Narrative:

Air carrier X was at 35000' MSL, approximately 60 mi east of pmm. We were cleared discretion to FL240 and were told to plan on 15 northwest pmm at FL200. This clearance was acknowledged by my first officer. At this time my first officer placed 24000 in the altitude alerter. He then switched over to company frequency and I monitored center. While I was monitoring we were cleared to cross 15 northwest pmm at FL200. I set 20000 in altitude selector. When my first officer returned to center frequency, I informed him of the new altitude, 20000'. About 8 mi northwest pmm we ere passing through 21500' when my first officer noticed air carrier Y on our right side about to pass underneath us. Air carrier Y seemed to be very close to our altitude. At this time we arrested our descent and informed the controller of our near proximity to air carrier Y. We were then instructed to climb to 21000'. After landing we called ZAU on phone. Once the tapes were reviewed, ZAU conveyed the following to us: before we began our climb to 21000', we were within 1 mi and 600' (vertical). The initial clearance (acknowledged by first officer) was discretion to FL240 and plan 15 northwest pmm at FL220. The second clearance (acknowledged by me) was to cross 15 northwest pmm at FL220. Both time the pilots read back, '15 northwest pmm at FL200.' these readbacks were not corrected by the controller. For this reason, I was told by the supervisor at chicago ARTCC that this incident would be filed as a center operational error and that no action would be taken against the pilots. Supplemental information from acn 143593: I was working the radio. At about 50 east of pmm at FL350 we were given a descent to FL200. I requested the descent at pilot's direction, which was approved, but plan to cross 15 northwest of pmm at FL200. I then advised the captain that I would be off the frequency to get ATIS and talk to the company. While I was talking to the company, the captain received clearance to cross 15 northwest of pmm at FL200, and he then set the altitude alerter. I returned to the center frequency, and about over pmm passing through FL210, I returned to the center frequency, and about over pmm passing through FL210, I noticed an medium large transport to my lower right and pointed it out to the captain. As the aircraft passed from right to left, we realized the altitude sep was less than normal. When I asked the controller about the aircraft ahead, he said it was at FL200, and we should climb and maintain FL220. Moments later he said to maintain FL210. I then asked him to confirm the altitude we were cleared to and he just gave us a phone # to call and a new frequency.

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Original NASA ASRS Text

Title: ACR X HAD LESS THAN STANDARD SEPARATION FROM ACR Y. SYSTEM ERROR. PLT DEVIATION.

Narrative: ACR X WAS AT 35000' MSL, APPROX 60 MI E OF PMM. WE WERE CLRED DISCRETION TO FL240 AND WERE TOLD TO PLAN ON 15 NW PMM AT FL200. THIS CLRNC WAS ACKNOWLEDGED BY MY F/O. AT THIS TIME MY F/O PLACED 24000 IN THE ALT ALERTER. HE THEN SWITCHED OVER TO COMPANY FREQ AND I MONITORED CENTER. WHILE I WAS MONITORING WE WERE CLRED TO CROSS 15 NW PMM AT FL200. I SET 20000 IN ALT SELECTOR. WHEN MY F/O RETURNED TO CENTER FREQ, I INFORMED HIM OF THE NEW ALT, 20000'. ABOUT 8 MI NW PMM WE ERE PASSING THROUGH 21500' WHEN MY F/O NOTICED ACR Y ON OUR RIGHT SIDE ABOUT TO PASS UNDERNEATH US. ACR Y SEEMED TO BE VERY CLOSE TO OUR ALT. AT THIS TIME WE ARRESTED OUR DSNT AND INFORMED THE CTLR OF OUR NEAR PROX TO ACR Y. WE WERE THEN INSTRUCTED TO CLB TO 21000'. AFTER LNDG WE CALLED ZAU ON PHONE. ONCE THE TAPES WERE REVIEWED, ZAU CONVEYED THE FOLLOWING TO US: BEFORE WE BEGAN OUR CLB TO 21000', WE WERE WITHIN 1 MI AND 600' (VERT). THE INITIAL CLRNC (ACKNOWLEDGED BY F/O) WAS DISCRETION TO FL240 AND PLAN 15 NW PMM AT FL220. THE SECOND CLRNC (ACKNOWLEDGED BY ME) WAS TO CROSS 15 NW PMM AT FL220. BOTH TIME THE PLTS READ BACK, '15 NW PMM AT FL200.' THESE READBACKS WERE NOT CORRECTED BY THE CTLR. FOR THIS REASON, I WAS TOLD BY THE SUPVR AT CHICAGO ARTCC THAT THIS INCIDENT WOULD BE FILED AS A CENTER OPERROR AND THAT NO ACTION WOULD BE TAKEN AGAINST THE PLTS. SUPPLEMENTAL INFO FROM ACN 143593: I WAS WORKING THE RADIO. AT ABOUT 50 E OF PMM AT FL350 WE WERE GIVEN A DSNT TO FL200. I REQUESTED THE DSNT AT PLT'S DIRECTION, WHICH WAS APPROVED, BUT PLAN TO CROSS 15 NW OF PMM AT FL200. I THEN ADVISED THE CAPT THAT I WOULD BE OFF THE FREQ TO GET ATIS AND TALK TO THE COMPANY. WHILE I WAS TALKING TO THE COMPANY, THE CAPT RECEIVED CLRNC TO CROSS 15 NW OF PMM AT FL200, AND HE THEN SET THE ALT ALERTER. I RETURNED TO THE CENTER FREQ, AND ABOUT OVER PMM PASSING THROUGH FL210, I RETURNED TO THE CENTER FREQ, AND ABOUT OVER PMM PASSING THROUGH FL210, I NOTICED AN MLG TO MY LOWER RIGHT AND POINTED IT OUT TO THE CAPT. AS THE ACFT PASSED FROM RIGHT TO LEFT, WE REALIZED THE ALT SEP WAS LESS THAN NORMAL. WHEN I ASKED THE CTLR ABOUT THE ACFT AHEAD, HE SAID IT WAS AT FL200, AND WE SHOULD CLB AND MAINTAIN FL220. MOMENTS LATER HE SAID TO MAINTAIN FL210. I THEN ASKED HIM TO CONFIRM THE ALT WE WERE CLRED TO AND HE JUST GAVE US A PHONE # TO CALL AND A NEW FREQ.

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.