Narrative:

ZAU cleared us to 19000'. I (PNF) read back '190' and set 19000' in altitude select window. First officer was flying and was off the frequency checking VOR (or something[?]). ATC then gave us a 090 degree vector. I then left the ATC frequency for fasten passenger in seats and arrival announcements. When I came back on frequency, we continued descent to FL180--the altitude in our altitude select window. I commented to first officer that we didn't get an altimeter setting, but that wasn't necessary since we were only going to FL180. My x-chk of altitude vs descent was the altitude select window (which is electronic, not mechanical). At approximately 18400' we got a traffic advisory from ATC: 'traffic 1 O'clock and 5 mi at FL180.' I immediately saw the traffic (when pointed out)--an small aircraft, 3-4 mi, slightly lower and approximately 1-2 O'clock. We were still descending to FL180. No evasive action was required and aircraft passed off our right side by 2-3 mi. About 1 min later ATC requested, 'verify your altitude.' I answered, '180 as assigned.' I was sure we'd been cleared to FL180. (Altitude select window was 180, an I'd previously made a remark about 180 to first officer.) we were vectored into ord with no further calls about altitude conflict. I feel the altitude select (electronic, not mechanical on EFIS) moved to 180 from 190 after I set it. This has been a recurring problem with electronic altitude select. I don't think I would have set 180 after just reading back 190 to ATC. I was distracted by other duties, and when I next checked the altitude vs descent, I saw the 180 in altitude select window and we used this as a reference from then on. Supplemental information from acn 144745: ZAU cleared flight to 18000'. Captain read back 18000' and I descended to 18000' out of 24000'. At 19000' center called traffic at 1 O'clock to 1:30 position, and at 18000'. I continued to 18000' and saw the traffic pass approximately 2 mi on our right side. Center asked what altitude we were at. Captain replied 18000', as assigned.

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Original NASA ASRS Text

Title: POTENTIAL CONFLICT GENERATED BY MISINTERP OF CLRNC IN ACR MLG DESCENT PROC.

Narrative: ZAU CLRED US TO 19000'. I (PNF) READ BACK '190' AND SET 19000' IN ALT SELECT WINDOW. F/O WAS FLYING AND WAS OFF THE FREQ CHKING VOR (OR SOMETHING[?]). ATC THEN GAVE US A 090 DEG VECTOR. I THEN LEFT THE ATC FREQ FOR FASTEN PAX IN SEATS AND ARR ANNOUNCEMENTS. WHEN I CAME BACK ON FREQ, WE CONTINUED DSNT TO FL180--THE ALT IN OUR ALT SELECT WINDOW. I COMMENTED TO F/O THAT WE DIDN'T GET AN ALTIMETER SETTING, BUT THAT WASN'T NECESSARY SINCE WE WERE ONLY GOING TO FL180. MY X-CHK OF ALT VS DSNT WAS THE ALT SELECT WINDOW (WHICH IS ELECTRONIC, NOT MECHANICAL). AT APPROX 18400' WE GOT A TFC ADVISORY FROM ATC: 'TFC 1 O'CLOCK AND 5 MI AT FL180.' I IMMEDIATELY SAW THE TFC (WHEN POINTED OUT)--AN SMA, 3-4 MI, SLIGHTLY LOWER AND APPROX 1-2 O'CLOCK. WE WERE STILL DSNDING TO FL180. NO EVASIVE ACTION WAS REQUIRED AND ACFT PASSED OFF OUR RIGHT SIDE BY 2-3 MI. ABOUT 1 MIN LATER ATC REQUESTED, 'VERIFY YOUR ALT.' I ANSWERED, '180 AS ASSIGNED.' I WAS SURE WE'D BEEN CLRED TO FL180. (ALT SELECT WINDOW WAS 180, AN I'D PREVIOUSLY MADE A REMARK ABOUT 180 TO F/O.) WE WERE VECTORED INTO ORD WITH NO FURTHER CALLS ABOUT ALT CONFLICT. I FEEL THE ALT SELECT (ELECTRONIC, NOT MECHANICAL ON EFIS) MOVED TO 180 FROM 190 AFTER I SET IT. THIS HAS BEEN A RECURRING PROB WITH ELECTRONIC ALT SELECT. I DON'T THINK I WOULD HAVE SET 180 AFTER JUST READING BACK 190 TO ATC. I WAS DISTRACTED BY OTHER DUTIES, AND WHEN I NEXT CHKED THE ALT VS DSNT, I SAW THE 180 IN ALT SELECT WINDOW AND WE USED THIS AS A REF FROM THEN ON. SUPPLEMENTAL INFO FROM ACN 144745: ZAU CLRED FLT TO 18000'. CAPT READ BACK 18000' AND I DSNDED TO 18000' OUT OF 24000'. AT 19000' CENTER CALLED TFC AT 1 O'CLOCK TO 1:30 POS, AND AT 18000'. I CONTINUED TO 18000' AND SAW THE TFC PASS APPROX 2 MI ON OUR RIGHT SIDE. CENTER ASKED WHAT ALT WE WERE AT. CAPT REPLIED 18000', AS ASSIGNED.

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.