Narrative:

In this situation, air carrier X was issued a descent clearance to FL290 and a frequency change to 124.57. Both air carrier X and air carrier Y read back the clearance simultaneously and then switched to the stated frequency. The descent of air carrier Y whom the clearance was never intended, put the aircraft in immediate peril with an overflt at FL290, aircraft Z. The issuance of an altitude and a frequency change in the same clearance resulted in the worst case scenario. 2 aircraft read back the altitude and not knowing that another aircraft was also reading back the same clearance, immediately switched to the assigned frequency. Even though the controller failed to get a positive readback, his immediate efforts to do so were in vain as both aircraft had switched. The issuance of an altitude and a frequency change in the same clearance should be prohibited. It has always been considered 'bad form' anyway. Supplemental information from acn 439894: facts: we were flying at our assigned altitude of FL310. We heard a clearance for our flight to descend to FL290. We leveled off at FL290. After several mins, the controller advised us to call ZID at our next stop about a possible pilot deviation. At FL290, we never saw any conflicting traffic, we never received a TCASII warning (TA or RA). We were never aware of any type of conflict. A subsequent phone call with ZID quality assurance office indicated that another flight with a similar flight number had been given the descent to FL290, and a frequency change in the same transmission. The controller received a garbled response (implying that 2 aircraft responded simultaneously). When the controller tried to clarify and ensure that only 1 aircraft had taken the clearance, there was no response -- both aircraft had already changed frequencys. Subsequently, a separation conflict occurred between my flight and another aircraft at FL290. Recommendations: 1) ATC should always advise pilots when aircraft with similar call signs are on the same frequency. We were not advised of the similar call signs. 2) ATC should avoid assigning an altitude change and frequency change in the same transmission. If we had been given the altitude change first, in a separate transmission from the frequency change, we would have still been in contact with the controller when he tried to verify the clearance. 3) pilots should be encouraged to momentarily delay a frequency change whenever they receive a frequency change and altitude change in the same transmission. Had we delayed 'flipping the switch' to the new frequency, we probably would have heard the controller's attempt to verify the clearance.

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Original NASA ASRS Text

Title: ARTCC CTLR AT ZID ISSUED A DSCNT CLRNC AND A FREQ CHANGE TO ACR X TO FL290. ACR Y WITH A SIMILAR CALL SIGN RESPONDED SIMULTANEOUSLY AND ALSO SWITCHED FREQ.

Narrative: IN THIS SIT, ACR X WAS ISSUED A DSCNT CLRNC TO FL290 AND A FREQ CHANGE TO 124.57. BOTH ACR X AND ACR Y READ BACK THE CLRNC SIMULTANEOUSLY AND THEN SWITCHED TO THE STATED FREQ. THE DSCNT OF ACR Y WHOM THE CLRNC WAS NEVER INTENDED, PUT THE ACFT IN IMMEDIATE PERIL WITH AN OVERFLT AT FL290, ACFT Z. THE ISSUANCE OF AN ALT AND A FREQ CHANGE IN THE SAME CLRNC RESULTED IN THE WORST CASE SCENARIO. 2 ACFT READ BACK THE ALT AND NOT KNOWING THAT ANOTHER ACFT WAS ALSO READING BACK THE SAME CLRNC, IMMEDIATELY SWITCHED TO THE ASSIGNED FREQ. EVEN THOUGH THE CTLR FAILED TO GET A POSITIVE READBACK, HIS IMMEDIATE EFFORTS TO DO SO WERE IN VAIN AS BOTH ACFT HAD SWITCHED. THE ISSUANCE OF AN ALT AND A FREQ CHANGE IN THE SAME CLRNC SHOULD BE PROHIBITED. IT HAS ALWAYS BEEN CONSIDERED 'BAD FORM' ANYWAY. SUPPLEMENTAL INFO FROM ACN 439894: FACTS: WE WERE FLYING AT OUR ASSIGNED ALT OF FL310. WE HEARD A CLRNC FOR OUR FLT TO DSND TO FL290. WE LEVELED OFF AT FL290. AFTER SEVERAL MINS, THE CTLR ADVISED US TO CALL ZID AT OUR NEXT STOP ABOUT A POSSIBLE PLTDEV. AT FL290, WE NEVER SAW ANY CONFLICTING TFC, WE NEVER RECEIVED A TCASII WARNING (TA OR RA). WE WERE NEVER AWARE OF ANY TYPE OF CONFLICT. A SUBSEQUENT PHONE CALL WITH ZID QUALITY ASSURANCE OFFICE INDICATED THAT ANOTHER FLT WITH A SIMILAR FLT NUMBER HAD BEEN GIVEN THE DSCNT TO FL290, AND A FREQ CHANGE IN THE SAME XMISSION. THE CTLR RECEIVED A GARBLED RESPONSE (IMPLYING THAT 2 ACFT RESPONDED SIMULTANEOUSLY). WHEN THE CTLR TRIED TO CLARIFY AND ENSURE THAT ONLY 1 ACFT HAD TAKEN THE CLRNC, THERE WAS NO RESPONSE -- BOTH ACFT HAD ALREADY CHANGED FREQS. SUBSEQUENTLY, A SEPARATION CONFLICT OCCURRED BTWN MY FLT AND ANOTHER ACFT AT FL290. RECOMMENDATIONS: 1) ATC SHOULD ALWAYS ADVISE PLTS WHEN ACFT WITH SIMILAR CALL SIGNS ARE ON THE SAME FREQ. WE WERE NOT ADVISED OF THE SIMILAR CALL SIGNS. 2) ATC SHOULD AVOID ASSIGNING AN ALT CHANGE AND FREQ CHANGE IN THE SAME XMISSION. IF WE HAD BEEN GIVEN THE ALT CHANGE FIRST, IN A SEPARATE XMISSION FROM THE FREQ CHANGE, WE WOULD HAVE STILL BEEN IN CONTACT WITH THE CTLR WHEN HE TRIED TO VERIFY THE CLRNC. 3) PLTS SHOULD BE ENCOURAGED TO MOMENTARILY DELAY A FREQ CHANGE WHENEVER THEY RECEIVE A FREQ CHANGE AND ALT CHANGE IN THE SAME XMISSION. HAD WE DELAYED 'FLIPPING THE SWITCH' TO THE NEW FREQ, WE PROBABLY WOULD HAVE HEARD THE CTLR'S ATTEMPT TO VERIFY THE CLRNC.

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.