Narrative:

Large transport at dqo is cleared to cross somto 130 descend and maintain 12000'. 3 1/2 mins later large transport is issued traffic 12 O'clock and 1 mi same direction at 11000'. Large transport responds, 'large transport roger looking.' mdt is later cleared to 7000' and asked, after his altitude readback, 'you got any traffic for us?' traffic was called passing off his right side at 12000' and 1/2 mi. 45 seconds later large transport, whose mode C had been intermittent and absent during the immediately preceding events, showed 105. When asked what his altitude was, large transport stated 'level 110,' (mode C still indicated 105). Large transport was told his assigned altitude was 12000'. His reply was that he read back '110.' the tape confirms this. Because of the lack of targets and loss of and maybe erroneous mode C readouts the incident is still under investigation. When large transport reports level at 11000' mdt had already left 9.9 in the descent. Supplemental information from acn 105264: the controller came back with a clearance to 'cross somto at 13000 or below, maintain 11000.' this is the clearance that was read back. Somewhere in our descent, 20000' to 16000', the controller gave us traffic, 'same direction, 11000', 1 mi.' we did not see traffic in that area although we did see traffic low at approximately 6-7 mi, on a slightly divergent heading. We continued our descent to 11000'. The aircraft we had seen moved out of our flight path to our left. As we leveled at 11000' I saw that the plane we had been watching earlier was about 1 mi or less to the left and appeared to be at our altitude. At this point we were just past somto intersection. The controller asked our altitude. I responded, '11000,' and reconfirmed that original clearance as 'somto 13000 or below, maintain 11000.' he said the read back altitude was 12000'. The arrival chart indicates that the pilot should expect to cross somto at 11000'. I have flown this arrival many times and have always received the 11000' crossing restriction. I was primed to hear, and acknowledge, 11000' by my past experience just as the controller was to issue the clearance by his. I believe the controller thought he gave us 12000'. The descent clearance was late, and I don't think the controller thought we could cross somto below 13000'. I believe the cause of this incident, and others like it, is excess controller workload in the high density climb and descent corridors in the us

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Original NASA ASRS Text

Title: LOSS OF SEPARATION BETWEEN ACR-LGT AND ACR-MDT WHEN CTLR FAILS TO CATCH WRONG ALT READBACK BY LGT.

Narrative: LGT AT DQO IS CLRED TO CROSS SOMTO 130 DSND AND MAINTAIN 12000'. 3 1/2 MINS LATER LGT IS ISSUED TFC 12 O'CLOCK AND 1 MI SAME DIRECTION AT 11000'. LGT RESPONDS, 'LGT ROGER LOOKING.' MDT IS LATER CLRED TO 7000' AND ASKED, AFTER HIS ALT READBACK, 'YOU GOT ANY TFC FOR US?' TFC WAS CALLED PASSING OFF HIS RIGHT SIDE AT 12000' AND 1/2 MI. 45 SECS LATER LGT, WHOSE MODE C HAD BEEN INTERMITTENT AND ABSENT DURING THE IMMEDIATELY PRECEDING EVENTS, SHOWED 105. WHEN ASKED WHAT HIS ALT WAS, LGT STATED 'LEVEL 110,' (MODE C STILL INDICATED 105). LGT WAS TOLD HIS ASSIGNED ALT WAS 12000'. HIS REPLY WAS THAT HE READ BACK '110.' THE TAPE CONFIRMS THIS. BECAUSE OF THE LACK OF TARGETS AND LOSS OF AND MAYBE ERRONEOUS MODE C READOUTS THE INCIDENT IS STILL UNDER INVESTIGATION. WHEN LGT RPTS LEVEL AT 11000' MDT HAD ALREADY LEFT 9.9 IN THE DSCNT. SUPPLEMENTAL INFO FROM ACN 105264: THE CTLR CAME BACK WITH A CLRNC TO 'CROSS SOMTO AT 13000 OR BELOW, MAINTAIN 11000.' THIS IS THE CLRNC THAT WAS READ BACK. SOMEWHERE IN OUR DSCNT, 20000' TO 16000', THE CTLR GAVE US TFC, 'SAME DIRECTION, 11000', 1 MI.' WE DID NOT SEE TFC IN THAT AREA ALTHOUGH WE DID SEE TFC LOW AT APPROX 6-7 MI, ON A SLIGHTLY DIVERGENT HDG. WE CONTINUED OUR DSCNT TO 11000'. THE ACFT WE HAD SEEN MOVED OUT OF OUR FLT PATH TO OUR LEFT. AS WE LEVELED AT 11000' I SAW THAT THE PLANE WE HAD BEEN WATCHING EARLIER WAS ABOUT 1 MI OR LESS TO THE LEFT AND APPEARED TO BE AT OUR ALT. AT THIS POINT WE WERE JUST PAST SOMTO INTXN. THE CTLR ASKED OUR ALT. I RESPONDED, '11000,' AND RECONFIRMED THAT ORIGINAL CLRNC AS 'SOMTO 13000 OR BELOW, MAINTAIN 11000.' HE SAID THE READ BACK ALT WAS 12000'. THE ARR CHART INDICATES THAT THE PLT SHOULD EXPECT TO CROSS SOMTO AT 11000'. I HAVE FLOWN THIS ARR MANY TIMES AND HAVE ALWAYS RECEIVED THE 11000' XING RESTRICTION. I WAS PRIMED TO HEAR, AND ACKNOWLEDGE, 11000' BY MY PAST EXPERIENCE JUST AS THE CTLR WAS TO ISSUE THE CLRNC BY HIS. I BELIEVE THE CTLR THOUGHT HE GAVE US 12000'. THE DSCNT CLRNC WAS LATE, AND I DON'T THINK THE CTLR THOUGHT WE COULD CROSS SOMTO BELOW 13000'. I BELIEVE THE CAUSE OF THIS INCIDENT, AND OTHERS LIKE IT, IS EXCESS CTLR WORKLOAD IN THE HIGH DENSITY CLB AND DSCNT CORRIDORS IN THE U.S.

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.