Separation was lost when an A321 flight crew accepted an ATC assigned altitude and crossing time that they were unable to meet.
Synopsis
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.
Second reporter narrative
We were climbing through FL340 in an Airbus 321 and ATC inquired how long to FL350. I replied 1 minute. He issued a clearance to be level at a zulu time corresponding to one minutes later. I put in a climb rate of 600 FPM and the aircraft lost speed and stopped climbing at about 34.6. I told the First Officer to tell ATC we can't make the restriction which he did. They vectored traffic away but we lost separation. They called it a loss of separation. I misjudged how fast I could climb that last 600 FT.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.