Narrative:

Both aircraft in this event were transiting to their requested altitudes. Small transport X was wbound climbing to 14000 ft. Air carrier Y was departing northbound, and was cleared on initial callup to 1000 ft below climbing small transport X. Air carrier Y leveled at this altitude, and on a subsequent transmission was cleared up 1000 ft, to stay below still climbing small transport X. Pilot read back a higher altitude, and climbed through small transport X assigned altitude. The cause of the system error was the missed readback, but upon review of the audio records, one other causative factor came to light. When the clearance altitude was issued, the altitude was unreadable on the audio tape due to a 'holler line' call from an adjacent facility. It was only by extracting the audio from the d-side tape that it could be determined that the controller had issued a lower altitude than was read back. The clarity of the transmission to the aircraft may have been very poor. The distrs of listening to at least a dozen holler lines, and even their interference in the audio xmissions from controller to aircraft, is certainly a factor in this incident.

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Original NASA ASRS Text

Title: ACR Y CLB THROUGH ASSIGNED ALT AND OCCUPIED ALT HAD LTSS FROM SMT X. PLTDEV. SYS ERROR. READBACK HEARBACK.

Narrative: BOTH ACFT IN THIS EVENT WERE TRANSITING TO THEIR REQUESTED ALTS. SMT X WAS WBOUND CLBING TO 14000 FT. ACR Y WAS DEPARTING NBOUND, AND WAS CLRED ON INITIAL CALLUP TO 1000 FT BELOW CLBING SMT X. ACR Y LEVELED AT THIS ALT, AND ON A SUBSEQUENT XMISSION WAS CLRED UP 1000 FT, TO STAY BELOW STILL CLBING SMT X. PLT READ BACK A HIGHER ALT, AND CLBED THROUGH SMT X ASSIGNED ALT. THE CAUSE OF THE SYS ERROR WAS THE MISSED READBACK, BUT UPON REVIEW OF THE AUDIO RECORDS, ONE OTHER CAUSATIVE FACTOR CAME TO LIGHT. WHEN THE CLRNC ALT WAS ISSUED, THE ALT WAS UNREADABLE ON THE AUDIO TAPE DUE TO A 'HOLLER LINE' CALL FROM AN ADJACENT FACILITY. IT WAS ONLY BY EXTRACTING THE AUDIO FROM THE D-SIDE TAPE THAT IT COULD BE DETERMINED THAT THE CTLR HAD ISSUED A LOWER ALT THAN WAS READ BACK. THE CLARITY OF THE XMISSION TO THE ACFT MAY HAVE BEEN VERY POOR. THE DISTRS OF LISTENING TO AT LEAST A DOZEN HOLLER LINES, AND EVEN THEIR INTERFERENCE IN THE AUDIO XMISSIONS FROM CTLR TO ACFT, IS CERTAINLY A FACTOR IN THIS INCIDENT.

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.