Narrative:

Air carrier X was climbing from FL310 to FL350. At approx xa:53 I noticed that there was a potential conflict between air carrier X and air carrier Y; which was westbound at FL340. I asked air carrier X how long to reach FL350. He was out of FL339 and he said about 1 minute. I then issued a valid time to climb restriction to reach FL350 by xa:55. At xa:54 the air carrier X pilot accepted the clearance. At xa:54.53 air carrier X stated that the aircraft was unable to climb further and that he would not meet the restriction and requested vectors. I could not issue a vector to air carrier X or it would negate the climb restriction. I then issued a 30 degree right turn to air carrier Y and issued traffic to both aircraft. Separation was lost. It is important to state that air carrier X actually had 2 minutes to level before the actual loss of separation. I think the issue is that aircraft are exceeding the operational limits trying to climb higher in order to save fuel. As it was; air carrier X accepted a climb to an altitude that the aircraft was not capable of making in a timely manner. Also the aircraft was not able to maintain at least a 500 ft per minute climb as required and did not advise ATC of the fact until it was too late to avoid a loss of separation. Also; I would like to see the restriction removed of turning an aircraft issued a climb restriction when the aircraft advises they will no longer be able to make the restriction. In this case I could not turn air carrier X without negating the climb restriction which would have transferred the responsibility for separation to me. If I could have turned the aircraft in this case I may have been able to maintain required separation.

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Original NASA ASRS Text

Title: Separation was lost when an A321 flight crew accepted an ATC assigned altitude and crossing time that they were unable to meet.

Narrative: Air Carrier X was climbing from FL310 to FL350. At approx XA:53 I noticed that there was a potential conflict between Air Carrier X and Air Carrier Y; which was westbound at FL340. I asked Air Carrier X how long to reach FL350. He was out of FL339 and he said about 1 minute. I then issued a valid time to climb restriction to reach FL350 by XA:55. At XA:54 the Air Carrier X pilot accepted the clearance. At XA:54.53 Air Carrier X stated that the aircraft was unable to climb further and that he would not meet the restriction and requested vectors. I could not issue a vector to Air Carrier X or it would negate the climb restriction. I then issued a 30 degree right turn to Air Carrier Y and issued traffic to both aircraft. Separation was lost. It is important to state that Air Carrier X actually had 2 minutes to level before the actual loss of separation. I think the issue is that aircraft are exceeding the operational limits trying to climb higher in order to save fuel. As it was; Air Carrier X accepted a climb to an altitude that the aircraft was not capable of making in a timely manner. Also the aircraft was not able to maintain at least a 500 FT per minute climb as required and did not advise ATC of the fact until it was too late to avoid a loss of separation. Also; I would like to see the restriction removed of turning an aircraft issued a climb restriction when the aircraft advises they will no longer be able to make the restriction. In this case I could not turn Air Carrier X without negating the climb restriction which would have transferred the responsibility for separation to me. If I could have turned the aircraft in this case I may have been able to maintain required separation.

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.