTACTED TWR. JUST PRIOR TO TOUCHDOWN, THE IN TRAIL ACFT CALLED 'RIDGE INBOUND' SO SPACING ABOUT 5 NM AND NO PROBLEM. WE SLOWED BASED ON VISUAL CONTACT AND CONFIRMED BY TCASII. WE WERE UNABLE TO ADVISE ATC PRIOR TO HIS INQUIRY DUE TO FREQ CONGESTION.

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.