Narrative:

While on arrival routing and level at FL220 into iad and on washington center; the controller stated that there was a problem with descending us due to an aircraft directly below us also going into iad. We were given instruction to turn right to a heading of 240; which was approximately a 180 degree turn off the arrival route and off of our present heading. We were also issued instructions to descend to FL210; which we acknowledged; and verified as per SOP. Once in the turn and about 90 degrees off the arrival route; and descending through 21;600 ft we got a TCAS climb; which we followed and reversed the descent and started an 800 FPM climb as commanded by the TCAS. We never got below 21500 ft. When we told the controller we were climbing for a TCAS; she asked what we were doing descending; and instructed us to immediately climb back to and maintain FL220. When we stated we were following her instructions; she claimed she never gave us a descent clearance to FL210. The first officer and myself both believe she gave us the descent clearance; as it was read it back to the controller; and verified in the cockpit; as per SOP. So; that is what happened; and now for my narrative. Understanding that there was an aircraft below us; it would make sense for a controller to vector us off the arrival route to descend us; as that is not an uncommon practice. That said; we were very surprised when we were 90 degrees off the airway; and still turning in a direction opposite of the arrival route; to find this aircraft now in front of us; with us descending into its path. Not quite as it was stated to us; as we expected this other aircraft the controller was talking about to be ahead of us on what our previous routing was; the arrival routing; moving in a direction opposite of ours; with both aircraft getting greater separation. After the event; both the first officer and I discussed the situation; and both recall the controller giving us the descent clearance; and clearly reading that descent clearance back to the controller. The controller was quite flustered; as denoted by the increased pitch in her voice. Once we stated we were responding to the TCAS; she initially gave us a heading; which would have reversed our turn; but then had us continue our turn further around to a heading of 270 degrees. She stated she never gave us the descent; but at that point it would have just gotten into a 'he said' 'she said' situation. Our best estimation is we were never any closer than approximately 500' vertically; and about 2 miles horizontally. The first officer said he saw the other aircraft; and we were not on a collision course with it. I never saw it; as I was focused inside on the TCAS. I realize that one of the main reasons for filing this report is to be able to take something away from the situation; and learn from it. I cannot say this is the case; as I do not know what we could have done differently given the circumstances. However; if the findings show that the clearance was indeed not given to us; I am puzzled as to how two crewmembers could hear the same thing; read back an unstated clearance; and not be corrected by ATC; when the erroneous clearance is stated over the radio.

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Original NASA ASRS Text

Title: An air carrier Captain describes beginning a descend from FL220 into IAD following a vector turn to clear traffic and getting a TCAS climb command from nearby traffic at FL210. The Controller denied issuing a descent clearance.

Narrative: While on arrival routing and level at FL220 into IAD and on Washington Center; the Controller stated that there was a problem with descending us due to an aircraft directly below us also going into IAD. We were given instruction to turn right to a heading of 240; which was approximately a 180 degree turn off the arrival route and off of our present heading. We were also issued instructions to descend to FL210; which we acknowledged; and verified as per SOP. Once in the turn and about 90 degrees off the arrival route; and descending through 21;600 FT we got a TCAS climb; which we followed and reversed the descent and started an 800 FPM climb as commanded by the TCAS. We never got below 21500 FT. When we told the Controller we were climbing for a TCAS; she asked what we were doing descending; and instructed us to immediately climb back to and maintain FL220. When we stated we were following her instructions; she claimed she never gave us a descent clearance to FL210. The First Officer and myself both believe she gave us the descent clearance; as it was read it back to the Controller; and verified in the cockpit; as per SOP. So; that is what happened; and now for my narrative. Understanding that there was an aircraft below us; it would make sense for a controller to vector us off the arrival route to descend us; as that is not an uncommon practice. That said; we were very surprised when we were 90 degrees off the airway; and still turning in a direction opposite of the arrival route; to find this aircraft now in front of us; with us descending into its path. Not quite as it was stated to us; as we expected this other aircraft the Controller was talking about to be ahead of us on what our previous routing was; the arrival routing; moving in a direction opposite of ours; with both aircraft getting greater separation. After the event; both the First Officer and I discussed the situation; and both recall the controller giving us the descent clearance; and clearly reading that descent clearance back to the Controller. The Controller was quite flustered; as denoted by the increased pitch in her voice. Once we stated we were responding to the TCAS; she initially gave us a heading; which would have reversed our turn; but then had us continue our turn further around to a heading of 270 degrees. She stated she never gave us the descent; but at that point it would have just gotten into a 'he said' 'she said' situation. Our best estimation is we were never any closer than approximately 500' vertically; and about 2 miles horizontally. The First Officer said he saw the other aircraft; and we were not on a collision course with it. I never saw it; as I was focused inside on the TCAS. I realize that one of the main reasons for filing this report is to be able to take something away from the situation; and learn from it. I cannot say this is the case; as I do not know what we could have done differently given the circumstances. However; if the findings show that the clearance was indeed not given to us; I am puzzled as to how two crewmembers could hear the same thing; read back an unstated clearance; and not be corrected by ATC; when the erroneous clearance is stated over the radio.

Data retrieved from NASA's ASRS site as of April 2012 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.