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.