Narrative:

Air carrier X checked on the frequency as follows: 'den, air carrier X with ya at 3-1 and clearance to 2-4.' first aircraft, air carrier Y, was issued and acknowledged holding at smity, X at 17000'. I transmitted 'air carrier X holding clearance, advise ready to copy.' response: 'air carrier X, go ahead.' clearance issued, 'air carrier X cleared to smity intersection, hold northeast as published,C ross smity at and maintain fl 1-8-0, expect further clearance 1-XX-9.' pilot did not acknowledge clearance. Air carrier X then again checked on at FL310 clearance to fl 2-4-0. Air carrier X was then given clearance to cross smity at FL180, which was acknowledged. At this time there were 16 aircraft in the sector, all of whom were going to be holding at vectored/speed reduced for delay into den. Air carrier X broadcast: 'for air carrier X, we don't have a hold yet.' I immediately issued hold to air carrier X, as aircraft was 3 mins from holding fix. 'Air carrier X cleared to smity intersection, hold northeast as published cross smity at and maintain fl 1-8-0, expect further clearance at 1-XX-7.' air carrier X read back, 'we'll hold at 1-7-ah-1-xx-7.' simultaneously, supervisor was instructing us to disregard the metering times on the 2 lowest turbo propellers. I looked at air carrier X coming out of FL183, 5 mi from smity. Next update read 178, while the next update read 174. I immediately cleared air carrier X to make an immediate right heading 130 degrees, and to maintain FL180. Pilot acknowledged and started the turn. Sep had reduced to less than 5 mi for 30 seconds. A lot of things went wrong here. First, obviously, was my inability to hear and recognize that the pilot intended hold at 1-7-ah-1-xx-7 to constitute an altitude readback. Another contributing factors, which will never be pursued, is the pilot's poor phraseology, and the fact that he didn't question what he thought was clearance to 17000', less than 2 mins before the last holding clearance was issued to air carrier X, the clearance was to cross smity at FL180, with the holding clearance. Had the pilot read back the clearance properly; i.e., 'cross smity at and maintain 17000',' etc, our opportunity to determine that a readback error had occurred would have been dramatically increased. I heard the pilot give the readback, but I thought he was stuttering through the efc (hold at 1-7-ah-1-xx-7). I've always wondered if pilots use the readback of clrncs as reassurances for something for which they are unsure; i.e., 'if we read it back and we're wrong, ATC will correct us.' true--we will, if we hear and recognize the incorrect readback, but the liability is squarely on ATC's backs, not the pilots'. If the pilot uses incorrect phraseology in the readback and makes a mistake and an incident results, the pilot is exonerated and the controller takes the error on his record. When a pilot is flying he has 1 call sign to listen to and be responsible for a controller has everyone to be responsible for.

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Original NASA ASRS Text

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

Narrative: ACR X CHKED ON THE FREQ AS FOLLOWS: 'DEN, ACR X WITH YA AT 3-1 AND CLRNC TO 2-4.' FIRST ACFT, ACR Y, WAS ISSUED AND ACKNOWLEDGED HOLDING AT SMITY, X AT 17000'. I XMITTED 'ACR X HOLDING CLRNC, ADVISE READY TO COPY.' RESPONSE: 'ACR X, GO AHEAD.' CLRNC ISSUED, 'ACR X CLRED TO SMITY INTXN, HOLD NE AS PUBLISHED,C ROSS SMITY AT AND MAINTAIN FL 1-8-0, EXPECT FURTHER CLRNC 1-XX-9.' PLT DID NOT ACKNOWLEDGE CLRNC. ACR X THEN AGAIN CHKED ON AT FL310 CLRNC TO FL 2-4-0. ACR X WAS THEN GIVEN CLRNC TO CROSS SMITY AT FL180, WHICH WAS ACKNOWLEDGED. AT THIS TIME THERE WERE 16 ACFT IN THE SECTOR, ALL OF WHOM WERE GOING TO BE HOLDING AT VECTORED/SPD REDUCED FOR DELAY INTO DEN. ACR X BROADCAST: 'FOR ACR X, WE DON'T HAVE A HOLD YET.' I IMMEDIATELY ISSUED HOLD TO ACR X, AS ACFT WAS 3 MINS FROM HOLDING FIX. 'ACR X CLRED TO SMITY INTXN, HOLD NE AS PUBLISHED CROSS SMITY AT AND MAINTAIN FL 1-8-0, EXPECT FURTHER CLRNC AT 1-XX-7.' ACR X READ BACK, 'WE'LL HOLD AT 1-7-AH-1-XX-7.' SIMULTANEOUSLY, SUPVR WAS INSTRUCTING US TO DISREGARD THE METERING TIMES ON THE 2 LOWEST TURBO PROPS. I LOOKED AT ACR X COMING OUT OF FL183, 5 MI FROM SMITY. NEXT UPDATE READ 178, WHILE THE NEXT UPDATE READ 174. I IMMEDIATELY CLRED ACR X TO MAKE AN IMMEDIATE RIGHT HDG 130 DEGS, AND TO MAINTAIN FL180. PLT ACKNOWLEDGED AND STARTED THE TURN. SEP HAD REDUCED TO LESS THAN 5 MI FOR 30 SECS. A LOT OF THINGS WENT WRONG HERE. FIRST, OBVIOUSLY, WAS MY INABILITY TO HEAR AND RECOGNIZE THAT THE PLT INTENDED HOLD AT 1-7-AH-1-XX-7 TO CONSTITUTE AN ALT READBACK. ANOTHER CONTRIBUTING FACTORS, WHICH WILL NEVER BE PURSUED, IS THE PLT'S POOR PHRASEOLOGY, AND THE FACT THAT HE DIDN'T QUESTION WHAT HE THOUGHT WAS CLRNC TO 17000', LESS THAN 2 MINS BEFORE THE LAST HOLDING CLRNC WAS ISSUED TO ACR X, THE CLRNC WAS TO CROSS SMITY AT FL180, WITH THE HOLDING CLRNC. HAD THE PLT READ BACK THE CLRNC PROPERLY; I.E., 'CROSS SMITY AT AND MAINTAIN 17000',' ETC, OUR OPPORTUNITY TO DETERMINE THAT A READBACK ERROR HAD OCCURRED WOULD HAVE BEEN DRAMATICALLY INCREASED. I HEARD THE PLT GIVE THE READBACK, BUT I THOUGHT HE WAS STUTTERING THROUGH THE EFC (HOLD AT 1-7-AH-1-XX-7). I'VE ALWAYS WONDERED IF PLTS USE THE READBACK OF CLRNCS AS REASSURANCES FOR SOMETHING FOR WHICH THEY ARE UNSURE; I.E., 'IF WE READ IT BACK AND WE'RE WRONG, ATC WILL CORRECT US.' TRUE--WE WILL, IF WE HEAR AND RECOGNIZE THE INCORRECT READBACK, BUT THE LIABILITY IS SQUARELY ON ATC'S BACKS, NOT THE PLTS'. IF THE PLT USES INCORRECT PHRASEOLOGY IN THE READBACK AND MAKES A MISTAKE AND AN INCIDENT RESULTS, THE PLT IS EXONERATED AND THE CTLR TAKES THE ERROR ON HIS RECORD. WHEN A PLT IS FLYING HE HAS 1 CALL SIGN TO LISTEN TO AND BE RESPONSIBLE FOR A CTLR HAS EVERYONE TO BE RESPONSIBLE FOR.

Data retrieved from NASA's ASRS site as of August 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.