Narrative:

Large transport X called on frequency descending to FL240. Descended large transport X to FL220. Large transport X requested lower. Descended large transport X to 11000. During pilot readback altitude I observed data block of medium large transport B climbing to FL230, putting him in conflict with large transport X. Instructed large transport X to amend altitude, maintain 16000. Had controller working medium large transport Y amend his altitude to 15000. Placed dodecagon on large transport X. Medium large transport Y called on frequency climbing to 15000. Issued traffic to medium large transport Y on large transport X. Issued traffic to large transport X on medium large transport Y. At 5 NM I observed data blocks of large transport X and medium large transport Y at 16000 and 15000 respectively. On the next 'hit' (update) observed altitude of large transport X at 15600. Asked pilot of large transport X to verify assigned altitude of 16000. Pilot replied in the negative stating his last clearance was down to 11000. Issued an immediate left turn to large transport X. Issued an immediate left turn to medium large transport Y and called the traffic on large transport X. No reply received. Reissued immediate left turn to medium large transport Y. Pilot replied in a left turn then stated 'traffic in sight.' large transport X returned to 16000 ft at which time both aircraft were cleared on course. I continued to work the position until being relieved. We found out later that the amended altitude clearance to large transport X was acknowledged by the word 'blocked' and not the word 'roger' as I had thought. Supplemental information from acn 254377: while descending, center called out traffic ahead of us at 15000 ft, 1 O'clock position. The copilot acknowledged saying 'looking for traffic.' at the time, we were mostly IMC, flying in and out of small buildups. At approximately 15300 ft center called and asked 'what is your altitude.' the copilot responded 'descending to 11000 ft as assigned.' the controller gave us an immediate turn to the left, also gave opposite traffic an immediate turn. I disconnected the autoplt, made an evasive left turn and began climbing. We leveled at 16000 ft. We had come within 1 mi of an air carrier medium large transport. I called ZID after landing. They reviewed the tapes. The supervisor told me that the controller had issued a descent to 11000 ft. He also said the controller reissued 16000 ft, but the tapes revealed the clearance was never received. Instead the phrase 'blocked' was heard on the tape, which he claimed sounded like 'roger' to him. After our fifth morning in a row of flying all night, fatigue may have set in. I guess my point here is, we should have caught the controller's mistake and queried him about the altitude. It also brings out the issue, why are cargo pilots unimportant when it comes to TCASII issues. Supplemental information from acn 254694: by the fifth day of flying, fatigue is a real concern and problem. The point here is that we should have caught the controller's mistake and questioned him about the assigned altitude.

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Original NASA ASRS Text

Title: LGT DSNDING MISSED AMENDED ALT ASSIGNMENT AND HAD LTSS WITH AN ACR MLG.

Narrative: LGT X CALLED ON FREQ DSNDING TO FL240. DSNDED LGT X TO FL220. LGT X REQUESTED LOWER. DSNDED LGT X TO 11000. DURING PLT READBACK ALT I OBSERVED DATA BLOCK OF MLG B CLBING TO FL230, PUTTING HIM IN CONFLICT WITH LGT X. INSTRUCTED LGT X TO AMEND ALT, MAINTAIN 16000. HAD CTLR WORKING MLG Y AMEND HIS ALT TO 15000. PLACED DODECAGON ON LGT X. MLG Y CALLED ON FREQ CLBING TO 15000. ISSUED TFC TO MLG Y ON LGT X. ISSUED TFC TO LGT X ON MLG Y. AT 5 NM I OBSERVED DATA BLOCKS OF LGT X AND MLG Y AT 16000 AND 15000 RESPECTIVELY. ON THE NEXT 'HIT' (UPDATE) OBSERVED ALT OF LGT X AT 15600. ASKED PLT OF LGT X TO VERIFY ASSIGNED ALT OF 16000. PLT REPLIED IN THE NEGATIVE STATING HIS LAST CLRNC WAS DOWN TO 11000. ISSUED AN IMMEDIATE L TURN TO LGT X. ISSUED AN IMMEDIATE L TURN TO MLG Y AND CALLED THE TFC ON LGT X. NO REPLY RECEIVED. REISSUED IMMEDIATE L TURN TO MLG Y. PLT REPLIED IN A L TURN THEN STATED 'TFC IN SIGHT.' LGT X RETURNED TO 16000 FT AT WHICH TIME BOTH ACFT WERE CLRED ON COURSE. I CONTINUED TO WORK THE POS UNTIL BEING RELIEVED. WE FOUND OUT LATER THAT THE AMENDED ALT CLRNC TO LGT X WAS ACKNOWLEDGED BY THE WORD 'BLOCKED' AND NOT THE WORD 'ROGER' AS I HAD THOUGHT. SUPPLEMENTAL INFO FROM ACN 254377: WHILE DSNDING, CTR CALLED OUT TFC AHEAD OF US AT 15000 FT, 1 O'CLOCK POS. THE COPLT ACKNOWLEDGED SAYING 'LOOKING FOR TFC.' AT THE TIME, WE WERE MOSTLY IMC, FLYING IN AND OUT OF SMALL BUILDUPS. AT APPROX 15300 FT CTR CALLED AND ASKED 'WHAT IS YOUR ALT.' THE COPLT RESPONDED 'DSNDING TO 11000 FT AS ASSIGNED.' THE CTLR GAVE US AN IMMEDIATE TURN TO THE L, ALSO GAVE OPPOSITE TFC AN IMMEDIATE TURN. I DISCONNECTED THE AUTOPLT, MADE AN EVASIVE L TURN AND BEGAN CLBING. WE LEVELED AT 16000 FT. WE HAD COME WITHIN 1 MI OF AN ACR MLG. I CALLED ZID AFTER LNDG. THEY REVIEWED THE TAPES. THE SUPVR TOLD ME THAT THE CTLR HAD ISSUED A DSCNT TO 11000 FT. HE ALSO SAID THE CTLR REISSUED 16000 FT, BUT THE TAPES REVEALED THE CLRNC WAS NEVER RECEIVED. INSTEAD THE PHRASE 'BLOCKED' WAS HEARD ON THE TAPE, WHICH HE CLAIMED SOUNDED LIKE 'ROGER' TO HIM. AFTER OUR FIFTH MORNING IN A ROW OF FLYING ALL NIGHT, FATIGUE MAY HAVE SET IN. I GUESS MY POINT HERE IS, WE SHOULD HAVE CAUGHT THE CTLR'S MISTAKE AND QUERIED HIM ABOUT THE ALT. IT ALSO BRINGS OUT THE ISSUE, WHY ARE CARGO PLTS UNIMPORTANT WHEN IT COMES TO TCASII ISSUES. SUPPLEMENTAL INFO FROM ACN 254694: BY THE FIFTH DAY OF FLYING, FATIGUE IS A REAL CONCERN AND PROB. THE POINT HERE IS THAT WE SHOULD HAVE CAUGHT THE CTLR'S MISTAKE AND QUESTIONED HIM ABOUT THE ASSIGNED ALT.

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.